Huntington disease is transmitted as an autosomal dominant with 100% penetrance. Disease-causing alleles are also recessive lethal during fetal development. What is the exact probability, accounting for the possibility of a lethal genotype.

*** Pedigree link used for problem: http://img827.imageshack.us/img827/3017/pedb.jpg

http://img827.imageshack.us/img827/3017/pedb.jpg

As far as I know, the disease is the about the same whether you have two copies of the dominant mutant gene or just one.

I don't think it's true that it's lethal in homozygote fetuses.

Please clarify your question, because I'm not sure what you are asking.

If just the likelihood of the 2 year old having the disease, calculate what it would be for each parent of the 2 year old and tell me what those numbers are.

This is a sort of trick question because it is possible to inherit this disease from both one's parents and still not be affected. The gene in question, which codes for the production of glutamine, has to replicate itself many times before the body produces toxic levels of the amino acid. In some people, the gene doesn't replicate itself at all and in others, it just does it a few times then stops, producing only mild symptoms, rather like those of mild forms of Parkinson's disease. If we knew how to stop the gene from replicating itself, we would have a cure for the disease.