BIOLOGY CLASS NOTES FOR CBSE Chapter 27. Principles of Inheritance 01. Introduction The transfer of characters from parents to offspring is known as inheritance. Progeny produced resembles the parents closely but is not identical in all the respects. The reason behind is variation. The branch of science which deals with the inheritance as well as the variation of characters from parents to offspring is Genetics.
02. Mendel’s Laws of Inheritance Mendel was born on July 22, in 1822. He worked on Pisum sativum (Garden pea of Edible pea) for 7 years (1856−1863) and proposed the law of inheritance in living organisms.
03. Selection of Pea Plant The (i) (ii) (iii) (iv)
main reasons for adopting garden pea for experiments were as follows : Pea has many distinct alternative traits (clear contrasting traits). It produces a large number of seeds and completes its life cycle in one season. Flowers show self (bud) pollination, so are true breeding. It is easy to artificially cross-pollination the pea flowers. The hybrids thus produced were fertile.
04. Reasons for Mendel’s Success (a) Mendel applied statistical method and mathematical logic for analysing his results. (b) He kept accurate records of his experiments, giving all the details of number and type of individuals, which are a necessity in the genetic studies. (c) Mendel experimented on a number of plants for the same trait and obtained hundreds of offspring. A large sampling size gave credibility to his results. (d) He tried to formulate theoretical explanations for the observed results. These explanations were further tested by conducting experiments for successive generation of the test plants.
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05. Inheritance of One Gene Study of inheritance of single pair of contrasting traits of a character at a time is called one gene inheritance. Mendel crossed true breeding tall variety (6−7 ft.) and true breeding dwarf variety (0.75−1 ft.) pea plants to study the inheritance of one gene. The plants used in initial cross are referred to as P1 and P2 or parents. Since pea is self-fertilising, the anthers should be removed from the female parent before maturity for the purpose of cross pollination. The method of removal of anthers from bisexual flowers of female parent plant is called emasculation. The pollens, then at the dehiscence stage, is brought from the male parent and is dusted on the stigma or emasculated flower. He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the filial1 (offspring) progeny or the F1.
06. Concept of Factors Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. We now call these factors as “genes”. Genes which code for pair of contrasting traits are known as alleles i.e. they are slightly different forms of the same gene. For example, if T is used for the ‘tall’ trait and t for ‘dwarf’ then T and t are alleles of each other.
07. Homozygous and Heterozygous Mendel proposed that in true breeding, tall or dwarf pea variety the allelic pair of genes for height are identical, TT and tt, respectively. This condition was termed as ‘homozygous’ by Bateson and Saunders. An individual having two different alleles (Tt) will be called gybrid. Bateson and Saunders termed this condition as ‘heterozygous’.
08. Genotype and Phenotype Genotype is representation of genetic complement of an individual with respect to one or more characters. e.g., TT, Tt, tt. Phenotype is observable morphological appearance. The phenotypes of an individual is determined by different combinations of alleles e.g., tallness, dwarfness.
09. Dominant and Recessive The one that expresses itself is called dominant factor while which fails to express is termed as recessive factor. In other words we can say that a dominant allele influences the appearance of the phenotype even in the presence of an alternative allele.
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10. Concept of Segregation The segregation of alleles is a random process and so there is a 50% change of a gamete containing either allele. In this way the gametes of the tall TT plants have the allele T and the gametes of the dwarf tt plants have the allele t. During fertilisation of the two alleles, T from one parent through the pollen (n), and t from the female parent through the egg (n) are united to produce zygotes (2n) that have one T allele and one t allele i.e. hybrid of heterozygous Tt plant (2n). Punnett square was developed by a British geneticist, Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
11. Test Cross To determine the genotype of a tall plant at F2, Mendel crossed the tall plant from F2 with a dwarf plant. This is called a test cross. In a typical test cross, an organism showing a dominant phenotype is crossed with the recessive parent instead of self-pollination. The progenies of such a cross can be easily analysed to predict the genotype of test organism.
12. Law of Dominance Mendel experimented with garden pea for seven characters. In each case he found that : (a) Every character is controlled by discrete units called factors. (b) The factors occur in pairs. (c) In a dissimilar pair of factors (e.g. Tt), only one is able to express its effect that called as dominant factor. The other factor which does not show its effect is known as recessive factor. The law of dominance is used to explain the expression of only one of the parental traits in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained in F2 generation. This law is not universally applicable.
13. Law of Segregation This law is based on the fact that the two factors of a character present in an individual do not get mixed up (blending) and both the traits are recovered as such in the F2 generation though one of these is not seen at the F1 stage. During gamete or spore formation, factors of a pair separate or segregate from each other, so that a gamete carries only one factor of a character. This ensures the purity of gametes.
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14. Exceptions to Mendelian Principles (a) Incomplete Dominance : After Mendelism, a few cases were observed where F1 phenotype is intermediate between dominant and recessive phenotype, it means F1 did not resemble either of the two parents and was in between the two. The Mendelian concept of a gene controlling a single character has also expanded to take into account genes which affect several characters simultaneously (pleiotropy). It means in pleiotropy, a single gene product may produce more than one effect or control several phenotypes depending on its position. (b) Multiple allelism : Mendel proposed that each gene has two contrasting forms i.e. allels. But there are some genes which are having more than two alternative forms (allele). Presence of more than two alleles for a gene is known as multiple allelism. A good example is different types of red blood cells that determine ABO blood grouping in human beings. (c) Co-dominance : Besides incomplete dominance, certain alleles show co-dominance. Here in F1 hybrid, both alleles express themselves equally and there is no mixing of the effect of the both alleles, therefore hybrid progeny (F1) resembles both parents. The alleles which do not show dominance-recessive relationship and are able to express themselves independently when present together are called co-dominant alleles.
15. Inheritance of Two Genes Law of Independent Assortment Based upon the results obtained in dihybrid crosses, Mendel proposed a second set of generalisations that we call Mendel’s Law of Independent Assortment. The law states that “when two pairs of traits are combined in a hybrid, segregation of one pair of traits is independent to the other pair of traits”. Segregation of one pair of factors will occur independently of the other pair or they will assort independently. According, the gametes must carry all possible combinations of the factors in equal frequency.
16. Two Genes Interaction (w.r.t. Post-Mendelism) (a) Complementary genes the complementary genes are two genes present on separate loci that interact together to produce dominant phenotypic character, neither of them if present alone, can express itself. It means that these genes are complementary to each other. (b) Duplicate genes If the dominant alleles of two gene loci produce the same phenotype, whether inherited together or separately, the 9:3:3:1 ratio is modified into a 15:1 ratio. (c) Epistasis
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(d) A gene which masks (hides) the action of another gene (non allelic) is termed as epistatic gene. The process is called epistasis. The gene whose effects are masked is called hypostatic gene. Epistasis is of two types. (i) Recessive epistasis : Here the recessive allele in homozygous condition masks the effect of dominant allele, e.g., in mice, the wild body colour is known as agouti (greyish) and is controlled by a gene say A which is hypostatic to recessive allele C, A gives rise to agouti. (ii) Dominant epistasis : In summer squash or Cucurbita pepo, there are three types of fruit colour yellow, green and white. White colour is dominant over other colours, while yellow is dominant over green. Gene for white colour (W) masks the effects of yellow colour gene (Y). So, yellow colour is formed only when the dominant epistatic gene is represented by its recessive allele (w). S.N o. (i) (ii) (iii) (iv) (v) (vi) (vii) (viii)
Types of non-allelic genetic interactions Complementary genes Duplicate genes Recessive epistasis Dominant epistasis Polymeric/Additive genes Inhibitory genes Supplementary genes Collaborative gene action
Dihybrid phenotypic ratios in F2 generation 9:7 15:1 9:3:4 12:3:1 9:6:1 13:3 9:3:4 9:3:3:1
17. Polygenic Inheritance or Quantitative Inheritance In humans we do not just have tall or short people as two distinct alternatives but a whole range of possible heights. Such traits are generally controlled by two or more genes and are thus called as polygenic traits. The inheritance of polygenic traits is called polygenic or quantitative inheritance. In quantitative inheritance, the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional polypeptide and full trait is shown when all the dominant alleles are present. Genes involved in quantitative inheritance are called polygenes. H. Nilsson-Ehle (1908) and East (1910) demonstrated segregation and assortment of genes controlling quantitative traits, e.g. Kernel colour in wheat and corolla length in tobacco. (a) Number of phenotype for polygenes = 2n+1 (b) Number of genotype for polygenes = 3n, where n represents pair of polygenes.
