BIOLOGY wild cows, we have well-known Indian breeds, e.g., Sahiwal cows in Punjab. We must, however, recognise that though our ancestors knew about the inheritance of characters and variation, they had very little idea about the scientific basis of these phenomena. 5.1 MENDEL’S LAWSOF INHERITANCE It was during the mid-nineteenth century that headway was made in the understanding of inheritance. Gregor Mendel, conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms. During Mendel’s investigations into inheritance patterns it was for the first time that statistical analysis and mathematical logic were applied to problems in biology. His experiments had a large sampling size, which gave greater credibility to the data that he collected. Also, the confirmation of his inferences from experiments on successive generations of his test plants, proved that his results pointed to general rules of inheritance rather than being unsubstantiated ideas. Mendel investigated characters in the garden pea plant that were manifested as two opposing traits, e.g., tall or dwarf plants, yellow or green seeds. This allowed him to set up a basic framework of rules governing inheritance, which was expanded on by later scientists to account for all the diverse natural observations and the complexity inherent in them. Mendel conducted such artificial pollination/cross pollination experiments Figure 5.1 Seven pairs of contrasting traits in pea plant studied by Mendel using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits. Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, inflated (full) or constricted green or yellow pods and tall or dwarf plants (Figure 5.1, Table 5.1). PRINCIPLES OF INHERITANCE AND VARIATION Table 5.1: Contrasting Traits Studied by Mendel in Pea S.No. Characters Contrasting Traits 1. Stem height Tall/dwarf 2. Flower colour Violet/white 3. Flower position Axial/terminal 4. Pod shape Inflated/constricted 5. Pod colour Green/yellow 6. Seed shape Round/wrinkled 7. Seed colour Yellow/green 5.2 INHERITANCEOF ONE GENE Let us take the example of one such hybridisation experiment carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene (Figure 5.2). 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 progeny or the F1. Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf (Figure 5.3). He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them. Mendel then self-pollinated the tall F1 plants and to his surprise found that in the Filial2 generation some of the offspring were ‘dwarf’; the character that was not seen in Figure 5.2 Steps in making a cross in pea the F1 generation was now expressed. The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offspring were either tall or dwarf, none were of in-71 between height (Figure 5.3). Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F1 generation while at the F2 stage both the traits were expressed in the proportion 3:1. The contrasting traits did not show any blending at either F1 or F2 stage. BIOLOGY Based on these observations, 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’. Now we call them as genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene. If we use alphabetical symbols for each gene, then the capital letter is used for the trait expressed at the F1 stage and the small alphabet for the other trait. For example, in case of the character of height, T is used for the Tall trait and tfor the ‘dwarf’, and T and t are alleles of each other. Hence, in plants the pair of alleles for height would be TT, Tt or tt. Mendel also proposed that in a true Figure 5.3 Diagrammatic representation of monohybrid cross breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt, respectively. TT and tt are called the genotype of the plant while the descriptive terms tall and dwarf are the phenotype. What then would be the phenotype of a plant that had a genotype Tt? As Mendel found the phenotype of the F1 heterozygote Tt to be exactly like the TT parent in appearance, he proposed that in a pair of dissimilar factors, one dominates the other (as in the F1 ) and hence is called the dominant factor while the other factor is recessive . In this case T (for tallness) is dominant over t (for dwarfness), that is recessive. He observed identical behaviour for all the other characters/trait-pairs that he studied. It is convenient (and logical) to use the capital and lower case of an alphabetical symbol to remember this concept of dominance and recessiveness. (Do not use T for tall and d for dwarf because you will find it difficult to remember whether T and d are alleles of the same gene/character or not). Alleles can be similar as in the case of homozygotes TT and tt or can be dissimilar as in the case of the heterozygote Tt. Since PRINCIPLES OF INHERITANCE AND VARIATION 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. From the observation that the recessive parental trait is expressed without any blending in the F2 generation, we can infer that, when the tall and dwarf plant produce gametes, by the process of meiosis, the alleles of the parental pair separate or segregate from each other and only one allele is transmitted to a gamete. This segregation of alleles is a random process and so there is a 50 per cent chance of a gamete containing either allele, as has been verified by the results of the crossings. 