Is there an error in the BRS explaination or am I just missing the concept here. They say the answer is 1/64 but I got 1/16 .
Is there an error in the BRS explaination or am I just missing the concept here. They say the answer is 1/64 but I got 1/16 .
Is there an error in the BRS explaination or am I just missing the concept here. They say the answer is 1/64 but I got 1/16 .
I got 1/16 too. Stage IV has a 50% chance of being a carrier. And the only way to get disease expression is two carriers having a baby (1/4 chance). So I got 1/4 * 1/2 * 1/2 = 1/ 16.
I guess my confusion is trying to wrap my head around how gen II to III is 1/2? I woudl think its 100% because one of the parents has the disease.It's 1/64.
It's (1/2)*(1/2)*(1/2)*(1/2)*(1/4)
Gen II for patient's mom = 1/2
Gen III for patient's mom = 1/2
Gen II for patient's dad = 1/2
Gen III for patient's dad = 1/2
Then a child has a 1/4 chance of being homo-recessive from two hetero parents.
Both generation II and generation III are 1/2 because the people are already born and must be heterozygous or there is no chance the unborn child will be affected. In a heterozygous cross you have 1 AA, 2 Aa, 1 aa. So 2/4 are heterozygous. Since the parents are both heterozygous, their child has a 1/4 chance of being homorecessive.
I guess my confusion is trying to wrap my head around how gen II to III is 1/2? I woudl think its 100% because one of the parents has the disease.
If II-2 is affected (aa) and has a child with a homozygous WT (AA) that would be:
AA x aa = 100% chance offspring is a carrier
Why is that not the case?
I appreciate your time and effort.Oh I see what you're all saying now. I agree, 1/16 looks right.
Edit: Nope, nevermind.. back to 1/64, here's why.
Generation III is 100% carrier (both maternal and paternal sides).
For some reason the tendency to assume heterozygosity resonates with me. I remember my genetics teacher explained this, but I don't remember why or how.
So, assuming all 4 maternal grandparents are heterozygotes, you now have (1/2)*(1/2) for BOTH maternal and paternal odds and another (1/2)*(1/2) for baby
Thus, (1/2)^6.
You get (1/2)^4 if you assume that 2 grandparents are homozygous dominant and the other 2 are heterozygotes though.