18. Chromosomal Theory of Inheritance Chromosomal theory of inheritance was proposed independently by Sutton and Boveri. The salient features of chromosomal theory of inheritance are as follows :
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(a) Like the hereditary traits the chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation. The two neither get lost nor mixed up. They behave as units. (b) Both chromosomes as well as genes occur in pairs in the somatic or diploid cells. The two alleles of a gene pair are located on homologous sites on homologous chromosomes. Both chromosomes as well as genes segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete. (c) A gamete contains only one chromosome of a type and only one of the two alleles of a trait. (d) The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization. Experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variations that sexual reproduction produced. Morgan worked with the tiny flies, Drosophila melanogaster, which were found very suitable for such studies.
19. Linkage and Recombination According to Mendel’s law of independent assortment, the gene controlling different characters get assorted independent to each other. It is correct if the genes are present on two different chromosomes, but if these genes are present on same chromosome they may or may not show independent assortment. If crossing over takes place between these two genes then the genes get segregated and they will assort independent to each other. But if there is no crossing over between these two genes there is no segregation, hence only parental combination will be found in gametes. Morgan carried out several dihybrid crosses in Drosophila to study genes that were X-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas. Morgan coined the term linkage to describe this physical association of genes on same chromosome and the term recombination to describe the generation of non-parental gene combinations.
20. Chromosomal Mapping Crossing over is important in locating the genes on chromosome. The genes are arranged linearly on the chromosome. This sequence and the relative distances between various genes is graphically represented in terms of recombination frequencies or cross over values (COV). This is known as linkage map of chromosome. Distance or cross over units are called centimorgan (cM) or map unit. Term centimorgan is used in eukaryotic genetics and map unit in prokaryotic genetics. Number of recombinants Recombination frequency or cross over value = × Total number of of fsprings The recombination frequency depends upon the distance between the genes. If the distance between the genes is lesser the chances of crossing over is less and hence recombination frequency is also lesser and vice versa.
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A.H. Sturtevant suggested that these recombination frequencies can be utilized in predicting the sequence of genes on the chromosome. On the basis of recombination frequency, he prepared first chromosomal map or genetic map for Drosophila.
21. Sex Determination Establishment of sex through differential development in an individual at an early stage of life is called sex determination. Different species use very different strategies for this purpose. The initial clue about the genetic or chromosomal mechanism of sex determination can be traced back to some of the experiments carried out in insects. (I) Chromosomal basis of sex determination : The foundation of this type was laid down by Henking (1891). He traced a specific nuclear structure all through spermatogenesis in a few insects. Henking also observed that only 50% of the sperm received this structure. This structure was termed ‘X body’ by him. ‘X body’ was actually a chromosome, therefore it was given the name X-chromosome. Stevens (1902) discovered Y-chromosome. X and Y chromosomes named as sex chromosomes by Wilson and Stevens (1905). Chromosomal basis of sex-determination is of the following types: (a) Male heterogamety : In this type male individul produces two different types of gametes. Thus, the sperm determines the sex of the offspring. It involves two types of sex determining mechanisms; XO type and XY type. (i) XO type (XX-XO type) : It is observed in large number of insects e.g. Grasshopper. Number of chromosomes are different in male and female individuals. It is clear that, all eggs (ova) bear an additional X-chromosome besides the autosomes while only 50% of the sperms bear X-chromosomes. In grasshopper, eggs fertilised by (A+X) type sperm become females while those fertilised by (A+O) type sperm become males. Therefore, sperm determines the sex of the offspring. Due to the involvement of the X-chromosomes in sex determination, it was designated to be the sex chromosome. (ii) XY type (XX-XY type) : In a number of other insects like Drosophila and mammals including human beings, the males contain two types of sex chromosomes (X and Y) while females possess two similar type of sex chromosomes (XX). Both male and females have same number of chromosomes. In males, Y-chromosome is often shorter than the X-chromosome.
Sex Determination in Humans Human beings have 22 pairs of autosomes and one pair of sex chromosomes. All the ova (haploid) formed by female are similar in their chromosome type (22+X). Therefore females are homogametic. male individual produces two types of sperms during the process of spermatogenesis. 50% of the total sperm produced possess the X-chromosome and the rest 50% has Y-chromosome besides the autosome.There is an equal probability of fertilisation of the ovum (22+X) with the sperm carrying either X or Y chromosome.
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If ovum fertilises with (22+X) type sperm, the zygote develops into a female (44+XX) and the fertilisation of ovum with (22+Y) type sperm results into a male individual (44+XY). Thus, genetic makeup of the sperm determines the sex of the child. It is also clear that in each pregnancy there is always 50% or 1/2 probability of either a male or a female child. (b) Female heterogamety : Female individual produces two different types of gametes. Thus, the egg determines the sex of the offspring. It involves two types of sex-determining mechanisms ZW type and ZO type. (i) ZW type (ZW-ZZ type): In birds, both the sexes possess two sex chromosomes. Unlike human beings, the females contain heteromorphic sex chromosomes while the males have homomorphic sex chromosomes. Because of having heteromorphic sex chromosomes, the females are heterogametic. (ii) ZO type (ZO-ZZ type): In butterflies, sex-determination is exactly opposite the condition found in grasshoppers. Here females produce two types of eggs (A+Z and A+O type).
22. Sex-determination in Honey bee The sex-determination in honey bee is based on the number of sets of chromosomes an individual receives. An offspring formed from the union of a sperm and an egg develops as a female (queen or worker), and an undertilised egg develops as a male (drone) by means of parthenogenesis. This means that the males have half the number of chromosomes than that of a female. The females are diploid having 32 chromosomes and males are haploid, i.e., having 16 chromosomes. This is called as haplodiploid sex-determination system. (a) Non-Allosomic genetic sex determination : Fertility factor of plasmid in bacteria determines sex. (b) Genic Balance or X/A Balance Theory of Sex Determination : Given by C.B. Bridges. According to him, Y chromosome plays no role in sex determination of Drosophila and it is the ratio between number of X-chromosome and set of autosomes which determines the sex of fly. Chromosome Constitution AA+XXX AA+XX AAA+XXY AA+XY AA+XO AAA+XY
X/A ratio 3/2 = 1.50 2/2 = 1.00 2/3 = 0.67 1/2 = 0.50 1/2 = 0.50 1/3 = 0.33
Sex Index Super ♀ Normal ♀ Intersex Normal ♂ (Fertile) ♂ (Sterile) Super ♂
It was concluded that X/A ratio of > 1.0 expresses super femaleness, 1.0 femaleness, below 1.0 and above 0.5 intersexes, 0.5 maleness and < 0.5 supermaleness. (c) Environmental Mechanism of Sex Determination : This mechanism is observed by F. Baltzer in Bonnelia viridis (marine worm). In this organism, the sex is undifferentiated in larva. The larva which settle down in mud grow up into mature female while those which settle down near the proboscis of female and become parasite develop into male.
