Qbank genetics question

[JayZee]

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Can someone please explain this to me:

A man and a woman are both affected by an autosomal dominant disorder that has 80% penetrance in all affected individuals. They are both heterozygotes for the disease-causing mutation. What is the probability that they will produce phenotypically normal offspring.

The answer given is 40% because they say 75% will receive one or both copies of the disease causing gene (25% homo, 50% hetero) so they say that 80% x 75% = 60% (phenotypically affected)
so 1-0.6= 0.4 or 40% will be the phenotypically normal.

that all makes sense. My question is why don't you factor in the homo recessive? aren't they phenotypically normal? 😕 😕 😕 😕

 

[nrosigh]

The stem says it's a DOMINANT disease.

Can someone please explain this to me:

A man and a woman are both affected by an autosomal dominant disorder that has 80% penetrance in all affected individuals. They are both heterozygotes for the disease-causing mutation. What is the probability that they will produce phenotypically normal offspring.

The answer given is 40% because they say 75% will receive one or both copies of the disease causing gene (25% homo, 50% hetero) so they say that 80% x 75% = 60% (phenotypically affected)
so 1-0.6= 0.4 or 40% will be the phenotypically normal.

that all makes sense. My question is why don't you factor in the homo recessive? aren't they phenotypically normal? 😕 😕 😕 😕

 

[JayZee]

yes i know...but if you have 2 Dd and Dd people then you have a 25% of getting a dd right? aren't they phenotypically normal???

 

[SkylineMD]

the recessive people are already factored into the answer

assume 100% penetrance and you will find that 25% will be unaffected correct?
its then logical to assume that since there is only 80% penetrance, the phenotypically unaffected kids will be more than 25% since not all will show the disease/illness

In the end, the 40% already takes into account the recessive children

 

[BDylan]

EDIT: yeah, what he said

You factor in the homo recessive when you establish that only 75% will be affected (leaving out the 25% homo recessive normals).

In other words, you're multiplying 80% penetrance by only 75% (not 100%) b/c the 25% normals have already been factored out.

 

[JayZee]

oh. duh!!

i'm so dumb...thanks you guys! 😀