Nail-patella syndrome in humans is characterized by congenital abnormalities of
ID: 81369 • Letter: N
Question
Nail-patella syndrome in humans is characterized by congenital abnormalities of the fingernails (and sometimes toenails) and the patellae (kneecaps). The gene for this disorder
is dominant and is located on chromosome 9 about 10 map units from the ABO locus. Suppose a man with nail-patella syndrome and type A blood marries a normal woman with type B blood. The mothers of both the husband and the wife are normal and have type O blood.
a) The husband and the wife have two children, one with type A blood and the other with type B. What is the probability that both children have normal fingernails and patellae?
b) The couple are now expecting another child. What is the chance that the child will have nail-patella syndrome and type O blood?
c) Amniocentesis reveals that the fetus has type AB blood. What is the chance that it has nail-patella syndrome?
Explanation / Answer
Please find the answers below:
Answer a: According to the information, the father suffers from the syndrome and the mother is normal. The couple has two children, one with type A and other with type B blood. Since the information suggests that the gene responsible for this disorder is located only 10 map units form ABO site, it is very likely that these genes might be linked (disease and blood group genes) and thus, it is very likely that the child with blood group A will be 100% affected from the disease. This is because the genes located less than 50 map units apart from each other have a very high tendency to be inherited together due to linkage.
Answer b: Since the information suggests that father is type A, mother is type O, and the children are type A and B, this means that both mother and father are heterozygous for blood group alleles. However, it is very important to note here that either the father should have genotype of AB or the mother should have genotype BO since one of the kids has type AB. Since B is dominant over O, it suggests that the allele for type B blood has been masked in these individuals. Thus, the child with type O will have chances to inherit the same allele B from either of the parents and along with it, the gene for this syndrome will be inherited. Since the syndrome is dominant in expression, a child with type O will be 100% affected from the disease.
Answer c: According to the information, the fetus has type AB blood. Since blood group alleles are inherited together with the disease, there are 100% chances that this child with type AB blood will have the syndrome.
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