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23. Sex Linked Inheritance Sex linked was discovered by Morgan, while working on inheritance of eye colour in Drosophila. Taking all the crosses into consideration, Morgan came to the conclusion that eye colour gene is linked to sex and is present on the X-chromosome. X-chromosome does not pass directly from one parent to the offspring of the same sex but follows a criss-cross inheritance, i.e., it is transferred from one sex to the offspring of the opposite sex. In other words, in criss-cross inheritance a male transmits his traits to his grandson through daughter (Diagynic), while a female transmits the traits to her granddaughter through her son (Diandric). S.No . I.
Sex limited traits
Sex Influenced traits
Holandric traits
The genes of these traits are autosomal and found in both sexes but express in one sex only.
These are those autosomal genes which are influenced by the sex of the bearer. These traits appear more frequently in one sex than in the other.
There are Y-linked traits those inherit from male to male only
II.
Examples : (i) Milk glands in female (ii) Beard in man (iii) Deep male voice (iv) Antlers in male deer (v) Brilliant plumage in pea-cock (vi) Female or male musculature
(i)
Pattern baldness (affected (i) by male sex hormone/testo-sterone) (ii) (ii) Short index finger in male (iii)
Porcupine skin TDF (Testes determining factor) Hypertrichosi s
Sex Linkage in Human Beings Colour blindness and haemophilia (Bleeder’s disease) are two common examples of sex-linked diseases in human beings.
24. Mutation Mutation is sudden, discontinuous variation in genotype and phenotype of an organism due to change in chromosomes and genes. Depending upon the cause, mutations are of three types : (I) Gene Mutation It is alteration of DNA due to change in nucleotide sequence. Gene mutation may occur due to change in a single base pair of DNA, known as point mutation.. (a) Frame-shift mutation (i) Deletion : Removal of one or more bases from nucleotide chain. (ii) Insertion or addition : Addition of one or more bases in a nucleotide chain.
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(b) Substitution : The replacement of one base by another. It is of two types. (i) Transition : When a purine base (A or G) is substituted by another purine base or pyrimidine base (T or C) is substituted by another pyrimidine base. (ii) Transversion : Substituted of a purine base with a pyrimidine base or vice versa. (II) Chromosomal Aberrations Loss (deletions) or gain (insertion/duplication) of a segment of DNA, results in alternation in chromosomes. We know that genes are located on chromosomes, so that alteration in chromosomes results in abnormalities or aberrations. These are commonly observed in cancer cells. (a) Deletion : Occurs when a part of the chromosome is lost. It can be divided into two types-terminal and intercalary. Terminal deletion is the loss of a terminal segment of a chromosome and is produced by a single break in the chromosome. During intercalary deletion there is the loss of an intercalary segment of a chromosome due to double break. (b) Translocation : It involves shifting of a part of one chromosome to another non-homologous chromosome. So new recombinant chromosomes are formed, as this induces faulty pairing of chromosomes during meiosis. An important class of translocation having evolutionary significance is known as reciprocal translocation or segmental interchanges, which involves mutual exchange of chromosome segments between non-homologous chromosome, i.e., illegitimate crossing over. Chronic myelogenous leukemia (CML) occurs due to translocation of segment of long arm from chromosome 22 to chromosome 9. Chromosome 22 is called Philadelphia chromosome. (c) inversion : Change in linear order of genes by rotation of a section of chromosome by 180°. Inversion occur frequently in Drosophila as a result of X-ray irradiation. They may be of two types. (III) Genomatic Mutation It is change in chromosome number that bring about visible effects on the phenotype. It is of two types. (a) Aneuploidy : In aneuploidy, any change in number of chromosomes in an organism would be different than the multiple of basic set of chromosomes. It commonly arises due to non-disjunction (absence of separation of two homologous chromosomes during cell division) of the two chromosomes of homologous pair so that one gamete comes to have an extra chromosome (n+1) while the other becomes deficient in one chromosome (n–1). (i) Trisomy : Down’s syndrome, Klinefelter’s syndrome (ii) Monosomy : Turner’s syndrome (b) Euploidy : In euploidy, any change in the number of chromosomes is the multiple of the number of chromosomes. (i) Haploidy : One set of chromosomes. Haploids are better for mutation experimental studies, because all mutations either dominant or recessive can express immediately in them, as there is only one allele of each gene present in each cell.
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(ii)
Polyploidy : More than two sets of chromosomes. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is called a polyploidy. It is ofter seen in plants.
25. Mutagens Mutations can be artificially produced by certain agents called mutagens or mutagenic agents. Following are two major types of mutagens. (I) Physical mutagens : Radiations are the most important physical mutagens. H.J. Muller who used X-rays, for the first time, to increase the rate of mutation in Drosophila, opened an entirely new field in inducing mutations. So, Muller is considered as father of Actinobiology. (a) Ionizing radiations (b) Non-ionizing radiations (c) NoEffects of ionizing radiations : The ionizing radiations include X-rays, γ-rays, α-rays and β-rays. Ionizing radiations cause breaks in the chromosome. These cells then show abnormal cell divisions. If these include gametes, they may be abnormal and even die prematurely. (d) NoEffects of non-ionizing radiations : The non-ionizing radiations have longer wavelengths but carry lower energy. This energy is insufficient to induce ionization. Therefore, non-ionizing radiations such as UV light do not penetrate beyond the human skin. Thymine (pyrimidine) dimer formation is a major mutagenic effect of UV rays that disturbs DNA double helix and thus, DNA replication. (II) Chemical mutagens : Large number of chemical mutagens are now known. These are more injurious than radiations. The first chemical mutagen used was mustard gas by C. Auerbach et. al. during world war II. Following are the effects of some of the chemical mutagens : (a) Nitrous acid : It is mutagenic to both replicating and non-replicating DNA. It acts directly by oxidative deamination on A, G and C bases which contain amino groups. A is deaminated to hypoxanthine which is complementary to cytosine. G is converted to xanthine which pairs whit C. Cytosine is converted to U which pairs with A. (b) Acridines : Acridines and proflavins are very powerful mutagens. These can intercalate between DNA bases and interfere with DNA replication, producing insertion or deletion or both of single bases respectively. This induces frame shift-mutations or gibberish mutation, e.g., Thalassaemia. (c) Base analogues : These have structures similar to the normal bases and are incorporated into DNA only during DNA replication. Base analogues cause mispairing and eventually give rise to 5-methyl cytosine, 5-hydroxymethyl cytosine, 6-methyl purine etc. The most commonly used artificial base analogues are 5-bromouracil and 2-aminopurine.
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26. Genetic Disorders (I)
Pedigree Analysis (Method of study of human genetic disorder) Human beings, like other living organisms, also follow the principles of inheritance but common Mendelian experiments cannot be carried out over us due to following reasons. (a) Controlled crosses are not possible in human beings. (b) Number of offspring per couple is small. Because of the reasons described above, human geneticist has to resort to slightly different methods of genetic analysis. Such an analysis of traits in a several of generations of a family is called the pedigree analysis. Unaffected male By pedigree analysis one can easily understand whether the trait in question is autosomal dominant or recessive. Similarly, the trait may also be linked to the sex chromosome as in case of haemophilia.
