Down Syndrome occurs when there is an extra chromosome #21.Such individuals therefore have 47 chromosomes.Assume that a mating occurs between a female with Down Syndrome and a normal male.Assuming normal disjunction during meiosis,what percentage of offspring would be expected to have Down Syndrome?
A) 25%
B) 50%
C) 75%
D) 100%
E) 0%

Question

Quizplus · Accepted Answer

In this scenario, the female with Down Syndrome has three copies of chromosome 21. During meiosis, her eggs will end up with either one or two copies of chromosome 21. A normal male will produce sperm with one copy of chromosome 21. Therefore, fertilization will result in embryos with either two copies (normal) or three copies (Down Syndrome) of chromosome 21. This means that there is a 50% chance for an offspring to inherit Down Syndrome.

Down Syndrome Occurs When There Is an Extra Chromosome #21