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A polygenic inherited trait is controlled by three pairs of genes. Two individuals which are heterozygous for three alleles, are crossed with each other. What will be the phenotypic ratio of offspring?

A

1:2:1

B

9:3:3:1

C

1:4:6:4:1

D

1:6:15:20:15:6:1

Answer

Incomplete dominance is the condition when none of factors of a gene is dominant, the phenotype of heterozygous dominant individual is blend of dominant and recessive traits. Here, monohybrid cross between two pure varieties gives 1 : 2 : 1 phenotypic ratio in F generation which is 3 : 1 in otherwise dominant traits.  Hence, option A is incorrect. The phenotypic ratio of F generation of a dihybrid cross is 9 : 3 : 3 : 1 provided that the alleles of both follow dominant-recessive relationship. Hence, option B is incorrect. A trait governed by more than one gene where dominant allele of each gene express only a part of trait and the full trait is expressed only in presence of dominant alleles of all multiple genes, is called polygenic inheritance. The phenotype of the organism depends upon number of dominant alleles. A cross between two individuals which are heterozygous for two alleles produce phenotypic ratio of 1 : 4 : 6 : 4 : 1. Hence, option C is incorrect. A cross between two individuals which are heterozygous for three alleles will produce phenotypic ratio of 1 (all dominant alleles): 6 (atleast one recessive allele) : 15 (atleast two recessive alleles) : 20 (three recessive alleles) : 15 (four recessive alleles) : 6 (five recessive alleles) : 1 (all recessive alleles). Hence, the correct option is D. 

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