(II) Mendelian Disorder : These are mainly determined by mutation in the single gene, therefore also called gene related human disorders. They are transmitted to the offspring as per Mendelian principles. The pattern of inheritance of such disorders can be traced in a family by the pedigree analysis. S.No.
Disorder
(i) (ii) (iii) (iv) (v) (vi) (vii)
Haemophilia Colour blindness Sickle cell anaemia Phenylketonuria Cystic fibrosis Thalassemia Myotonic dystrophy
Dominant/Recessive
Autosomal/Sex linked
Recessive Recessive Recessive Recessive Recessive Recessive Dominant
X-linkled X-linkled Autosomal Autosomal Autosomal Autosomal Autosomal
(a) Colour blindness : Colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene for normal vision is dominant. The normal gene and its recessive allele are carried by X-chromosome. In female, colour blindness appears only when both the sex chromosomes carry the recessive gene (XCXC). The females have normal vision but function as carrier if a single recessive gene for colour blindness is present (XXC). However, in human males the defect appears in the presence of a single recessive gene (XCY) because Y-chromosomes of males do not carry any gene for colour vision. Colour blindness, like any other sex-linked trait, shows criss-cross inheritance (i.e., a male transmits his trait to his grandson through daughter, while a female transmits the traits to her grand-daughter through her son or it is transfer of trait from one sex to the offspring of the opposite sex). (b) Haemophilia : It is X-linked recessive trait therefore shows its transmission from normal carrier female (heterozygous) to male progeny. Due to presence of defective form of blood clotting factor (protein), exposed blood of affected individuals fails to coagulate.
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The person suffering from this disease cannot synthesize a normal blood protein called antihaemophilic globulin (AHG) required for normal blood clotting (Haemophilia A - more severe). Therefore, even a very small cut may lead to continuous bleeding for a long time. This gene is located on X chromosome and is recessive. It remains latent in carrier females. Haemophilia-B (Christmas disease) : plasma thromboplastin is absent, Inheritance is just like Haemophilia A. (c) Sickle-cell anaemia : As it is autosomal recessive disease therefore it can be transmitted from parents to the offspring when both male and female individuals are carrier (heterozygous) for the gene. The disease is controlled by a single pair of allele, HbA and HbS. Thus three genotypes are possible in population. (i) HbAHbA (Normal, homozygous) (ii) HbAHbS (Normal, carrier) (iii) HbSHbS (Diseased, die before attaining maturity) Heterozygous (HbAHbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50% probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait. (d) Phenylketonuria : This inborn error of metabolism is also inherited as the autosomal recessive trait. The affected individual lacks a liver enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney. (e) Thalassemia : Thalassemia is a recessive autosomal genetic defect, originated in Mediterranean region by their mutation or deletion recessive autosomal. Thalassemias are a group of disorders caused by defects in the synthesis of globin polypeptide in RBC. Absence or reduced synthesis of one of the globin chains results in an excess of the other. (i) Alpha (α) thalassemia : The α-thalassemias involve the genes HBA1 and HBA2, inherited in a Mendelian recessive fashion. There are two gene loci and so four alleles. It is also connected to the deletion of the 16p (short-arm) chromosome. α-Thalassemias result in decreased α-globin production, therefore, fewer α-globin chains are produced, resulting in an excess of β chains in adults and excess γ chains in newborns. (ii) Beta (β) thalassemia : β-Thalassemias are due to mutations in the HBB gene on chromosome 11, also inherited in an autosomal-recessive fashion. The severity of the disease depends on the nature of the mutation. Mutations are characterised as (β° or β thalassemia major) if they prevent any formation of β chains (which is the most severe form of β thalassemia); they are characterised as (β+ or β thalassemia intermedia) if they allow some β chain formation to occur.
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(iii) Delta (δ) thalassemia : Just like β thalassemia, mutations can occur which affect the ability of this gene to produce δ chains. α and β chains are present in hemoglobin but about 3% of adult hemoglobin is made of α and δ chains.
(III) Chromosomal Disorder : Disorder can also be created by imbalance in chromosome number and chromosomal rearrangement. These are called as chromosomal disorders. Down‘s syndrome, Klinefelter‘s syndrome and Turner‘s syndrome are common examples or chromosomal disorders. (a) Down’s syndrome : The disorder develops due to trisomy of chromosome number 21. Trisomic condition arises due to the formation of n+1 male or female gamete by non-disjunction and the subsequent fertilisation by a normal (n) gamete. It is characterised by. (i) Short stature (ii) Small round head (iii) Furrowed tongue. (iv) Partially open mouth (v) Broad palm with characteristic palm crease (vi) Many ‘loops’ in finger tips (vii) Big and wrinkled tongue (viii) Physical (underdeveloped gonads and genitals, loose jointedness), psychomotor and mental development is retarted. (b) Klinefelter’s syndrome : It is caused due to the presence of an additional copy of X-chromosome resulting into 44+XXY type chromosome complement. The defect appears due to union of an abnormal egg (22+XX) and a normal sperm (22+Y) or normal egg (22+X) and abnormal sperm (22+XY). Such persons are sterile males with overall masculine development and some female characteristics (e.g., Feminine pitched voice, development of breast or gynaecomastia). (c) Turner’s syndrome : The disorder is due to monosomy. It appears due to fusion of abnormal egg (22+0) and a normal sperm (22+X) or a normal egg (22+X) and abnormal sperm (22+0). Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.
Cytoplasmic Inheritance Some self replicating genes (DNA) are present in the cytoplasm (mitochondrial DNA and chloroplast DNA) also. These are called plasmagenes and all the plasmagenes together constitute plasmon (like genome). The inheritance of characters by plasmagenes is called extranuclear or extrachromosomal inheritance.
Maternal inheritance : The amount of nuclear hereditary material contributed by the two sexes is almost equal but the cytoplasm in egg is always much more than that of the sperm. So, in extranuclear inheritance contribution of female parent is more. This is called maternal inheritance. The evidence of maternal inheritance is the coiling of shell in snails.
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Organelle inheritance : The DNA is present in mitochondria and chloroplast which controls the inheritance of some characters. A well known example of the characters controlled by chloroplasts is plastid inheritance in Mirabilis jalapa (4 O‘clock plant), discovered by Correns.
Mendel’s Laws of Inheritance Ÿ Ÿ Ÿ
Ÿ
Genetics is the branch of biology, which deals with inheritance and variation of characters from parents to offspring. Inheritance is the process by which characters or traits are transferred from one generation to the next. Variation is the degree by which progeny differs from each other and with their parents. Humans knew from as early as 8000-1000 B.C., that one of the causes of variation was hidden in sexual reproduction. Gregor Johann Mendel for the first time conducted experiments to understand the pattern of inheritance of variation in living beings.
