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  #1  
Old 04-04-2012
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Genetics Fully penetrant autosomal recessive

i know the question sound easy, but i really can't understand the answer.
if someone could plz explain it to me in simple way


Q: a man who is a know heterozygous carrier of oculocutaneous albinism marries his half cousin they share one common grandparent as shown in the figure. this trait is transmitted as fully penetrant autosomal recessive. what is the probability that this couple will produce a child with this disorder?

Fully penetrant autosomal recessive-pedigree.jpg
click image to enlarge

the answer is 1/64 ( .5 *.5 *.5* .5 * .25 = 1/64?

can someone explain why?
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If the man is a heterozygous carrier, then he received a recessive allele from one of his two parents... so one of his parents must be a heterozygous carrier. There's a 50% chance that the heterozygous carrier is his dad.

By the same logic, if his dad is a heterozygous carrier, there's a 50% chance that his dad's dad is a heterozygous carrier. So, 0.5 * 0.5 = 0.25, so there's a 25% chance that the grandfather in this diagram is a heterozygous carrier.

If grandpa is a heterozygous carrier, then there's a 50% chance that he passed the recessive alleles to his daughter. So, 0.5 * 0.5 * 0.5 = 1/8 chance that the patient's aunt is a heterozygous carrier.

If the aunt is a heterozygous carrier, then there's a 50% chance that her daughter is a carrier. So, (0.5)^4 = 1/16 chance that the wife is a carrier.

So... 1/16 chance that the wife is a carrier, and 100% chance that the man is a carrier.

If both parents are heterozygous carriers, there's a 1/4 chance that their child is homozygous recessive.

So, (1/16) * (1/4) = 1/64 chance that the child is affected.
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Old 04-04-2012
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i love u dude thanks alot, but tiny thing, from grandpa to aunt only two generations..why did u multiply 0.5 three times?

i mean the grandpa passing the allele is 0.5 and then aunt passing the allele is 0.5. this is 0.5 * 0.5 only? where is the last 0.5 coming from? sorry mate for the trouble it is just i listened to kaplan videos twice in this chapter and i couldn't still figure it out
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very well explained
you deserve a pat on your back
i tried it but wasnt able to figure out how
thankyou
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Old 04-04-2012
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I'm not sure if I understand what you're asking, but I'll give it a shot:

You have to consider the chances of whether or not the grandpa even has the allele. You're correct to say that if grandpa has the allele, then it's just 0.5 * 0.5 = 0.25. But just because your patient has the allele doesn't mean that grandpa has it... remember that your patient could have gotten the allele from his mother or his grandmother, which would mean that his father/grandfather/aunt/wife wouldn't have it.

There's a 50% chance that he got the allele from his dad, and a 50% chance that he got it from his mom. If he got it from dad, there's a 50% chance that dad got it from grandpa, and a 50% chance that dad got it from grandma. So overall, there's a 25% chance that grandpa even has the allele.

Then there's a 50% chance that grandpa passes the allele to aunt, so now the overall chance that aunt has the allele is 1/8.

Then there's a 50% chance that aunt passes the allele to her daughter, so the chance that the daughter has the allele is 1/16.

Did that make sense?
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Old 04-05-2012
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Cool now i got it

thanks alot mate, so the third 0.5 simply came from just assuming that even the grandpa might have it or not.

finally i understood this question....i don't know how to thank you for paying sometime explaining in depth ;D

wishing ya the triple 99 u deserve
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