Genetic probability Q

[roady]

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I heard these were huge on Step 1, so I'd like to hear you reason out how you would do this one:

A 50-year-old man who has had a myocardial infarct was
subsequently diagnosed as having familial
hypercholesterolemia, an autosomal dominant disorder.
His son has not been tested for hypercholesterolemia.
What is the probability that the patient's
granddaughter, through his son, will have
hypercholesterolemia?

I take it you pretty much assume the wives for both men are hh--so do you also assume the 50yo dude is Hh--since he could be HH, but with the allele being so (presumably) rare..? With the above assumptions, then, the chance his son sick is 50%. The chance his daughter would have it would therefore also be 50%. So 50% x 50% = 25% chance of ill granduaghter. What do you think, since if we started with the 50yo as HH, then everything would be different (not to mention if one of the wives was Hh or HH...)?

Please post some more such genetic probability type questions if you've got 'em (reason as stated above being they're supposedly everywhere on some exams). Thanks in advance!

 

[Stinger86]

I heard these were huge on Step 1, so I'd like to hear you reason out how you would do this one:

A 50-year-old man who has had a myocardial infarct was
subsequently diagnosed as having familial
hypercholesterolemia, an autosomal dominant disorder.
His son has not been tested for hypercholesterolemia.
What is the probability that the patient's
granddaughter, through his son, will have
hypercholesterolemia?

I take it you pretty much assume the wives for both men are hh--so do you also assume the 50yo dude is Hh--since he could be HH, but with the allele being so (presumably) rare..? With the above assumptions, then, the chance his son sick is 50%. The chance his daughter would have it would therefore also be 50%. So 50% x 50% = 25% chance of ill granduaghter. What do you think, since if we started with the 50yo as HH, then everything would be different (not to mention if one of the wives was Hh or HH...)?

Please post some more such genetic probability type questions if you've got 'em (reason as stated above being they're supposedly everywhere on some exams). Thanks in advance!

You assume he's Hh, because if was HH, he would've been dead long, long ago. The fact that's he's made it to 50 is a good indicator that he has heterozygous FH. So you're looking at a 50% chance that the father passed the H to his son, and then a 50% chance that the son passed the H to his daughter. 50% x 50% = 25% chance. I'm doing this off the top of my head, so if I'm wrong, someone please correct me, but I think this is correct.


edit: and yes, if not otherwise specified, you have to assume that the wives are not affected

 

[Idiopathic]

This is correct. Homozygotes have MI's before the age of 30, usually. But the concept that homozygotes for AD diseases are rare and usually fatal early holds true across the board.

 

[roady]

Thanks! That helps a lot Idiopathic and Stinger86. I should have plenty more in the days ahead as I unravel. Err...I mean I unravel these questions!!

 

[BombayBombshell]

ok, another oh so "random" genetics q... 🙂

so, B-Hexosaminidase is made of A and B units...tay-sachs has a mutated A subunit and sandhoff dz is due to a mutatated B subunit gene. if a TS's carrier marries a SD's carrier what is the probability that the 1st child will have either dz??? 0, 12.5, 25, 50, 100%

ugh i hate hate hate genetics problems.... 🙁
Brin

 

[KluverB]

so, B-Hexosaminidase is made of A and B units...tay-sachs has a mutated A subunit and sandhoff dz is due to a mutatated B subunit gene. if a TS's carrier marries a SD's carrier what is the probability that the 1st child will have either dz??? 0, 12.5, 25, 50, 100%

"It's a trick! Get a axe!"

Or, so I think... A multi-unit protein generally means it is assembled from more than one prot. each one coded for by a different gene. The parents-to-be are both carriers. That makes them heterozygotes, but each one for a DIFFERENT gene. TS is AR, and I assume so is Sandhoffs. (Think of a 4x4 Punett square: AaBB x AABb.) So all told, the chances of them producing a homoz-recessive (affected) child is nil, ie. 0%.

 

[Idiopathic]

Yes, its 0%. They may end up with some unnamed disease process, but they will not get either Tay-Sachs or sandhoff's...

 

[newbie5]

I had a 2 or 3 questions on step 1, you know the 2/3 kinds....AR disease and you know already know that they are not affected, so the chance they are carrier is 2/3.