Genetics question; if none of the parents have it, assumption

[onedirection]

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So

A couple comes to you for genetic counseling. They are considering having children, but the man is colorblind and a hemophiliac (both X-linked traits). The woman does not have relatives with either disease. The couple wants to know how likely it is that they will have a child with either disease. What should be your response?




The solution key says that since none of the relatives have the disease, the woman has two normal X chromosomes

But how do you know that? The woman could still be a carrier couldn't she

and if that's the case
X*Y x X*X --> X*X*; X*X; X*Y; XY

So that makes two potential punnet squares here, there's the one where she's homozygous for the dominant trait and another one where she's a carrier

In the carrier one, it's a 50% chance the kids will have the disease

so 0.5*0.5 = 0.25

so 25% chance they will have a child with one of the two diseases could be the answer couldn't it? This was choice B


The answer key said: There is essentially no chance that they will have a child with either disease.

 

[Synapsis]

So

A couple comes to you for genetic counseling. They are considering having children, but the man is colorblind and a hemophiliac (both X-linked traits). The woman does not have relatives with either disease. The couple wants to know how likely it is that they will have a child with either disease. What should be your response?




The solution key says that since none of the relatives have the disease, the woman has two normal X chromosomes

But how do you know that? The woman could still be a carrier couldn't she

and if that's the case
X*Y x X*X --> X*X*; X*X; X*Y; XY

So that makes two potential punnet squares here, there's the one where she's homozygous for the dominant trait and another one where she's a carrier

In the carrier one, it's a 50% chance the kids will have the disease

so 0.5*0.5 = 0.25

so 25% chance they will have a child with one of the two diseases could be the answer couldn't it? This was choice B


The answer key said: There is essentially no chance that they will have a child with either disease.

You can essentially always assume that someone with no relatives that have a "rare" recessive disease is homozygous dominant (wild-type). This is typically the case with rare recessive diseases. If the disease is rare, and you have no family that ever displayed that disease, it's incredibly unlikely that you (or essentially anyone in your family for that matter) is a carrier. The chances are so small that they're negligible. Hemophilia and colorblindness can be considered relatively rare for women. Of course, since they're X-linked diseases, they're more likely to affect men, who are hemizygous for the X chromosome.

 

[doctorswole]

no relatives having the disease basically means she is going to be homozygous
imagine it as being no ancestors or relatives ever had the disease, even though such a case might not be realistic

so from there, we have a homozygous dominant female and a male with 2 X-linked diseases

from this, we can see that any regular male child will neither have the disease nor be a carrier because the X-chromosome will come from the mother

this also means that any regular female child will be a carrier, but disease free, because one X-chromosome is the father's that has hemophilia and colorblindness


neither boys nor girls will have the disease