ICSE Class 10 Answered

1. Pair of genes responsible for a particular characteristic of an individual – Alleles

2. The faulty ‘gene’ for colour blindness is found only on the X chromosome. So, for a male to be colour blind the faulty colour blindness ‘gene’ only has to appear on his X chromosome. For a female to be colourblind it must be present on both of her X chromosomes.

Now,

• If the father is colour blind and mother is non-colour blind, there will be no colourblind sons. But all the daughters will be colourblind and carriers.
• If the father is non colour blind and mother is colourblind carrier, 50% of sons would be colourblind while 50% daughters will be colour blind carriers.
• If the father is colourblind and the mother is colourblind carrier, 50% of sons would be colourblind. 50% daughters would be colourblind while 50% would be colourblind carriers.
• If the father is non colour blind and mother is colour blind, all sons would be colourblind while all daughters would be colourblind carriers.

Thus, we can see that the possibility of having a colourblind progeny is more when the mother is colourblind as compared to when the father is colourblind. Hence, it is better to have a colourblind father than to have a colourblind mother.