Simple genetics question (TPRH SW Passage 6)

[circulus vitios]

Xeroderma pigmentosum is a recessive skin condition that can lead to cancer at an early age. A woman homozygous for the dominant gene and a homozygous man with the condition wish to have a child. What is the probability that the child will get xeroderma pigmentosum?

A. 0%
B. 25%
C. 50%
D. Cannot be determined

Answer is A.

I have a feeling that this is super simple but the explanation makes absolutely no sense at all. Could someone walk me through this, preferrably with a Punnet square? The solution says something about the disease being caused by a recessive trait.

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[Bru]

The father is homozygous for the dominant allele which is not the defective one. The mother is homozygous for the recessive, defective trait.

P P
p Pp Pp
p Pp Pp

So each child is heterozygous, carrier who do Not have the condition.

 

[circulus vitios]

The father is homozygous for the dominant allele which is not the defective one. The mother is homozygous for the recessive, defective trait.

P P
p Pp Pp
p Pp Pp

So each child is heterozygous, carrier who do Not have the condition.

Thank you, that is dead simple and makes perfect sense. My brain is fried because I re-read the question a half dozen times and couldn't figure it out.

 

[dudewheresmymd]

Thank you, that is dead simple and makes perfect sense. My brain is fried because I re-read the question a half dozen times and couldn't figure it out.

It happens. I was doing electric fields and couldn't even get the right hand rule for electron right w/ respect to a Lenz law passage.