Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal complement of 46 chromosomes plus one (extra) chromosome 21. Such individuals therefore have 47 chromosomes
Assume that a mating occurs between a female with Down syndrome and a normal 46-chromosome male. What proportion of the offspring would be expected to have Down syndrome? Justify your answer.
One-half of the offspring would be expected to have Down syndrome because of 2 × 1 segregation of chromosome 21 at anaphase I.
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