19) Trisomy 21, or Down syndrome, occurs when there is a nom complement but one
ID: 271517 • Letter: 1
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19) Trisomy 21, or Down syndrome, occurs when there is a nom complement but one (extra) chromosome 21. Although fertility is reduced in both sexes, females have higher fertility rates than males. Van Dyke 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? A) One-third of the offspring would be expected to have Down syndrome. B) Two-thirds of the offspring would be expected to have Down syndrome. C) All the children would be expected to have Down syndrome. D) None of the offspring would be expected to have Down syndrome. E) One-half of the offispring would be expected to have Down syndrome. 20) Although the most frequent forms of Down syndrome are caused by a random error, nondisjunction of chromosome 21, Down syndrome occasionally runs in families. The cause of this form of familial Down syndrome is A) an inversion involving chromosome 21 B) a chromosomal aberration involving chromosome 1 C) too many X chromosomes D) a translocation between chromosome 21 and a member of the D chromosome group E) a maternal age effect 21) A genomic condition that may be responsible for some forms of fragile-X syndrome, as well as Huntington disease, involves A) F plasmids inserted into the FMR-1 gene B) various lengths of trinucleotide repeats C) multiple breakpoints fairly evenly dispersed along the X chromosomeExplanation / Answer
19. E) One-half of the offspring would be expected to have Down syndrome.
Explanation: Recombination during meiotic crossing over of a trisomic female with a normal male results in an offspring with two normal carrier females and two males with down syndrome. This happens only if the female parent contains the dominant allele on both X chromosomes.
20. D) A translocation between chromosome 21 and a member of D chromosome group.
Explanation: D group chromosomes are medium acrocentric (13-15). Individuals with familial Down syndrome have an extra part of chromosome 21 as the result of a translocation. If the normal chromosome 21 segregates with translocated chromosome, the gamete produced will have two copies of chromosome 21 and the offspring suffers from familial Down syndrome.
21. B) Various lengths of trinucleotide repeats.
Explanation: Fragile X-syndrome is caused due to variable expansions of CGG trinucleotide repeats in FMR1 gene whereas the variable expansions of CAG trinucleotide repeats in HTT gene results in Huntington's disease.
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