Read the passage and answer the following questions:
Mendel published his work on inheritance of characters in 1865 but for several reasons, it remained unrecognised till 1900. Firstly, communication was not easy (as it is now) in those days and his work could not be widely publicised. Secondly, his concept of genes (or factors, in Mendel's words) as stable and discrete units that controlled the expression of traits and, of the pair of alleles which did not 'blend' with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature. Thirdly, Mendel's approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time. Finally, though Mendel's work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of.
Mendel's discrete factors were not accepted as an explanation for the apparently continuous variation seen in nature. Such variation is seen in human skin colour, a polygenic trait. Assume that three genes A, B and C control skin colour, 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. Which of the following genotypes will have the same skin colour as AaBbCc?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → AABbcc
Human skin colour is a polygenic trait. The effect of each allele is additive, so the colour depends on the number of dominant alleles, not on which gene they belong to. AaBbCc has three dominant alleles (A, B and C) and three recessive ones, which gives an intermediate skin colour.
Option (1) AaBBCC — Incorrect. AaBBCC has five dominant alleles (A, B, B, C, C). Two more dominant alleles than AaBbCc make the skin darker.
Option (2) AABbcc — Correct. AABbcc has three dominant alleles (A, A, B) and three recessive ones, the same count as AaBbCc. The alleles sit at different genes, but as their effects add up, the skin colour is the same intermediate shade.
Option (3) aaBbcc — Incorrect. aaBbcc has only one dominant allele (B), so it is much lighter, close to the lightest colour of aabbcc.
Option (4) AaBbcc — Incorrect. AaBbcc has two dominant alleles (A, B). One dominant allele less than AaBbCc makes the skin a little lighter.
NCERT Extract:
Polygenic Inheritance – “… 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. 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 lightness of the skin in an individual.”