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A cloned DNA fragment, when used as a probe, reveals a restriction fragment length polymorphism (RFLP) in the region adjacent to the centromere of chromosome 21. Four haplotypes exist: A, B, C, and D. An AB woman and a CD man have a child with trisomy 21 who is ACC. Nondisjunction occurred in what process?
A)the child during the first mitotic division
B)the father during meiosis I
C)the father during meiosis II
D)the mother during meiosis I
E)the mother during meiosis II
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schammy thankyou for the Q's.
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schammy can you pls post the explanation for this too?
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It's always my pleasure to share.
The RFLP detects a region near the centromere of chromosome 21. The region around the centromere exhibits a phenomenon called crossover suppression. Since genetic exchange cannot happen in this area, the probe is a reliable marker for the individual chromosomes. During meiosis II, sister chromatids, which are two identical copies of the same chromosome, should separate. If a nondisjunction event occurs in this division, 2 copies of the same chromosome are passed to the progeny. In this case, both parents are heterozygous for the probe. The child received an A from the mother and 2 Cs from the father, leading us to conclude that the problem occurred in the father during meiosis II.
If a nondisjunction event of chromosome 21 occurs early in development, a child that is a mosaic for trisomy 21 is the result. This accounts for approximately 1% of children with trisomy 21. Since some of their cells are normal, these individuals show only a mild expression of the trisomy 21 phenotype.
During meiosis I, homologues that carry similar but not identical information separate. If a failure occurred in this division, we would expect the man to pass CD and the woman to pass A or B, producing a child that was ACD or BCD.
If an AB woman had a failure in meiosis I, an AB gamete would be produced. When fertilized by the man's C or D sperm, a child that was ABC or ABD would result.
If an AB woman's sister chromatids failed to disjoin during meiosis II, AA or BB gametes would result. When fertilized by the CD male's sperm, a child that was AAC, AAD, BBC, or BBD would result.
