Question

A polygenic inheritance in human beings is

• A. Skin colour
• B. Phenylketonuria
• C. Colourblindness
• D. Sickle cell anaemia

Answer 1

Answer: (A)

• 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 classical example for this. Height in human is a continuous variation.
• In a polygenic trait the phenotype reflects the contribution of each allele, i.e. the effect of each allele is additive.
• To understand this better let us assume that three genes $A,B,C$ control skin colour in human with the dominant forms $A,B$ and $C$ responsible for dark skin colour and the recessive forms a, b and c for light skin colour.
• The genotype with all the dominant alleles $(AABBCC)$ will have the darkest skin colour and that with all the recessive alleles ($aabbcc$) will have the lightest skin colour.
• As expected the genotype with three dominant alleles and three recessive alleles will have an intermediate skin colour.
• In this manner the number of each type of alleles in the genotype would determine the darkness or light ness of the skin in an individual.
• In quantitative inheritance $F_2$ ratio obtained in a dihybrid cross is $1:4:6:4:1$.
• In a polygenic trihybrid cross ($AaBbCc\times AaBbCc$) the phenotypic ratio is $1:6:15:20:15:6:1$.