Question

A. Yeast cells have a haploid chromosome number of 16. A haploid cell disomic for two different chromosomes has the same chromosome number as a:

B. Two people with Trisomy 21 fall in love and decide to get married. What is the probability that their first child will be euploid?

C.

A scientist who studies Trisomy 21 in humans wants to know whether the disomic gamete that resulted in Trisomy 21 came from the mother or the father, as well as whether non-disjunction chromosome 21 occurred at Meiosis I or Meiosis II. She finds that there are specific DNA sequences near each chromosome 21 centromere in the two parents (shown below). She then determined the DNA sequences at this location on the three copies of chromosome 21 present in the affected child. The results are shown below:

From these data, one can conclude:

A. The disomic gamete resulted from Meiosis I nondisjunction in the mother.

B. The disomic gamete resulted from Meiosis II nondisjunction in the mother.   

C. The disomic gamete resulted from Meiosis I nondisjunction in the father

D. The disomic gamete resulted from Meiosis II nondisjunction in the father.

Answer 1

Q.A.

A. Yeast cells have a haploid chromosome number of 16. A haploid cell disomic for two different chromosomes has the same chromosome number as a:  

Given that yeast has 16 chromosomes (n). When a cell is disomic for two chromosome, it will have 2 extra chromosome, for a total of 18. Same is the case for a haploid cell with trisomy in one chromosome, as it would also lead to 18 chromosome.

QB. Two people with Trisomy 21 fall in love and decide to get married. What is the probability that their first child will be euploid?   

When both parents are trisomic for 21, then the probability that child will be euploid (have normal number of chromosomes, 46), =1/2×1/2 =1/4