Question

3. A husband and wife have normal vision. Both of their fathers are colorblind. What is the probability that their first child will be a) Normal son? b) Normal daughter? c) Colorblind son?

Answer 1

Let, X°Y - normal man

X'Y - colourblind man

X'X' - colourblind woman

X°X' - normal woman

X°X° - normal woman

Given, both husband (X°Y) and wife have normal visions. And both of their fathers are colourblind (X'Y).

We assume that the both of their mothers were normal/non-carrier (since both husband and wife have normal vision)

Wife's genotype: since her father was colourblind and mother was normal (non carrier) , the only possibility is that she is a carrier.

Now, CROSSING: Husband (X°Y) X wife (X°X')

	X°	Y
X°	X°X°	X°Y
X'	X°X'	X'Y

OFFSPRING:

X°X° - normal non-carrier female

X°X' - normal carrier female

X°Y - normal male

X'Y - colourblind male

a) probability (1st child normal son) = 1/2*1/2

= 1/4

b) probability ( 1st child normal daughter) = 1/2*2/2

= 1/2

c) probability ( colourblind son) = 1/2*1/2

=1/4