27. Mendel’s Experiment Mendel’s Experimental Material He conducted experiments on garden pea plant (pisum sativum) for seven years. (1856-1863) and proposed the laws of inheritance in living organisms. (ii) He selected garden pea plant as experimental material because of: (a) easy availability on a large scale. (b) many varieties are available with distinct characteristics. (c) they are self-pollinated and can be cross-pollinated easily in case self-pollination does not occur. (d) pea plant has a shorter life cycle. It is because short life cycle enables the genetecists to study many generations of the organism in a short time period. (iii) Mendel selected 14 truer-breeding (a breeding line which has undergone continuous self-pollination and show stable trait inheritance and expression for several generations) pea plant varieties, as pairs which were similar except for one character with contrasting traits. Seven contrasting characters and their traits as taken by Mendel are listed in the table given below: Contrasting Characters Studied by Mendel in Pea (i)
Character Stem height Flower colour Flower position Pod shape Pod colour Seed shape Seed colour
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Contrasting character (Dominant/Recessive) Tall/Dwarf Violet/White Axial/Terminal Inflated/Constructed Green/Yellow Round/Wrinkled Yellow/Green
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CLASS NOTES FOR CBSE – 27. Principle of Inheritance
Mendel’s Experimental Procedure (i)
He studied one trait or character at a time, e.g. he crossed tall and dwarf pea plants to study the inheritance of one gene that confers tallness or dwarfness. (ii) Mendel hybridised plants with alternate forms of a single trait (monohybrid cross). The seeds produced by these crosses were grown to develop into plants of Filial1 progeney of F1-generation. (iii) He then self-pollinated the tall F1 plants to produce plants of Filial2 progeny or F2generation. (iv) In later experiments, Mendel also crossed pea plants with two contrasting characters known such as cross is known as dihybrid cross. (v) Mendel self-pollinated the F2 plants also. (vi) Mendel used emasculation and begging like procedures to avoid unwanted pollination in his experiments.
Mendel’s Observation in his Experimental (i) (ii) (iii) (iv) (v)
(vi) (vii)
In F1- generation, Mendel found that all pea plants were tall and non wes dwarf. He also observed other pair of traits and found that F1 always resemble one of its parents while the trait of other parent was always masked. In F2-generation, he found that some of the offspring were dwarf, i.e. the characters which were not seen in F1-generation were expressed in F2-generation. These contrasting traits (tall/dwarf) did not show any mixing either in F1 or in F2generation. Similar results were obtained with the other traits that he studied. Only of the parental traits was expressed in F1-generation, while at F2-generation stage, both the ratis were expressed in the ratio of 3:1, Mendel also found identical results in dihybrid cross as in monohybrid cross. On self-pollinating F2 plants, he found that dwarf F2 plant continued to generate dwarf plants in F3 and F4-generations.
Inferences of Mendel’s Experiments (i)
Mendel observed that something was being passed down, from parents to offsprings through the gametes over successive generations. He called these as ′factors′. The Mendelian ′factors′ are now known as genes. (ii) Genes are considered to be the units of inheritance. They contain the information required to express a particular trait. (iii) Genes which codes for a pair of contrasting traits are called alleles or allelomorphs, i.e. they are slightly different forms of the same gene or two alternative forms of the same gene or two alternative forms of a gene are known as allele. (iv) Mendel also proposed that in a true breeding variety. the allelic pair of genes are identical. For example, TT and tt for tall or dwarf pea variety respectively. (v) TT and tt represent genotype of a trait. (vi) The observable external features, e.g. tall and dwarf represent the phenotype. (vii) TT and tt represent genotype of a trait. (viii) The observable external features, e.g. tall and dwarf represent the phenotype. (ix) When the tall (TT) and dwarf (tt) pea plants produce gametes, the alleles of the parental pair segregate from each other and only one allele is transmitted to a gamete. (x) The gametes of the tall TT plants have the allele T and the dwarf tt plant have the allele.
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(xi) This segregation of alleles is a random process and so there is a 50% chance of a gamete containing either allele, as verified by the results of crossings. After fertilisation of plant with TT and tt traits, hybrids are formed that contain Tt. (xii) Mendel found the phenotype of Tt to be similar as TT parent in appearance. he proposed that in a pair of dissimilar factors, one dominates the other (T in this case) and hence, is called the dominant factor, while the other factor (t) is recessive. In other words. in a pair of dissimilar factor, one factor is able to express itself and is knonw as recessive. (xiii) Allele can be similar in case of homozygous TT or tt and dissimilar in case of heterozygous Tt. (xiv) Dominant character is expressed in homozygous as well as in heterozygous condition, while recessive is expressed only in its homozygous condition. (xv) Since, the Tt plant is heterozygous for genes controlling one character (height). it is a monohybrid and the cross between TT and tt is a monohybrid cross.
Punnett Square (i) The production of gametes by the parents, the formation of the zygotes, the F1 and F2-generations can be understood by a diagram called Punnet square developed by a British geneticists RC Punnett. (ii) The Punnett square is a graphical representation to calculate the probability of all possible genotypes of offsprings in a genetic cross. (iii) The 1/4:1/2:1/4 genotypic ratio of TT: Tt: tt is mathematically condensable to the form of binomial expression (ax + by)2, that has the gametes bearing genes T or t in equal frequency of 1/2. (viii) The expression can be expanded as
T T T t × T t TT Tt tt
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Figure A Punnett square used to understand a typical monohybrid cross conducted by Mendel between true-breeding tall plants and true-breeding dwarf plants
28. Mendel’s lows of Inheritance Mendel’s laws of inheritance are based on his observations on monohybrid and dihybrid crosses. He proposed three laws: Low of Dominance (first low) Low of dominance states that characters are controlled by genes which occur in pairs, when two alternate forms of a trait or character (genes or alleles) are present in an organism, only one factor (dominant) expresses itself in F1-generation While, the other factor (recessive) remains hidden. This is known as law of dominance. It explains expression of genes in a monohybrid cross. In such a cross the F2-generation. Phenotypic ratio is 3:1 while genotypic ratio is 1 : 2 : 1 There are two exceptions of law of dominance. These include incomplete dominance and codominance. Low of Segregation (Second Low) Low of segregation states that the factors or alleles of a pair segregate from each other during gamete formation, in a way that a gamete receive only one of the two factors. They do not show any blending or mixing. It is also known exception to law of segregation.
Law of Independent Assortment (Third low) Law of independent assortment is based on inheritance of two genes, i.e. dihybrid cross. It states that when two pairs of contrasting traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. Thus, the genes randomly rearrange in the offsprings producing both parental and new combinations of characters. Therefore, the inheritance of the character does not affect the inheritance of another character and both the characters of another and both the characters assort independently. The Punnett square can be used to understand the independent segregation of the two pairs of genes during meiosis. Linkage is the exception to low of independent assortment.
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Incomplete Dominance Incomplete dominance is a phenomenon in which the F1-hybrid shows characters intermediate of the parental genes. In incomplete dominance, dominant allele is not able to mask the characters of recessive allele completely hence, an intermediat phenotype is obtained. In this process, the phenotypic ratio of F2- generation deviates from the Mendel’s monohybrid ratio. Incomplete dominance is an exception of Mendel’s law of dominance. Example, inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) and four O’ clock plant (Mirabilis jalapa). As a result of a cross between red flower (RR) and white flower plant (rr), the F1 containing pink (Rr) flower was obtained. When F1 plants were self-pollinated, the F2 resulted into red, pink and white flowers in the ratio of 1 : 2 : 1. In incomplete dominance genotypic ratio of F2 remains same as Mendel’s monohybrid cross, i.e. 1 : 2 : 1, but phenotypic ratio changes from 3 : 1 to 1 : 2 : 1.
