Question

the parents of a child with unusual disease symptoms take the child to a doctor for help. the doctor suspects that the condition might have a genetic basis. she recommends that the child be taken to a specialty clinic where physicians and staff members are trained to diagnose genetic diseases and counsel parents. ultimately, the child is diagnosed with a rare recessively inherited disease. the parents are tested for the gene, and both are found to be heterozygous. the parents want to have another child but are afraid this child will also be affected. what would genetic counselors say is the probability that the second child will have the disease? please read the following scenario to answer the following question. the parents of a child with unusual disease symptoms take the child to a doctor for help. the doctor suspects that the condition might have a genetic basis. she recommends that the child be taken to a specialty clinic where physicians and staff members are trained to diagnose genetic diseases and counsel parents. ultimately, the child is diagnosed with a rare recessively inherited disease. the parents are tested for the gene, and both are found to be heterozygous. the parents want to have another child but are afraid this child will also be affected. what would genetic counselors say is the probability that the second child will have the disease? 1/4 1/8 1/2 1/16

Answer 1

The probability that the second child will have the disease depends on whether the parents have one or two copies of the recessive disease-causing allele. Since both parents are heterozygous carriers, they each have one copy of the disease-causing allele and one normal allele.

When they have a child, each parent randomly passes one allele to their offspring. There are four possible allele combinations that can result in the child inheriting the disease:

The child inherits the disease-causing allele from both parents (homozygous recessive)

The child inherits one disease-causing allele from one parent and one normal allele from the other parent (heterozygous carrier)

The child inherits the normal allele from both parents (homozygous dominant)

The probability of each of these combinations depends on the probabilities of the parents passing on each of their alleles. Since the parents are each heterozygous carriers, there is a 25% chance that they will both pass on the disease-causing allele, a 50% chance that they will each pass on one disease-causing allele, and a 25% chance that they will both pass on the normal allele.

Therefore, the probability that the second child will have the disease is 25%.