A trait shows a whole range of phenotypes rather than two distinct alternatives, and it is also influenced by the environment. Which of the following traits fits this description?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Skin colour in humans
Mendel’s traits had two distinct forms, such as tall or dwarf. Some traits are not like this. They show a whole range of phenotypes spread across a gradient. Such traits are controlled by three or more genes whose effects add up, and so they are called polygenic traits. Polygenic inheritance also takes the influence of the environment into account.
Option (1) Pod shape in garden pea — Incorrect. Pod shape is one of the seven pea traits Mendel studied. It has only two distinct forms, inflated and constricted, and is controlled by a single gene.
Option (2) Sex in honey bees — Incorrect. In honey bees, sex depends on the number of chromosome sets. A fertilised egg develops into a female (queen or worker) and an unfertilised egg into a male (drone). There are only two clear sexes, so this is not a range of phenotypes.
Option (3) Flower colour in snapdragon (Antirrhinum sp.) — Incorrect. Flower colour in snapdragon shows incomplete dominance. One gene with two alleles gives three distinct colours, red, pink and white, not a continuous range.
Option (4) Skin colour in humans — Correct. Human skin colour is the book’s classic example of a polygenic trait. It is controlled by several genes, it varies from very light to very dark with many shades in between, and it is also affected by the environment.
NCERT Extract:
Polygenic Inheritance – “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 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.”