Question

You are a genetic counselor to a couple with a Down's child. You used flanking primers to a centromere-linked SSR locus on chromosome 21 to do the following PCR analysis on DNA extracted from the mother, father and son. you conclude that nondisjunction occurred in the ____X____ at _____Y_____.

Answer 1

Down syndrome is caused by trisomy 21, which is a result of nondisjunction of chromosome 21 during meiosis, more likely in a mother's egg due to increased risk with maternal age.

Step-by-step explanation:

In the context of providing genetic counseling for a couple with a Down's child, nondisjunction refers to the incorrect separation of chromosomes during cell division. Specifically, Down syndrome is caused by having three copies of chromosome 21, also known as trisomy 21. This extra chromosome results from chromosomal nondisjunction occurring during meiosis. The occurrence of nondisjunction increases with maternal age, particularly in women over 36, making the condition more likely in eggs than in sperm.

The correct answer to the PCR analysis question, 'Based on the karyotype provided, the nondisjunction of which chromosome causes Down Syndrome?' is a. chromosome 21. This chromosome's nondisjunction can occur in either parent, but it is more common in the mother's egg than in the father's sperm. As genetic counselors completing this analysis with flanking primers to a centromere-linked SSR locus on chromosome 21, the conclusion would indicate whether the nondisjunction occurred in the mother or the father and at which stage of meiosis.