A man who is red-green colour-blind marries a woman with normal vision. They have three sons and two daughters. One of the sons is red-green colour-blind. All the other children have normal colour vision. Draw a genetic diagram to suggest an explanation for this.
Answers
Answer:
The genotype for normal vision may be symbolized by (XX), and colour blindness by (XX). X indicates the sex-linked recessive gene for colour blindness. If a colour blind man 0(Y) marries a normal woman (XX), in the F
1
generation all-male progeny (sons) will be normal (XY).
The female progeny (daughters) though will show normal phenotype, but genetically they will be heterozygous (XX). Since these daughters bear the recessive gene of colour blindness, they are the carriers of the trait.
If such a carrier woman with normal vision (heterozygous for colour blindness) marries a normal man (XY), the followingprogeny may be expected in the F
2
generation: among the daughters, 50% is normal and 50% are carriers for the diseases; among sons, 50% are colour blind and 50% are with normal vision.
All sex-linked genes, therefore, pass from male to female and then come back to a male of F
2
generation (grand-children generation). That is there is a chance of 1 in 2 (50% or 0.5) of her grandson being colour blind.
solution
Assuming that all the F1 progeny from the above cross marry only normal individuals with no genes for color-blindness we can say that in the next generation, the progeny of all their sons will be normal as the wife is not a carrier of color-blindness. Therefore 0% of grandsons will be colorblind.