Likelihood of inheriting autosomal dominant diseases

[Qube]

If one of your parents has Huntington's disease, you have a 50% chance of getting the gene for the disease.

This IS wrong. This only holds true if one of your parents is HETEROZYGOUS dominant.

Hh x hh => 50% chance of offspring having Huntington's disease.

However, if one of your parents is HOMOZYGOUS DOMINANT ..

HH x hh => 100% chance of offspring having Huntington's diseases.

Simple Punnett squares will verify the above.

Therefore, the probability should be 75% if one parent has Huntington's disease. Am I right?

 

[epenguin]

This IS wrong. This only holds true if one of your parents is HETEROZYGOUS dominant.

Hh x hh => 50% chance of offspring having Huntington's disease.

However, if one of your parents is HOMOZYGOUS DOMINANT ..

HH x hh => 100% chance of offspring having Huntington's diseases.

Simple Punnett squares will verify the above.

Therefore, the probability should be 75% if one parent has Huntington's disease. Am I right?

I don't know what you are quoting or context, but I'd guess this is an approximation based on homozygous being rare in the population.

The dynamics of Huntingdon's is a bit more complex than just the inheritance of a stable chromosome, and was explained in a recent thread on PF.

 

[Ygggdrasil]

You are making the faulty assumption that heterozygotes and homozygotes are equally abundant. In reality, the probability is 0.5p+(1-p), where p is the proportion of Huntington's patients who are heterozygous for the disease allele. As mentioned by epenguin, the disease allele is rare, so there are many more heterozygotes than homozygotes, and p~1.