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 the two alleles, T from one parent say, through the pollen, and t from the other parent, then through the egg, are united to produce zygotes that have one T allele and one t allele. In other words the hybrids have Tt. Since these hybrids contain alleles which express contrasting traits, the plants are heterozygous. The production of gametes by the parents, the formation of the zygotes, the F1 and F2 plants can be understood from a diagram called Punnett Square as shown in Figure 5.4. It 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. The possible gametes are written on two sides, usually the top row and left columns. All possible combinations are represented in boxes below in the squares, which generates a square output form. The Punnett Square shows the parental tall TT Figure 5.4 A Punnett square used to understand a typical monohybrid (male) and dwarf tt (female) plants, the gametes cross conducted by Mendel produced by them and, the FTt progeny. The F 11 between true-breeding tall plants plants of genotype Tt are self-pollinated. The and true-breeding dwarf plants symbols & and % are used to denote the female (eggs) and male (pollen) of the F1 generation, respectively. The F1 plant of the genotype Ttwhen self-pollinated, produces gametes of the genotype T and t in equal proportion. When fertilisation takes place, the pollen grains of genotype T have a 50 per cent chance to pollinate eggs of the genotype T, as well as of genotype t. Also pollen grains of genotype t have a 50 per cent chance of pollinating eggs of genotype T, as well as of PRINCIPLES OF INHERITANCE AND VARIATION Figure 5.5 Diagrammatic representation of a test cross Based on his observations on monohybrid crosses Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses. Today these rules are called the Principles or Laws of Inheritance: the First Law or Lawof Dominance and the Second Law or Law of Segregation. 5.2.1 Law of Dominance (i) Characters are controlled by discrete units called factors. (ii) Factors occur in pairs. (iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive). The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2. 5.2.2 Law of Segregation This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage. Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors. Of course, a homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes each having one allele with equal proportion. BIOLOGY 5.2.2.1 Incomplete Dominance When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F 1 had a phenotype that did not resemble either of the two parents and was in between the two. The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance. In a cross between true-breeding red-flowered (RR) and true- breeding white-flowered plants (rr), the F1(Rr) was pink (Figure 5.6). When the F1 was self-pollinated the F 2 resulted in the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White. Here the genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio. What happened was that R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white) . Explanation of the concept of dominance: What exactly is dominance? Why are some alleles dominant and some recessive? To tackle these questions, we must understand what a gene does. Every gene, as you know by now, contains the information to express a particular trait. In a diploid organism, there are two copies of each gene, i.e., as a pair of alleles. Now, these two alleles need not always be identical, as in a heterozygote. One of them may be different due to some changes that it has undergone (about which you will read further on, and in the next chapter) which modifies the information that particular allele contains. Let’s take an example of a gene that contains Figure 5.6 Results of monohybrid cross in the plant Snapdragon, where the information for producing an enzyme. Now there are two copies of this gene, the two allelic one allele is incompletely forms. Let us assume (as is more common) that dominant over the other allele the normal allele produces the normal enzyme that is needed for the transformation of a 76 substrate S. Theoretically, the modified allele could be responsible for production of – (i) the normal/less efficient enzyme, or (ii) a non-functional enzyme, or (iii) no enzyme at all PRINCIPLES OF INHERITANCE AND VARIATION Figure 5.7 Results of a dihybrid cross where the two parents differed in two pairs of contrasting traits: seed colour and seed shape PRINCIPLES OF INHERITANCE AND VARIATION Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance. Following this synthesis of ideas, experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation (a) (b) that sexual reproduction produced.Morgan worked with the tiny Figure 5.10 Drosophila fruit flies, Drosophila melanogaster(Figure 5.10),which were melanogaster (a) Male found very suitable for such studies. They could be grown on (b) Female simple synthetic medium in the laboratory. They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies. Also, there was a clear differentiation of the sexes – the male and female flies are easily distinguishable. Also, it has many types of hereditary variations that can be seen with low power microscopes. 