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Codominance Codominance is a phenomenon is which two alleles are able to express themselves Independently when present together, means both alleles are dominant. These alleles are called codominant alleles. The offsprings show resemblance to both the parents. (i) A common example of codominance is ABO blood groups in humans, which is controlled by genes gene I. (ii) The gene for blood group exist in three allelic forms IA, IB, and I. However, each person can contain any two of the three I alleles. (iii) IA and IB produce RBC surface antigens A and B, respectively, whereas ‘i’ does not produce any antigen. (iv) IA, and IB, both are dominant alleles, whereas ‘i’ is the recessive allele. (v) When IA and IB are present together, both express equally and produce both the surface antigens A and B. Allele from parent 1 IA IA IA IB IB IB i
Allele from parent 2 IA IB i IA IB i i
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Genotype of offspring IAIA IAIB IAi IAIB IBIB IBi ii
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Blood types of offspring A AB A AB B B O
CLASS NOTES FOR CBSE – 27. Principle of Inheritance
(vi) These there different alleles, may produce six different genotypes of human ABO blood group that may show four phenotype A, B, AB and O. Multiple Allelism When more then two alleles are present for a character than this conditions is known as multiple allelism. It can be explained by ABO blood grouping. In this case, more then two, i.e. three alleles are governing the same character. Multiple alleles can be found only when population studies are made since, an individual can have only two alleles.
29. Test Cross It is a method devised by Mendel to determine the genotype of an organism. Test cross is performed to know whether an organism is homozygous dominant. (T) A cross is conducted between unknown dominant genotype and the recessive parent. (i) For example, F1 hybrid (Tt) heterozygous (of a pure tall plant, i.e. TT and a pure dwarf plant, i.e. tt) is crossed with a pure dwarf plant.
(ii)
In this example, the progeny consists of tall and dwarf plants in the ratio of 1:1 Thus, monohybrid test cross ratio is 1:1 In case of both homozygous parents, i.e. TT, the progeny obtained will have all tall plants.
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(iii) In case of dihybrid test cross, where two traits are taken, a heterozygous individual is crossed with a homozygous recessive parent.
30. Pleiotropy Pleiotropy is the phenomenon is which a single gene exhibits multiple phenotypic expressions. It means that a single pleiotropix gene may produce more than one effect. For example, (i) Phenylketonuria a disorder caused by mutation in the gne encoding for enzyme phenylalanine hydroxylase. In the absence of this enzyme, phenylalanine is not converted into tyrosine and axxumlation of phenylalanine takes place. The affected individuals show hair and skin pigmentation and mental problems. (ii) Starch synthesis in pea seeds is controlled by one gene with two alleles (B and b). (a) Starch is synthesised effectively by the homozygotes, BB and hence, the starch rains are round. (b) The homozygotes, bb are less efficient in starch synthesis, hence they have small starch grains and the seeds are wrinkled.
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(c) The heterozygotes, Bb produce round seeds, indicating that B is the dominant allele, but the starch grains are intermediate in size and hence, for the starch grains size, the alleles show incomplete dominance. It is an example of pleiotropy as the same gene controls two traits, i.e. seeds shaped and size of starch grains. (d) Here, it is to be mentioned that dominance is not an autonomous feature of the the gene or its product, but it depends on the production of a particular phenotype from the gene product.
31. Polygenic Traits Polygenic inheritance was given by Galton in 1833. In this, traits are controlled by three or more genes (multiple genes). Such traits are called polygenic traits. The phenotype is produced as a result of participation of several gnes and is also fluenced by the environment and is called quantitative inheritance as the character/phenotype can be quantified. For example, human skin colour is caused by a pigment melanin. The quantity or melanin is due to three pairs of polygenes (A, B Band C). If a black or very dark (AA BB CC) and white or very light (aa bb cc) individuals marry each other, the offspring shows intermediate colour often called mulatto (Aa Bb Cc.) A total of eight allele combination are possible in the gametes forming 27 distinct genotypes. Complementary Genes Complement the effect of each other to produce a phenotype. For example, in case of sweet pea, the flower colour is due to complements the expression of another gene.
32. Rediscovery of Mendel’s Laws (i)
Though, Mendel published his work on inheritance of characters in 1885, it remained unrecognised for several reasons till 1900. Some of these reasons are as follows: (a) Communication was difficult, so his work could not be widely publicised. (b) His concept of genes as stable unit that controlled the expression of traits and of the pair of allleles which did not blend was not accepted. (c) He idea of using mathematics to expalin biological phenomenon was new and unacceptable. (d) He could not provide any physical proof for the existence of factors that where these factors are located in the cell. (ii) In 1900, Hugo de Vries, Correns and von Tschermak rediscovered Mendel’s results independently. Due to microscopy, they carefully observed cell division. (iii) This led to discovery of chromosomes (structure in the nucleus that appeared to double and divide just before each cell division).
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33. Chromosomal Theory of Inheritance Chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902. They united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance. The main point of this theory are as follows: (i) Gametes (sperm and egg) carry and transmit hereditary characters from one generatin to another. (ii) Nucleus is the site where hereditary factors are present. (iii) chromosomes as well as genes are found in pairs. (iv) The two alleles of a gene pair are located on homologous sites on the homologous chromosomes. During meiotic anaphase-I, separation of homologous chromosomes takes place. (v) The sperm and egg having haploid sets of chromosomes fuse to regain the diploid state. (vi) Homologous chromosomes synapse during meiosis and get separated to pass into different cells. It is the basis of segregation and independent assortment during meiosis, Sutton and Boveri said that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factor they carry.
Experimental Verification of the Chromosomal Theory of Inheritance Experimental verification of the chromosomal theory of inheritance was done by Thomas Hunt Morgan and his colleagues. (i) Morgan selected fruit fly, Drosophila melanogaster for his experiments because of following reasons: (a) they could be grown on simple artificial medium in the laboratory. (b) their life cycle is only about two weeks. (c) a single mating could produce a large number of flies. (d) there was a clear differentiation of the sexes, i.e. male (smaller) and female (bigger). (e) It has many types of hereditary variation that can be easily seen through low power microscopes. (ii) Linkage and Recombination (a) The physical association of two or more genes on a chromosome is called linkage. In other words when two or more genes are closely located on a chromosome, then both the genes try to go together in the next generation. This type of inheritance of genes is known as linkage. It is the exception of Mendel’s law of independent assortment. (b) Recombination explains the generation of non-parental gene combinations. (c) To explain the phenomena of linkage and recombination. Morgan carried out several dihybrid crosses in Drosophila to study gene that were sex-linked, i.g. the genes are located on X-chromosome. (d) He observed that : Ÿ two closely located genes did not segregate independently of each other. Ÿ the proportion of parental gene combinations were much higher than the non-parental types, when two genes in a dihybrid cross were situated on the same chromosome. Morgan concluded this as a physical association or linkage. Cross A show crossing between genes y and w.
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Ÿ
(e)
(f) (g) (h)
Cross B show crossing between genes w and m. Here, dominant wild type alleles are represented with [+] sign. Linkage∝ Distance between two genes Means if two genes are very close to each other then they are tightly linked and there are very are tightly linked and there are very less chances of recombination. However, if two genes are located far from each other, then they are loosely linked and there are more chances of recombination. Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (very low recombination.) Recombination of liked genes is by crossing over, i.e. exchange of corresponding parts between the chromatids of homologous chromosomes. Alfred Sturtevant (Morgan’s student used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome. Genetic maps are now used as a starting point in the sequencing of whole genomes as done in case of Human Genome Sequencing Project.
34. Mechanism of Sex-Determination (i)
The establishment of sex through differential development in an individual at the time of zygote formation is called sex-determination. (ii) Henking (1891) could trace a specific nuclear structure all through spermatogenesis in few insects.
