How do you guys solve Pedigrees?

[Caffine]

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Pedigrees are one of those things that really annoy me. Chances are if I saw it on the MCAT, I'd probably mark it and move on to another problem. Choosing the correct Genetic Disease for Pedigrees seem to either require: a lot of generalized assumptions (ie. Dominant traits don't skip generations, where as Recessive traits typically do), but these assumptions are sometimes wrong. If I actually went through each answer choice and proved each answer wrong one by one, I would probably end up with the right answer but it would take ages to do (for me, like 3 or 4 minutes ...time better spent elsewhere). I was just wondering how everyone else feels about these types of problems. I'm hoping it's not something that'll show up on the MCAT, but I just want to cover my bases. If it wasn't in the AAMC outline, I wouldn't care to ask, lol.

Also, if someone who took the MCAT recently could shed some light on how complex a pedigree problems were (2,3, or more generations ...or even a passage), it would help a lot.

Thanks guys for the input.

 

[aarsdam]

Pedigrees are one of those things that really annoy me. Chances are if I saw it on the MCAT, I'd probably mark it and move on to another problem. Choosing the correct Genetic Disease for Pedigrees seem to either require: a lot of generalized assumptions (ie. Dominant traits don't skip generations, where as Recessive traits typically do), but these assumptions are sometimes wrong. If I actually went through each answer choice and proved each answer wrong one by one, I would probably end up with the right answer but it would take ages to do (for me, like 3 or 4 minutes ...time better spent elsewhere). I was just wondering how everyone else feels about these types of problems. I'm hoping it's not something that'll show up on the MCAT, but I just want to cover my bases. If it wasn't in the AAMC outline, I wouldn't care to ask, lol.

Also, if someone who took the MCAT recently could shed some light on how complex a pedigree problems were (2,3, or more generations ...or even a passage), it would help a lot.

Thanks guys for the input.

Start with the parental generation when looking at a pedigree. If neither of the parents expresses the trait, it will not be autosomal dominant. Move on to the next generation. Look to see if only males express the trait, if so, it is likely sex-linked recessive. If that generation does not express the trait at all, then it is likely autosomal recessive (it will almost never be THAT easy). For clarity you want to make a judgement, and then begin inserting genotypes for each individual to ensure that your hypothesis is correct.

Of course these rules don't always apply (except for the first one), but the MCAT is NOT trying to trick you. Sometimes, a certain pedigree will correspond to more than one possibility, but trust me, only ONE answer choice will be possible.

It seems like you understand pedigrees, but get caught up in the details, which is dangerous for the MCAT. Some of my best students have the most trouble because they are too analytical. Stick with exactly what is presented to you and don't make assumptions and you should be fine.

 

[kevo246]

Start with the parental generation when looking at a pedigree. If neither of the parents expresses the trait, it will not be autosomal dominant. Move on to the next generation. Look to see if only males express the trait, if so, it is likely sex-linked recessive. If that generation does not express the trait at all, then it is likely autosomal recessive (it will almost never be THAT easy). For clarity you want to make a judgement, and then begin inserting genotypes for each individual to ensure that your hypothesis is correct.

Of course these rules don't always apply (except for the first one), but the MCAT is NOT trying to trick you. Sometimes, a certain pedigree will correspond to more than one possibility, but trust me, only ONE answer choice will be possible.

It seems like you understand pedigrees, but get caught up in the details, which is dangerous for the MCAT. Some of my best students have the most trouble because they are too analytical. Stick with exactly what is presented to you and don't make assumptions and you should be fine.

Good input, +1 on the identity of autosomal vs sex-linked.

I would assume very low probability of them asking about a y-linked trait considering the very low freq of appearance, so this leaves us with Autosomal dominant and recessive, and Sex-linked (X chromosome). As stated above, just look for the characteristic patterns. A bit of practice with pedigrees makes them quite obvious at times, esp when they give you the heterozygotes!!
I would not risk guessing on a pedigree, these are too simple to blow freebie points.
At least we are not having to deal with weird dihybrid crosses involving complex epistasis and non-"Mendedlian" ratios(12:3:1, 9:3:4).