5.3.3 Linkage and Recombination Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas. For example Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent). Morgan and his group knew that the genes were located on the X chromosome (Section 5.4) and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations (Figure 5.11). Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) (Figure 5.11, Cross A) while others were loosely linked (showed higher recombination) (Figure 5.11, Cross B). For 83 example he found that the genes white and yellow were very tightly linked and showed only 1.3 per cent recombination while whiteand miniature wing showed 37.2 per cent recombination. His student Alfred Sturtevant 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. Today genetic maps PRINCIPLES OF INHERITANCE AND VARIATION 5.4 SEX DETERMINATION The mechanism of sex determination has always been a puzzle before the geneticists. The initial clue about the genetic/ chromosomal mechanism of sex determination can be traced back to some of the experiments carried out in insects. In fact, the cytological observations made in a number of insects led to the development of the concept of genetic/chromosomal basis of sex-determination. Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, and it was also observed by him that 50 per cent of the sperm received this structure after spermatogenesis, whereas the other 50 per cent sperm did not receive it. Henking gave a name to this structure as the X body but he could not explain its significance. Further investigations by other scientists led to the conclusion that the ‘X body’ of Henking was in fact a chromosome and that is why it was given the name X-chromosome. It was also observed that in a large number of insects the mechanism of sex determination is of the XO type, i.e., all eggs bear an additional X-chromosome besides the other chromosomes (autosomes). On the other hand, some of the sperms bear the X-chromosome whereas some do not. Eggs fertilised by sperm having an X-chromosome become females and, those fertilised by sperms that do not have an X-chromosome become males. Do you think the number of chromosomes in the male and female are equal? Due to the involvement of the X-chromosome in the determination of sex, it was designated to (a) (b) (c) Figure 5.12 Determination of sex by chromosomal differences: (a,b) Both in humans and in Drosophila, the female has a pair of XX chromosomes (homogametic) and the male XY (heterogametic) composition; (c) In many birds, female has a pair of dissimilar chromosomes ZW and male two similar ZZ chromosomes be the sex chromosome, and the rest of the chromosomes were named as autosomes.Grasshopper is an example of XO type of sex determination in which the males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes. These observations led to the investigation of a number of species to understand the mechanism of sex determination. In a number of other insects and mammals including man, XY type of sex determination is seen where both male and female have same number of chromosomes. BIOLOGY change or alteration. However, changes or alteration do take place occasionally. Such an alteration or change in the genetic material is referred to as mutation. A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes. 5.6.2 Mendelian Disorders Broadly, genetic disorders may be grouped into two categories – Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive. By pedigree Figure 5.13 Symbols used in the human analysis one can easily understand whether the pedigree analysis trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome as in case of haemophilia. It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny. A representative pedigree is shown in Figure 5.14 for dominant and recessive traits. Discuss with your teacher and design pedigrees for characters linked to both autosomes and sex chromosome. (a) (b) Figure 5.14 Representative pedigree analysis of (a) Autosomal dominant trait (for example: Myotonic dystrophy) (b) Autosomal recessive trait (for example: Sickle-cell anaemia) PRINCIPLES OF INHERITANCE AND VARIATION Haemophilia: This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non-stop bleeding. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father should be haemophilic (unviable in the later stage of life). The family pedigree of Queen Victoria shows a number of haemophilic descendents as she was a carrier of the disease. Sickle-cell anaemia: This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele, HbA and HbS. Out of the three possible genotypes only homozygous individuals for HbS (HbSHbS) show the diseased phenotype. Heterozygous (HbAHbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50 per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure 5.15). The defect is caused by the substitution of Glutamic acid (Glu) by Figure 5.15 Micrograph of the red blood cells and the amino acid composition of the relevant portion of β-chain of haemoglobin: (a) From a normal individual; (b) From an individual with sickle-cell anaemia