Question

In quantitative inheritance $\mathop F\nolimits_2$ ratio obtained in a dihybrid cross is
A. 1:4:6:4:1
B. 15:1
C. 12:3:1
D. 9:7

Answer 1

In quantitative inheritance, the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional polypeptide and full trait is shown when all the dominant alleles are present Genes involved in quantitative inheritance are called polygenes.

Complete answer:
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 do not just have tall or short people as two distinct alternatives but a whole range of possible heights. Such traits are generally controlled by two or more genes and are thus called polygenic traits. The inheritance of polygenic traits
H. Nilsson-Ehle (1908) and East (1910) demonstrated segregation and assortment of genes controlling quantitative traits, e.g., Kernel colour in wheat and corolla length in tobacco Kernel colour in wheat.
Now let us match this with given options :-
A. 1:4:6:4:1 :- Swedish geneticist, H. Nilsson-Ehle (1908) crossed red kernelled variety with white kerneled variety of wheat. Grains of $\mathop F\nolimits_1$, were uniformly red but intermediate between the red and white of parental generation. When members of $\mathop F\nolimits_1$, were self-crossed among themselves, five different phenotypic classes appeared in $\mathop F\nolimits_2$ showing the ratio of 1: 4: 6:4:1.
B. 15:1 :- In duplicate genes If the dominant alleles of two gene loci produce the same phenotype, whether inherited together or separately the 9: 3:3:1 ratio is modified into a 15: 1 ratio. This occurs in case capsules of the shepherd's purse.
C. 12:3:1 :- In case of dominant epistasis a cross between a pure breeding white summer squash, (WWYY) with a pure breeding green summer squash, (wwyy) yields white fruits in the $\mathop F\nolimits_1$, generation. Upon selfing of $\mathop F\nolimits_1$, the $\mathop F\nolimits_2$ generation comes to have 12 white fruit: 3 yellow fruit: 1 green fruit.
D. 9:7 :- In case of complementary genes Bateson and Punnett have demonstrated that in sweet pea (Lathyrus odoratus) purple colour of flowers develop as a result of interaction of two dominant genes C and P. The ratio comes to be 9:7.

So our required option of a that is 1:4:6:4:1

Note: Skin colour is also an example of polygenic inheritance . The presence of melanin that is colour determining pigment in the skin determines the skin colour. The amount of melanin developing in the individual is determined by three (two also) pairs of genes. The genes for pigment melanin or for skin colour are present at three different loci and each dominant gene and not recessive one is responsible for the synthesis of fixed amounts of melanin. The effect of all the genes additive and the amount of melanin produced is always proportional to the number of dominant genes.