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(a) He observed that 50% of sperms received this specific structure after spermatogenesis, whereas the other 50% sperms did not receive it. (b) He named this structure as X-body. Scientists further explained this X-body as X-chromosome. (iii) There are different types of sex-determination mechanism observed in various organisms. These mechanisms are mainly dependent on whether the parents are homogametic, i.e. with similar gametes or heterogametic, i.e. with different gametes some of these mechanisms are as follows : (a) XO type sex-determination is found in a large number of insects, e.g. grasshopper, etc. It includes homogametic females and heterogametic males. Ÿ In this type, all the ova bear a pari of X-chromosome, while sperms bears only oneX-chromosome along with the other chromosomes (autosomes). Ÿ Egg. fertilised by sperms having an X-chromosome become females and those fertilised by sperm that do not have C-chromosome become males. Ÿ Due to the involvement of the X-chromosome in sex-determination, it was named as Sex chromosome and rest chromosomes were named as autosomes. (b) XY type of sex-determination is present in many insects like Drosophila melanogaster and in mammals including man. This type of sex-determination includes homogametic female and heterogametic males. Ÿ In males, an X-chromosome is present along with another chromosome, which is very small and called as Y-chromosome. Ÿ Female have a pair of X-chromosomes. Ÿ Both males and females bear same number of autosomes. The males have autosomes plus XY and femaleh have autosomes plus XX-chromosomes. So, male is responsible for determination of the sex of the child. (c) XO type and XY type of sex-determination shows the example of male heterogamety. Because in both cases, males produce two different types of gametes: Ÿ Either with or without X-chromosome. Ÿ Some gametes with X-chromosome Ÿ and some with Y-chromosome. (d) ZW type of sex-determination is found in certain birds, fowls and fishes. Ÿ Females have Z and W-chromosomes along with autosomes and the males have a pair of Z-chromosomes. Ÿ In this type, sex is determined by the type of ovum that is fertilised to produce offspring. (e) ZO Type of sex-determination is seen in butterflies and moths. In this type the female have only one Z-chromosome, while the male have a pair of Z-chromosomes.
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(f) ZW type and ZO type of sex-determination shows the example of female heterogamety. Sex-Determination in Humans (i) In humans, 23 pairs of chromosomes are present, out of which 22 pair are exactly same in both males and females. These are known as autosomes. (ii) A pair of X-chromosome (XX) is present in females, whereas one X and one Y-chromosome (XY) is present in males. (iii) In males, during spermatogenesis, two types of gametes are produced. 50% of the total sperms carry the X-chromosome while the rest 50% carry Y-chromosomes beside autosomes. (iv) Females produce only one type of ovum with an X-chromosome. (v) In case, the ovum fertilies with a sperm carrying X-chromosome, the zygote develops into a female (XX) and if ovum fertilised the Y-chromosome carrying sperm, zygote formed is a male (XY).
(vi) Hence, the genetic make up of sperm, which fertilises the ovum determines the sex of a child. (vii) There are 50% chances of having male and 50% chances of having female in each pregnancy. So, women should not be blamed for the sex of a child.
Sex-Determination in Honeybee (i) It is known as haplo-diploidy method in which an unfertilised egg develops into male (Arrhenotoky) while fertilised egg develops into female. (ii) This type of sex-determination is seen in certain insects like honeybees. ants etc.
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35. Mutation It is a phenomenon, which causes alteration of DNA sequences, resulting in changes in the genotype and the phenotype of an organism. It leads to variation in DNA in addition to recombination. (i) Loss (deletion) or gain (insertion/duplication) of a segment of DNA, results in alteration in chromosomes. As genes are located on chromosome. As genes are located on chromosome, alteration in chromosome results in abnormalities, these are known as chromosomal alterations, which are common in cancer cells. (ii) Mutation also occurs due to change in a single base pair of DNA. This is called point mutation, e.g. sickle-cell anaemia. (iii) Deletions and insertion of base pairs of DNA. causes frameshift mutations. (iv) These are many physical and chemical factors that induce mutation, which are called mutagens. For example, UV radiation is a mutagen.
36. Pedigree Analysis It is an analysis of traits in several generations of a family. In this analysis, the inheritance of a particular trait is represented in the family tree over generations. (i) Pedigree study provides a strong tool in human genetics, which is utillised to trace the inheritance of a specific trait, abnormality or disease. (ii) Pedigree analysis is performed for human population because Mendel’s monohybrid and dihybrid cross with pure lines are not possible in huaman. (iii) The symbols used in pedigree analysis are given below.
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36. Genetic Disorders A number of disorders in human beings are associated with the inheritance of changed or altered genes or chromosome. These are called genetic disorders. Genetic disorders can be divided into following types: Mendelian Disorders
Mendelina disorders are mainly determined by alternation or mutation in the single gene. Here, chromosome number and their structure do not change. These disorders are transmitted in next generation according to the principle of inheritance and can be studied through pedigree analysis. They may be dominant or recessive. It means Mendelian disorder inherit according to Mendel’s law of inheritance. Some common Mendelian disorders and as follows: Haemophilia
Haemophilia is a sex-linked recessive disease, which is transmitted from an unaffected carrier female to some of the male offsprings. Ÿ In this disease, a single protein that is a part of cascade of proteins involved in the clotting of blood is affected. Ÿ Due to this, in an affected individual, a small cut results in non-stop bleeding. Ÿ The heterozygous female (carrier) may transmit the disease to sons. The possibility of a female become haemophilic is extremely rare because mother of such a female has to be atleast carrier and father and male has only one X-chromosome. So, even a single allele will make a male haemophilic. Ÿ Ecample, the family pedigree of queen Victoria, shows a number of haemophilic descendents as she was a carrier of the disease. Colour Blindness
It is a recessive sex-linked trait in which eyes fail to distinguish red and green colours. Ÿ The recessive allele is carried on X-chromosomes. Ÿ In female, it appears only when both the sex chromosome carry the gene (XcXc). Ÿ The females functions as carriers in the presence of a single recessive gene (XXc). Ÿ In males, the defect may appear in the presence of a single recessive gene (XcY) because Y-chromosome does not carry any gene for colour vision. Ÿ Haemophilia and colour blindness show criss-cross inheritance pattern, in which inheritance of sex-linked characters is transmitted is from father to daughter or from mother to son. Sickle-cell Anaemia
Sickle-cell anaemia is an autosome linked recessive trait that can be transmitted from parents to be offspring when both the partners are carrier for the gene (heterozygous). In this disorder due to a pont mutation abnormal haemoglobin is produced, which leads to sickle-cell RBC. is destroyed hence, person become anaemic.
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This disease is controlled by a single pair of allele, HbA and Hbs. HbA codes for normal haemoglobin, while Hbs codes for sickle-cell haemoglobin. Ÿ Only homozygous individuals for Hbs (HbsHbs) shows the diseased phenotype. Ÿ Heterozygous ((HbAHbs) individuals appear unaffected, but they are carrier of the disease as there is 50% changes of transmission of the mutant gene to the progeny leading to sickle-cell trait. Ÿ It is caused by the substitution of glutamic acid (Glu) by Valine at the sixth position of the β-globin chain of the haemoglobin molecule due to single base substitution from GAG to GUG. Ÿ The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle-like structure. Ÿ
Thalassemia
Thalassemia is an autosome linked recessive disease, which occurs due to either mutation or deletion of genes, resulting in reduced rate of synthesis of one of the globin chains of haemoglobin. Ÿ Haemoglobin consists of an α and a β-protein. If body does not produce engouh of either of these two protein, the RBC do not form properly and cannot carry sufficient oxygen. Ÿ Anaemia is the main feature of this disease. Phenylketonuria
Phenylketonuria is an inborn error of metabolism, which is inherited as the autosomal recessive trait.
