Question

34. Fred and Mary have a child with Down syndrome. Fred and Mary are each heterozygous for alleles of genetic marker located on chromosome 21 and their alleles are different from each other. (The genetic marker is called "X71" and the alleles are A, B, C, or D.) Below are the genotypes for this marker as well as their child's: Fred X71B; X71C Mary X71A; X71D Child X71A; X71B; X71D In which parent and in which phase of meiosis did the non-disjunction occur.

Answer 1

First of all it is very important to understand that Non-disjunction arises due assymmetrical division of sister chromosomes while transferring it to the dughter cell. Down syndrome happens when Non-disjunction happens at chromosome 21.There are 23 pairs of chromosomes. In the above given information the father (Fred ) has alleles : B and C while mother Mary has alleles : A and C .For normal process , for every each single allele of female parent, two alleles are given by the male parent. Example, if female gives A,male gives both C and B and if female gives D, male gives two B. From the given information, it displays that the child has acquired extra allele from mother. This shows that Non-disjunction has occured in mother during Meosis