Question

Heart disease is thought to be linked with an autosomal inheritance. A carrier man marries a carrier woman. They have 3 children, one boy with heart disease and another boy and girl, both carriers. The healthy daughter marries a completely healthy man. What is the probability they will have heterozygous children?

Answer 1

since there is carrier, so you know this is an autosomal recessive inheritance

Carrier daughter is : Aa
marries a completely healthy man: AA

it is 50% to have carriers (heterozygous ) and 50% to be normal

Answer 2

Hmm. The problem says that the carrier daughter appears healthy - so we deduce that the trait is completely dominant. That is, by looking at an individual with the dominant (healthy) phenotype, there is no way to infer whether they are not a carrier (AA) or a heterozygous carrier (Aa).

That allows two possible solutions:

As @cmkc109 says, if the carrier (Aa) daughter marries a non-carrier (AA) man, each of their offspring has a 0.5 probability of not being a carrier (AA) and a 0.50 probability of being a carrier (Aa).

The other case is that the carrier daughter marries another carrier. In that case the cross will be Aa x Aa, in which each offspring has a 0.25 chance of being a healthy non-carrier (AA), a 0.50 chance of being a carrier (Aa) and an 0.25 chance of developing heart disease.