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Ÿ
The disease is due to the lack of an enzyme phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine. In the lack of this enzyme phenylalanine is not converted into tyrosine. Ÿ The phenylalanine is accumulated and gets converted into phenyl pyruvic acid and other derivatives. Ÿ Accumulation in brain results in mental retardation. Ÿ These are also excreted through urine because of its poor absorption by kidney. Chromosomal Disorders Chromosomal disorders are caused by the absence or excess or abnormal arrangement of one or more chromosome. These disorders do not follows Mendel’s law of inheritance. Ÿ Failure of segregation of chromatids during cell division resulting in the gain or loss of chromosome (s) is called aneuploidy, e.g. Down’s syndrome. Ÿ Failure of cytokinesis after telophase stage resulting in an increase in whole set of chromosomes, called polyploidy. Often seen in plants. Some example of chromosome disorders are as follows: Down’s Syndrome
Down’s syndrome occurs due to the presence of an additional copy of the chromosome number 21. This condition is called trisomy of 21. So, it is an example of autosomal trisomy. Here, total number of chromosomes become 47 as there is an extra copy of 21 chromosome. Ÿ The disorder was first described by Langdon Down (1866). Ÿ Affected individuals are short statured with small round head. furrowed tongue and partially open mouth. Ÿ Palm is broad with characteristic palm crease. Ÿ Physical, psychomotor and mental development is retarded. Turner’s Syndrome
Turner’s syndrome is a disorder caused due to the absence of one of the X-chromosome. In this case, the number of chromosomes is 45 with XO. SO, it is an example of sex chromosomal monosomy. It is represented by (2n–1). Ÿ The effected females are sterile as ovaries are rudimentary. Ÿ Luck of secondary sexual characters, short stature. Klinefelter’s Syndrome
Klinefelter’s syndrome is caused due to the presence of an additional copy of X-chromsome (XXY), resulting into 47 chromosome. It is an example of trisomy of sex chromosome. It is represented by (2n+1). Ÿ Individuals have masculine development, but feminine development (development of breast, i.e. gynaecomastia) also occurs. These are males. Ÿ The individuals are sterile. All these chromosomal disorders can be easily studied via the analysis of karyotypes, i.e. an organised profile of an individual’s chromosome according to their shape, size and number.
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CBSE Pattern Exercise (1) (Q 1 to 3) One Mark 1. Give an example of polygenic trait in humans. 2. Mention any two contrasting traits with respect to seeds in pea plant that were studied by Mendel. 3. Give an example of a sex-linked recessive disorder in humans. (Q 4 to 6) Two Marks 4. Why did TH Morgan select Drosophila melanogaster to study sex-linked genes for his lab experiments. 5. The F2 progeny of a monohybrid cross showed phenotypic and genotypic ratio as 1 : 2 : 1, unlike that of Mendel‘s monohybrid F2 ratio. With the help of a suitable example, work out a cross and explain how it is possible. 6. Why is the possibility of a human female suffering from haemophilia rare? Explain. (Q 7 to 8) Three Marks 7. (i) Explain the phenomena of multiple allelism and codominance taking ABO blood group as an example. (ii) What is the phenotype of the following? (a) IAi
(b) ii
8. Differentiate between ‘ZZ’ and ‘XY’ type of sex-determination mechanisms.
(Q 9 to 10) Five Marks 9. Differentiate between the following. (i) Polygenic inheritance and pleiotropy. (ii) Dominance, codominance and incomplete dominance. 10. (i) (ii)
Why is haemophilia generally observed in human males? Explain the conditions under which a human female can be haemophilic. A pregnant human female was advised to undergo MTP. It was diagnosed by her doctor that the foetus she is carrying has developed from a zygote formed by an XX-egg fertilised by Y carrying sperm. Why was she advised to undergo
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CLASS NOTES FOR CBSE – 27. Principle of Inheritance
ANSWER Q1 An example of a polygenic trait is skin colour. It is considered to be a polygenic trait because it is under the control of many genes. Q2 Two contrasting seed traits are: (i) Seed shape Round and wrinkled. (ii) Seed colour Yellow and green. Q3 Haemophilia is sex-linked recessive disorder in humans. Q4 The scientific name of fruit fly is Drosophila melanogaster. TH Morgan preferred this organism for this study because of the following reasons: (i) It has fast and short life cycle. (ii) It has only four pairs of chromosomes. (iii) It reproduces quickly. Q5 In snapdragon, the inheritance of flower colour shows incomplete dominance. Neither of the alleles of gene for flower colour is completely dominant over the other and hybrid shows an intermediate phenotype. That’s why F2 phenotypic and genotypic ratio are same in a cross between red flowered snapdragon and white flowered snapdragon plants. it can be explained with the help of cross given below:
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CLASS NOTES FOR CBSE – 27. Principle of Inheritance
Q6 Haemophilia is an X-linked recessive disease therefore, the female having haemophilic allele on single X-chromosome do not produce haemophilic phenotype. The human females are thus, rarely haemophilic. Q7 (i)
(ii)
Inheritance pattern of ABO blood groups in human population. (a) Codominance IA and IB both are codominant. as both of them express themselves Independently in blood group AB (IA IB). (b) Multiple allelism A phenomenon in which genes exist in more than two allelic forms or combinations. e.g. the gene for blood group exists in there allelic forms IA, IB and I. Phenotype (c) IA i―’A’ blood group (d) ii―’O’ blood group
Q8 Sex-determination in humans are: (i) The males have 22 pair autosomes and a pair of XY- chromosome. (ii) The females have 22 pair autosomes and a pair of XX-chromosomes (iii) In males, 50% of sperm carry X-chromosomes and other 50% carry Y-chromosome. (iv) In females, all ova contains X-chromosomes. (v) The sex of an individual is determined by the type of sperm fertilising the ovum. (vi) If the ovum is fertilised by Y-chromosome, the zygote (XY) develops into a male and it the ovum is fertilised by X-chromosome, zygote (XX) develops into a female.
Sex-determination in birds: (i) It is ZW type. (ii) Males are homogametic (ZZ) and females are heterogametic (ZW.) The type of ovum fertilised determines the sex of the individual.
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CLASS NOTES FOR CBSE – 27. Principle of Inheritance
Q9 (i) Pleiotropy Single gene product confers many physiological effects. The genes involved are called genes pleiotropic genes. e.g. phenylketonuria.
Polygenic inheritance Single phenotypic effect is under the control of many genes. The genes involved are called polygenes. e.g. human skin colour.
(ii) Dominance It is a relationship between alleles of a single gene. in which one allele masks the phenotypic expression of another allele at the same gene locus, e.g. Tallness in pea plant.
Incomplete Dominance It is also known as (partial or mosaic) dominance where none of the two contrasting allels or factors is dominant, e.g. incomplete dominance in four ‘O’ clock plant.
Codominance It is the phenomenon of expression of both the alleles in heterozygous condition. In this, alleles do not show dominancerecessive relationship and are able to express themselves independently. e.g. ABO blood group in humans
Q10 (i) The genes for haemophila are present on X-chromosome. A male has only one X-chromosome and bears only one allele for the trait. He is hemizygous for he trait as Y-chromosome does no have a corresponding allele. A female contains two X-chromosomes. She has to be homozygous recessive to be haemophilic. It means her father must be a sufferer and mother must be either a carrier or sufferer to carry forward the disease. (ii) The zygote will have XXY- chromosomes. It will develop into a male with Klinefelter’s syndrome. Such males are sterile and show feminine characters. That is why, female was advised to undergo
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