Question

A polygenic inheritance in human beings is
(А) Skin colour
(B) Sickle cell anaemia
(С) Colour blindness
(D) Phenylketonuria

Answer 1

Hint: 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.
Now let us match this with given options :-
(А) Skin colour :- this is 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.
(B) Sickle cell anaemia:- this is an example of autosomal recessive disorder. As it is an autosomal recessive disease therefore it can be transmitted from parents to the offspring when both male and female individuals are carriers (heterozygous) for the gene.
(С) Colour blindness :- this is an example of criss cross inheritance. Colour blindness is a recessive that depends on recessive traits only and not dominant traits. It is sex-linked trait in which the eye fails to distinguish red and green colours. The gene for normal vision is dominant. The normal gene and its recessive allele are carried by X-chromosome.
 (D) Phenylketonuria :- this is an example of an autosomal recessive trait. The affected individual lacks a liver enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenyl pyruvic acid and other derivatives. Accumulation of these in the brain results in mental retardation.

Thus our required option is A) skin colour.

Note: 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. 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.