Assertion (A): In humans, height does not occur as two distinct alternatives (tall or short); there is a whole range of possible heights.
Reason (R): Human skin colour is another classic example of polygenic inheritance.
In the light of the above statements, choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are correct, but (R) is not the correct explanation of (A).
Assertion (A): In humans, height does not occur as two distinct alternatives (tall or short); there is a whole range of possible heights. — True. A is about what we see: height in humans is not of two kinds (tall and short). It is spread across a whole range of values.
Reason (R): Human skin colour is another classic example of polygenic inheritance. — True. R is about another trait. Human skin colour also shows a range of shades and is a classic example of polygenic inheritance.
Why R does not explain A? The cause of the range in height is that such a trait is controlled by three or more genes, each allele adding its own small effect, with the environment also playing a part. R does not give this cause. It only names a second trait of the same kind. Knowing that skin colour is polygenic does not tell us why height varies, so R is true but is not the correct explanation of A.
NCERT Extract:
Polygenic Inheritance – “… 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.”