Question

A polygenic trait is controlled by 3 genes, A, B and C. In a cross, AaBbCc ×AaBbCc, the phenotypic ratio of the offspring was observed as 1:6:X:20:x:6:1 what is the possible value of x?
A.3
B.9
C.15
D.25

Answer 1

Hint: Polygenic Inheritance of characteristic that is controlled by more than one gene as the name suggests full stop these phenotypes do not fit with mendel’s expected outcomes based on the principle of independent assortment of recessive and dominant alleles.

Complete answer:
Polygenic Inheritance of colour when one characteristic of an organism is controlled by two or more genes. Polygenic inheritance can produce unexpected phenotypes. This phenomenon shows the participation of every allele and includes the influence of the environment and is called quantitative inheritance as phenotype can be quantified. Human skin colour is the perfect example of polygenic inheritance which is caused by the pigment melanin. There are three pairs of poly genes A, B and C . he quantity of melanin is dependent on these three pairs of polygenes. If a black skin colour person (AABBCC) , marries with white skin coloured person( aabbcc) , the child of both the parents will show an intermediate colour which is called mulatto (AaBbCc)When more than two alternative alleles of the gene occupy the same locus on chromosome it is known as multiple alleles full stop it is also termed as more than two contrasting alleles for a gene.
In polytechnic inheritance a particular trait is controlled by three different genes. The given problem is of polygenic inheritance. In this phenotypic ratio is 1:6:15:20:15:6:1 instead of 27:9:9:9:3:3:3:1 in F2 generation. In the given example the genotype of the parent is AaBbCc and AaBbCc. They will produce gametes as ABC, ABc, ABC, ABC, ABC, aBc, abC, AbC. The phenotypic ratio will be 1:6:15:20:15:6:1

Hence the correct answer is option(C)

Note: Traits which are influenced by phenotypic inheritance correspond to quantitative characters are opposed to enter two characters which contain monogenic determinism. Multiple alleles contain one or two dominant alleles along with one recessive allele.