Autosomal Dominance

plzNOCarribbean

Ok, I get that when a gene has a dominant mode of inheritance that you only need 1 copy to express that trait, and the fact that it is autosomal means that it is not sex linked. However, I am confused about how we figure the probability of passing these traits to offspring.

Say, for instance, that a male is carrying the gene for polycystic kidney disease, which is an autosomal dominant trait. If he has a son, the son has a 50% chance of inheriting the disease? They say its 50%, but this is assuming that the father has only 1 dominant allele, well call it P, for this trait, as opposed to both allele.

So, if the father were Pp, and the mother carried the normal allele, the offspring would have a 50% chance of having the disease?

and if the father were PP, then ALL of the offspring would have this disease?

So basically, for autosomal dominant traits, the chance that offspring carry that trait depends on whether the parent carrying the alleles has 2 copies of the dominant allele or 1 copy of the dominant allele? is this correct?

kkentm

10+ Year Member
Ok, I get that when a gene has a dominant mode of inheritance that you only need 1 copy to express that trait, and the fact that it is autosomal means that it is not sex linked. However, I am confused about how we figure the probability of passing these traits to offspring.

Say, for instance, that a male is carrying the gene for polycystic kidney disease, which is an autosomal dominant trait. If he has a son, the son has a 50% chance of inheriting the disease? They say its 50%, but this is assuming that the father has only 1 dominant allele, well call it P, for this trait, as opposed to both allele.

So, if the father were Pp, and the mother carried the normal allele, the offspring would have a 50% chance of having the disease?

and if the father were PP, then ALL of the offspring would have this disease?

So basically, for autosomal dominant traits, the chance that offspring carry that trait depends on whether the parent carrying the alleles has 2 copies of the dominant allele or 1 copy of the dominant allele? is this correct?
google "Punnet squares" and learn the method of how genotypes are predicted. both kaplan and EK books include this method so i assume it must be relatively high yield on the mcat

plzNOCarribbean

I understand how to predict genotypes; My specific question was regarding SPECIFICALLY predicting the chances of inheriting an AUTOSOMAL DOMINANT trait.

i know it BB x bb....all offspring are Bb
if Bb x bb, 50% chance of having an offspring thats bb and 50% thats Bb, with no homozygous dominants. All individuals expressing the B phenotype will be heterozygotes

paul411

ANES
7+ Year Member
So basically, for autosomal dominant traits, the chance that offspring carry that trait depends on whether the parent carrying the alleles has 2 copies of the dominant allele or 1 copy of the dominant allele? is this correct?
It depends on the genotype of BOTH parents.

Punnet squares give you the correct answer every time.

plzNOCarribbean

It depends on the genotype of BOTH parents.

Punnet squares give you the correct answer every time.
if both are heterozygous, does the kid have a 50% chance of inheriting the trait?

since 1/2 +1/2 = 2/4 = 1/2

PingPongPro

7+ Year Member
Use the punnet square. It seems like you don't really understand how inheritance works.

If both parents are heterozygous, the kid has 75% chance of inheriting the trait.
1/4 of the offspring will be Homozygous dominant
2/4 of offspring heterozygous
1/4 homozygous recessive.

3/4 = 75% will have chance of inheriting the trait.

muhali3

10+ Year Member

I think you might be confusing phenotype and genotype.

If the dad was homozygous dominant, and the mom was homozygous recessive, then 100% of the kids would have the disease (phenotype).

However, 50% would be PP and 50% Pp. Different genotypes, same phenotype.