Genetic problem
Schilder's disease, which causes a progressive degeneration of the nervous system leading to death at about two years of age, behaves as a simple autosomal recessive trait. If a couple loses their first two children due to Schilder's disease, what is the chance that their third child will develop the disease? S=normal and s=Schilder's disease
I would assume that both have the recessive gene, which would make the probability 1/4, regardless of the bad luck they had with the previous two births. The fact that the parents lived to adulthood indicates that neither has two genes for the disease.
aids
To determine the chance that the third child of the couple will develop Schilder's disease, we need to consider their genetic makeup and the inheritance pattern of the disease.
Given that Schilder's disease is an autosomal recessive trait, it means that both parents must carry at least one copy of the disease-causing allele (s) and pass it on to the child for them to develop the disease. Normal individuals are homozygous for the normal allele (S).
Let's analyze the possibilities:
1. If both parents are carriers (Ss) and have lost their first two children to Schilder's disease:
- For each child, there is a 25% (1/4) chance of inheriting two copies of the disease-causing allele (ss), resulting in the child developing the disease.
- Therefore, for each child, there is a 75% (3/4) chance of inheriting at least one copy of the normal allele (S), resulting in the child being unaffected.
2. If one of the parents is affected by Schilder's disease (ss) and the other parent is normal (SS):
- In this case, the affected parent can only pass on the disease-causing allele (s) to the child.
- Therefore, each child will inherit one copy of the disease-causing allele (s) and one copy of the normal allele (S).
- The child will become a carrier (Ss) but will not develop the disease.
Given that the first two children were affected by Schilder's disease, it is reasonable to assume that both parents are carriers (Ss). In this case, for the third child, there is a 25% (1/4) chance of inheriting two copies of the disease-causing allele (ss), resulting in the child developing the disease. There is also a 75% (3/4) chance of inheriting at least one copy of the normal allele (S), resulting in the child being unaffected.
Therefore, the chance that their third child will develop Schilder's disease is 25% or 1 in 4.