Aim: Preparation and analysis of Pedigree Charts
Principle: The Mendelian concept of dominance and segregation can also be studied in humans by preparing and then analysing the pedigree charts. The internationally approved symbols for indicating males and females, marriages, various generations (I, II, III), etc., are given below.
Requirement: Information about characters/traits in a family for more than one generation
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
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the cheek, hypertrichosis of ear, hitch-hiker's thumb, etc., is found. Ask the person exhibiting the trait to tell in which of his/her parents, grand parents (both maternal and paternal), their children and grand children the trait in question is present. Among surviving individuals the trait may also be examined. The information made available is the basis for the preparation of pedigree chart using the appropriate symbols. A careful examination of the pedigree chart would suggest whether the gene for the character is autosomelinked dominant or recessive, X - chromosome linked dominant or recessive, Y- chromosome linked or not.
Autosome Linked Dominant traits: These are the traits whose
encoding gene is present on any one of the autosomes, and the wildtype allele is recessive to its mutant allele, i.e., the mutant allele is dominant.
The pedigree-chart can be of the undernoted pattern (Fig. 11.2), where the female being interviewed is exhibiting the trait, and is indicated by an arrow-mark in the chart.
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.
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.
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X-Linked Dominant traits: These are the traits whose encoding gene is present on the X- chromosome, and the mutant allele of...