calculating probabilities and pedigree help?

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

typicalindian

Full Member
10+ Year Member
Joined
Dec 25, 2010
Messages
2,338
Reaction score
17
When a passage is given to you and it includes a pedigree and it asks you what the probability of family member X having a certain genotype is, is there a quick way to calculate the probability? I spend an inappropriate amount of time trying to make punnet squares and I know there has to be a faster way around it?

also, what about when it asks you what the genotype of a certain individual is? are there shortcuts for these types of questions? I'm not referring to any one passage or question in particular but if needed I guess I could find one from one of my books.
 
When a passage is given to you and it includes a pedigree and it asks you what the probability of family member X having a certain genotype is, is there a quick way to calculate the probability? I spend an inappropriate amount of time trying to make punnet squares and I know there has to be a faster way around it?

also, what about when it asks you what the genotype of a certain individual is? are there shortcuts for these types of questions? I'm not referring to any one passage or question in particular but if needed I guess I could find one from one of my books.

It depends if the trait is autosomal or x-linked. If x-linked, then it depends if the individual is male or female.

I don't think there is a general shortcut, but if you had a specific example I might be able to show you one.
 
OtZpX.jpg


RogEN.jpg


5My5m.jpg
 
Two diseases on one pedigree? Never done that, and I just confused myself trying to. Sorry 🙁
 
Two diseases on one pedigree? Never done that, and I just confused myself trying to. Sorry 🙁

aw man lol thanks for trying. I got the entire passage correct (my face was like 😱 ) but I took much longer than 8 minutes to do it so I didn't exactly consider it to be a victory...
 
aw man lol thanks for trying. I got the entire passage correct (my face was like 😱 ) but I took much longer than 8 minutes to do it so I didn't exactly consider it to be a victory...

Well, for pedigrees that have only one disease you can figure out what kind it is (autosomal dominant, recessive, x-linked dominant, recessive, etc) by comparing the generations. You shouldn't need any punnet squares to determine what kind of disease it is.

As far as figuring out the likelyhood that a certain individual will have it, you either have to know the ratios, or make a punnet square. Making a square shouldn't take long considering it will probably be a single allele on the MCAT.
 
there are no real shortcuts. you do enough punett squares such that it becomes a mental calculation rather than a written one (plus they're mostly just simple 1/2^n fractions anyway)

how to determine genotype? look at parents, look at siblings. again, there is no shortcut.
 
there are no real shortcuts. you do enough punett squares such that it becomes a mental calculation rather than a written one (plus they're mostly just simple 1/2^n fractions anyway)

how to determine genotype? look at parents, look at siblings. again, there is no shortcut.

gah ok 🙁 thanks anyways guys
 
You can always multiply fractions as in if a person one person is heterozygous and another is homozygous recessive, there is a 1/2 probability of a child receiving either allele from the heterozygous parent and a 1 probability of receiving whatever the allele the homozygous parent has.

Therefore the odds are 1/2 x 1 = 1/2 for the child of being Aa and 1/2 x 1 = 1/2 of being aa.

This is only really a shortcut when you're dealing with more than one trait though.
 
Top