Mike and Stephanie are first cousins. Their common grandfather had a rare autsom

Question

Mike and Stephanie are first cousins. Their common grandfather had a rare autsomal recessive condition.

Part A - What is the probability that both Mike and Stephanie are carriers for this rare autosomal recessive condition at the same time?

A) 1/4, B) 1/16, C) 1, D) 4/9, E) 1/2, F) 1/9, J) 1/8

Part B - What if Mike and Stephanie have a child together and that child does not show the condition. Given this new information, what is the probability that both Mike and Stephanie are carriers for this rare autosomal recessive condition at the same time?

A) 1/5, B) 1/9, C) 1/8, D) 1/4, E) 1/16, F) 1/6

Explanation / Answer

Part A:  Their father has 50% chance to get it from your grandfather and they have another 50% to get it from your father Or in other words

1/2 % risk to pass mutant allele from genration I to genration II
1/2 % risk to pass mutant allele from genration II to genration III

Therefore, the probablity for them to be carrier = 1/2 * 1/2 = 1/4

Part B:  1/2 % risk to pass mutant allele from genration I to genration II
1/2 % risk to pass mutant allele from genration II to genration III

1/2 % risk to pass mutant allele from genration III to genration IV

So the probablity of the thier child to be carrier = 1/2 * 1/2 * 1/2 = 1/8

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