Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal

Question

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.   While there is impaired fertility of both sexes, females are more likely to be fertile than males. VanDyke et al. (1995. Down Syndrome Research and Practice 3(2):65-69) summarize data involving children born of Down syndrome individuals. Assume that children are born to a female with Down syndrome and a normal 46-chromosome male. What proportion of the offspring would be expected to have Down syndrome?

***I know that the answer is 1/2 would have down syndrome, but I am looking for an explanation as to why that is the case....Thanks!

Explanation / Answer

Down's syndrome arises in humans due to non-disjunctioning of 21st pair of autosome during gametogenesis resulting in presence of one extra chromosome in the zygote. According to the information, the mother has Down's syndrome whereas the father does not.

When the female will be producing her eggs during gametogenesis, non-disjunctioning will take place and one of the ovum will have one extra 21st pair of chromosome. This ovum with extra chromosome will fuse with the sperm with normal chromosomal count and thus result in generation of a zygote with a total of 46+1 = 47 chromosomes. Since gametogenesis in females gives rise to two haploid ova, there are 50% chances that the ovum which fuses with the normal sperm will be affected by Down's syndrome. Thus, in total, there are 50% or 1/2 chances that the offspring will be affected by Down's syndrome.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.