A karyotypically normal couple has a child with trisomy 21 Down syndrome. DNA an

Question

A karyotypically normal couple has a child with trisomy 21 Down syndrome. DNA analysis if 21q polymorphic markers reveals the following results. (Letters refer to polymorphic loci; numbers refer to alleles at that locus.) In which meiotic division did nondisjunction occur?

Marker Locus Father Mother child

A                      1, 2      2, 2      1, 2, 2

B                      1, 1      2, 2      1, 2, 2

C                      1, 1      1, 1      1, 1, 1

D                     1, 1      1, 2      1, 2, 2

(A)      Maternal meiosis I

(B)      Maternal meiosis II

(C)      Paternal meiosis I

(D)      Paternal meiosis II

(E)      Paternal meiosis I or II

Explanation / Answer

b. Maternal meiosis II.

Since the mother has contributed the 2, 2 marker to the child in almost all the cases, we can say that the non disjunction has occurred in the mother. Since the diviison of chromosomes into cells occurs at meiosis II

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