A recessive allele t is resposible for a condition called distonia. A man has it ,the woman doesn't.?

Question

They have 4 children 1 ends up infected. What are the possible genoyypes?

Answer 1

The possible phenotypes are; 50% Tt, 50% tt.
From your question we know the man has tt, and that the woman also has one t, because one of the children has the condition. Therefore the woman has t and T since she does not have the the condition.

Answer 2

Or the genotypes could be TT and tt, all kids are carriers, and one unlucky one has a second hit at some point, which promotes the disease. This seen commonly in cancer, for example the retinoblastoma gene; many kids are born +/- (-/- being lethal I think), and the 'good' gene gets mutated early in life, and promotes cancer formation, specifically in the eye (we're still not sure why this happens specfically in the eye, it's now know that Rb is an important cell cycle protein).

Answer 3

Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father.

If the man has the recessive condition, he has a recessive copy of the allele on both genes. If the woman does not have it, but her children do, then she has only one copy of the recessive allele and a dominant allele on the other gene-.