Quick Color blind question

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

BiomajorPreDent

Senior Member
10+ Year Member
Joined
Dec 18, 2008
Messages
653
Reaction score
3
This is from achiever..and im annoyed because I always get these correct..but I think its possible this is a typo..

A man with color blindness, a recessive sex-linked trait,
married a woman with normal color vision. What parental genotypes
can be deduced if all of their boys and girls have been born color-blind?


A.Xc Y and XC XC
B.Xc Y and XC Xc
C.XC Y and XC XC
D.XC Y and XC Xc
E.Xc Y and Xc Xc

Now Acheiver says B is the answer. but here is my problem with this

first of all, its saying that (and we know that) this is a recessive trait..if the mother has C c, this is the results of the cross

.........X­c.........Y
XC XCX­c ..XCY
Xc XcXc ....XcY

As you can clearly see, both sons have it, and the daughter in the botton left has it, BUT the daughter in the top left would be not colorblind, because it is a recessive disease and she is heterozygous for the trait.

So how does this make sense?


Thanks


ps: the explanation from achiever even confirms my beliefs:

"C and D can be eliminated because genotype for the color-blind father can only be XcY. Essentially, E can also be eliminated because genotype for the mother with normal vision can either be XCXC or XCXc. Finally, A is discarded because none of the boys [XCY] and girls [XCXc (carrier)] will be born color-blind. In the case of B, each boy or girl born has had a 50% chance of inheriting color blindness and it is not uncommon to have all children ecessively affected under this 50-50 situation."

notice the boy born in the top right of the punnet square with their "correct answer", he isnt XcY...and they say in the first sentance that colorblind male has to be XcY....

AND it even says a woman with normal vision will be heterozygous in the second sentance..and the girl born in the top left is heterozygous..and the question wants all children colorblind!

whats going on????????

 
Last edited:
There is a 50% chance that their daughters will be color blind, and a 50% chance that their sons will be color blind. This is just a probability. Even if they have 10 sons or daughters, there is a probability that they will all be color blind or all be normal, though that would be very rare - a 0.5^10 chance. The fact that the question says that all their children were color blind only allows you to conclude that the mother is heterozygous.
 
Although the probability is 50% for boys and girls, you can randomly have all your kids with colorblindness just because the probability is there. Even when Mendel did his experiment.. they weren't perfect ratio of 3:1 or 9:3:3:1.
 
Top