Schilders disease is a progressive degeneration of the central nervous system that leads to death by age 2. The disease is caused by a simple autosomal recessive mutation. A couple loses its first two children to Schilders disease. If they decide to have a third child, what is the probability that it will have the disease

The probability will be 25% (1/4)

Explanation:

Schilder's syndrome is a degenerative and progressive lethal disease caused by the demyelination of nerve cells in the brain and spinal cord. Since it is a lethal recessive autosomal disease, both the father and the mother have a single copy of the mutant allele, thereby the probability of obtaining a lethal combination will be:

SN X SN >>

1/2 x 1/2 = 1/4 = 25%

where N and S are normal and lethal alleles, respectivelyvely

a symbiotic relationship with the mitochondria