4. Give complete genetic explanations for each of these situations: (a) Phenotyp

Question

4. Give complete genetic explanations for each of these situations: (a) Phenotypically normal parents who have red-green colorblind XO daughters, and phenotypically normal parents who have red-green colorblind XXY sons. [Remember that this form of colorblindness is X-linked.] (b) A pair of otherwise identical twins, one of whom is normal and the other has Down syndrome. 4. Give complete genetic explanations for each of these situations: (a) Phenotypically normal parents who have red-green colorblind XO daughters, and phenotypically normal parents who have red-green colorblind XXY sons. [Remember that this form of colorblindness is X-linked.] (b) A pair of otherwise identical twins, one of whom is normal and the other has Down syndrome.

Explanation / Answer

Trisomy 21 (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males) is caused by a failure of the 21st chromosome to separate during egg or sperm development. As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21. About 88% of cases of trisomy 21 result from nonseparation of the chromosomes in the mother, 8% from nonseparation in the father, and 3% after the egg and sperm have merged. The ywo twins are zygoitic twins instead of monozygotic.

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