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Question : Describe polygenic Inheritance.

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Solution : Mendel.s studies mainly described those traits that have distinct alternate forms such as flower colour which are EITHER purple or white.
= But if you look around you will find that there are many traits which are not so distinct in their occurrence and are spread across a gradient.
= For EXAMPLE in humans we don.t just have tall or short people as two distinct alternatives but a whole range of possible heights such traits are generally controlled by three or more genes and are thus called as polygenic traits.
= Besides the involvement of multiple genes polygenic inheritance also takes into account the influence of environment.
= Human skin colour is another classic example for this.
= In a polygenic trait the phenotype reflects the contribution of each allele i.e. the effect of each allele is additive.
= To understand this better let US assume that three genes A, B, C control skin colour in human with the dominant forms A, B and C responsible for dark skin colour and the recessive forms a, b, and c for light skin colour.
=The genotype with all the dominant alleles [ AA BB CC] will have the darkest skin colour and that with all the dominant alleles (AA, BB, CC) will have the lightest skin colour.
=As expected the genotype with three dominant alleles and three recessive alleles will have an INTERMEDIATE skin colour.
=In this manner the number of each type of allelaes in the genotype would determine the darkness or lightness of the skin in an INDIVIDUAL.


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