Question

2. (5 PTS) You are presented with a patient, named Gerry, who has Down syndrome (trisomy 21). You determine that the nondisjunction event occurred in his mother, named Mary, during meiosis I. a. You are interested in the TPTE gene, which is near the centromere of chromosome 21. Virtually no recombination occurs between the centromere and the TPTE gene. Mary has two different alleles of the TPTE gene. Would Gerry's two chromosome 21s from his mother have the same or different alleles of the TPTE gene? Or do you not have enough information to decide? [1 pt] b. Explain your answer by drawing meiosis I and Il for Mary's chromosome 21. Use "A" for one allele and "a" for the other allele of the TPTE gene. Only draw the chromosomes relevant to this question (Do not draw all 46 chromosomes). [4 pts]

Answer 1

Non-disjunction is a phenomenon where the chromosomes fail to separate properly during meiosis. It is more common during anaphase I of meiosis I. at the centromere the chromosome consists of 2 sister chromatids held together. During anaphase, one daughter receives both the chromosomes whereas normally sister chromatids separate and go towards opposite poles.

This nondisjunction event affects the whole process as the other one gets none. when these cells divide into 2 cells during meiosis II all 4 daughter cells will have a chromosomal abnormality.

Zygote produced was aneuploid, containing 1 to too many chromosomes called as trisomy (2n + 1). The same events occurred in Gerry’s mom and it resulted in an extra copy of chromosome 21 which caused down syndrome.