calculating probabilities and pedigree help?

[typicalindian]

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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.

 

[MedPR]

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.

 

[typicalindian]

 

[MedPR]

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

 

[typicalindian]

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...

 

[MedPR]

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.

 

[chiddler]

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.

 

[typicalindian]

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

 

[chiddler]

🙁

 

[Buttafuoco]

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.

 

[pfaction]

We did this in class. **** that passage. I got 4/6, guessed on the last 3.