Question 26

[matthew]

Dr. Ferdinand,

When I did the punnett sq. during the exam all boys were affected b/c mom had the defective gene for nomal and abnormal. and dad gave the Y chromosome.


I picked I only b/c I still don't know how a boy can show negative for PKU if 100% of the male offspring is affected, assuming the bad gene being passed around on X only?

[Guest]

i'm a bit confused on this one too... i thought PKU was a recessive AUTOSOMAL disease... so does sex matter?

[admin]

According to the table, mom only has one defective gene because the "reaction" to a normal specific test means that allele is normal. Mom has one normal and one abnormal allele. Dad has one abnormal and one non reactive allele which is not determinate.

Yes, PKU is an autosomal recessive disease as also mentioned in P1 in the passage. So, it is not sex-linked and the way the Punnett square was done was only a simplification. If you wanted to, you could do the square using AA, Aa, and aa then say that any of the above could possibly be a male and therefore a male (which would be the same as for a female because as you suggested, sex is not an issue in this problem), either has the disease or does not. Being a carrier but disease negative is not an option for an autosomal recessive disease.

[admin]

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