A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On the basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

For this question topper says that it can be concluded that light colour is dominant. But my doubt is that if both parents are homozygous recessive that is if both have 2 alleles for light coloured eyes then their offspring will have light coloured eyes. So how can we conclude that light colour is dominant. Please explain the correct answer.

Asked by joshiraghavendra53 | 27th Feb, 2015, 01:35: PM

Expert Answer:

Eye colour is a polygenic trait, i.e., it is affected by multiple genes. The more the number of dominant genes you get, the darker is the colour of your eyes. So, if both the parents have light coloured eyes, it implies that they both have either only one or two or infact, no dominant genes. They possess only recessive genes. Therefore, the parents cannot pass on any dominant genes for eye colour to the child. So, the child must have either the same eye colour or a lighter colour than that of the parents.

From the given study, we cannot be sure whether light eye colour is dominant or recessive. However, since both, the children and the parents have light eye colour, there is a possibility that light eye colour is a recessive trait. If light eye colour would have been a dominant trait, then the homozygous light eyed parents would have only light eyed children. However, heterozygous light eyed parents might have had atleast one recessive dark eyed child considering 3:1 ratio of Mendelian inheritance.

So, it would be difficult to conclude that light eye colour is dominant or recessive. For considering a trait as dominant or recessive, we need data of at least three generations. The data provided is about for two generations.

Answered by Sheetal Kolte | 27th Feb, 2015, 02:53: PM