genetics question..

[beastly115]

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. XcY and XCXC
B. XcY and XCXc
C. XCY and XCXC
D. XCY and XCXc
E. XcY and XcXc


The answer is B, but when I do that cross, I only get 1 boy and 1 girl being color blind. The question asks if _all_ of their boys and girls have been born colorblind. The woman with normal vision can either be homozygous dominant or heterozygous. Either way, not _all_ of the children would be color blind.

So what am I missing here? Thanks.

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[tinman831]

A woman can have all colorblind girls even if she is heterozygous. She would just be contributing the Xc that has the recessive gene every single time she has a child. If you do the cross, there is a 50% chance the girl will be color blind (if she gets the Xc from the mother) and a 50% chance the girl will be a carrier (if she gets the normal X from the mother). The numbers don't mean that it is a definite fact that the mother will have half normal and half colorblind children. They are just probabilities.

The answer choice wouldnt be E since the mother in that instance would be color blind (the question states that she is normal).

*edit* and a mother who is homoygous dominant would not have any girls that are colorblind

 

[beastly115]

Thanks, that helps.