Exercise 11 Select a family in which any one of the monogenic traits such as tongue rolling, widow's peak, blood groups’, red-green colour blindness, dimple in The characteristic features of inheritance of such type of traits are: (a) Transmission of traits occurs from parents of either sex. (b) Males and females are equally affected. (c) The pedigree is vertical, i.e., the trait is marked to be present in each of the generations. (d) Multiple generations are characteristically affected. Brachydactyly, polydactyly, dimple in the cheek are some of the common traits of this type. 40 EXERCISE 11 2. Autosomal Recessive trait: These are the traits whose mutant allele is recessive to its wild type allele. The pedigree chart can be more or less of the pattern given below (Fig. 11.3), where the lady (marked by the arrow) is showing the trait. The bar in the example represents the presence of corresponding dominant or recessive allele for the specific trait. Suppose the given trait is albinism. Denote its dominant allele as ‘A’ that produces pigments, and the recessive allele as ‘a’ that fails to synthesise the pigment, melanin. The female (our subject in generation III) is therefore of genotype aa. She must have received each of her ‘a’ allele from both the parents (generation-II), who are therefore themselves normal but are definitely of genotype Aa, and are carriers of the trait. The allele a must also have been present in her grand parents too, of course in heterozygous condition also to make them carriers (generation-I) Albinism in the subject’s children (generation-IV) suggests her husband too to be of genotype Aa, a carrier. Marriage of her albino daughter to an albino man is bound to produce all her grand-children albino (gen-V). The following are the salient features of the inheritance of such type of traits. (a) Occur in equal proportions in multiple male and female siblings, whose parents are normal but carriers; (b) The siblings are homozygous for the defective allele, but their parents, though some may appear normal, are obviously heterozygous, i.e., are merely carriers of the trait. (c) Consanguinity (marriage between man and woman genetically related to each other, such as cousins) occasionally results in the appearance of such traits. The possible genotypes of the above pedigree can be written as follows (Fig 11.5): 42 EXERCISE 11 Here, the dominant mutant allele is denoted by ‘D’, and its recessive wild type allele is denoted by ‘d’. Remember that human females have two X-chromosomes (XX), and the males have only one X and one Y chromosome. Males receive their lone X-chromosome from their mother, and the Y-chromosomes from their father, whereas females receives one of her X-chromosome from her mother, and the other X from her father. The characteristics of such inheritance are: (a) The trait appears in almost all the generations, and the inheritance is vertical. (b) If the female is affected, then about half of her sons are affected. (c) If the male is affected then all of his daughters would be affected, but none of his sons are affected. (d) In short, the pedigree resembles the pattern of inheritance of autosomal dominants, except that there is no male-to-male transmission. 4. X-linked Recessive traits: These are the traits whose encoding gene is present on the X-chromosome and its mutant allele is recessive to its wild-type allele. Red-green colour blindness and hemophilia, are some of its well known examples. The characteristic features of such inheritance are: (a) Females express the trait only when they are homozygous for the mutant allele, whereas the males do so even when they are hemizygous for it. The pedigree chart would appear as the following one (Fig. 11.6): LABORATORY MANUAL: BIOLOGY (b) About half of the sons of the carrier (heterozygous for the trait) females are affected. In case of homozygous females showing the trait, fifty percent of her daughters and all of her sons are likely to be affected. Therefore, the males are most affected in the population. (c) Affected persons are related to one another through the maternal side of their family. (d) Any evidence of male-to-male transmission of the trait rules out the X- linked inheritance. 5. Y-chromosome linked traits: These are the traits whose gene is present on the Y-chromosome. The females do not have any Y-chromosome, whereas all the males must have a Y-chromosome to be a male, and this Y-chromosome they get from their father. Therefore, any trait linked to the Y- chromosome must be present only in males, and certainly not in any of the females. This is why these traits are also called male-sex limited traits. All the sons of the affected male would express the trait whereas none of his daughters would do so. The pattern of the pedigree chart would be as follows (Fig 11.7): Hypertrichosis of the ear (presence of hairs on pinna) is one most common example of such traits. Note: Students may be asked to prepare the pedigree-chart from given data and analyse the pattern of inheritance. The work may be done as a project. Questions 1. How will you differentiate between autosome linked dominant and sex chromosome linked dominant pedigree chart? Explain. 2. Discuss the differences in the patterns of autosome linked recessive and sex-chromosome linked pedigree.