Question

Baker’s yeast Saccharomyces cerevisiae is a unicellular organism with n=16. It can stably exist either as a haploid or as a diploid. Haploid cells can divide by mitosis and produce more haploid cells. Occasionally, two haploid cells can fuse together to produce a diploid cell. Diploid cells can divide by mitosis and produce more diploid cells. Occasionally, a diploid cell can undergo meiosis to produce haploid cells.

(a) If two haploid yeast cells fuse to produce one diploid cell, and then this diploid cell undergoes meiosis to produce four haploid cells, what is the probability that an individual haploid cell resulting from this meiosis will have a complete parental set of chromosomes (that is, all chromosomes from one parental haploid cell, or all chromosomes from the other parental haploid cell)?

Disregard the effect of crossing over; consider only the assortment of chromosomes.

(b) A normal diploid yeast cell went through meiosis, and a nondisjunction event occured in meiosis I. Out of the 4 haploid cells that resulted from this meiosis, how many will have normal chromosomal content?

(c) A normal diploid yeast cell went through meiosis, and a nondisjunction event occured in meiosis II. Out of the 4 haploid cells that resulted from this meiosis, how many will have normal chromosomal content?

Would appreciate help on all these parts! Thanks!

Answer 1

A. If two haploid yeast cells fuse to produce one diploid cell, and then this diploid cell undergoes meiosis to produce four haploid cells, then the probability that an individual haploid cell resulting from this meiosis will have a complete parental set of chromosome is (1/2)^n. Because, for any one pair, the probability of a centromere from one parent going into a specific gamete is 1/2. So for n pairs, the probability of all the centromeres being from one parent is 1/2)^n.

B. Nondisjunction in meiosis I, out of four haploid cells, no cell will contain normal chromosomal content. Because, in meiosis I, nondisjunction of homologous chromosomes occurs, both homologs move towards the same pole.

C. Nondisjunction in meiosis II, out of four haploid cells, two will contain the normal chromosomal content and one will contain extra chromosomal set and other will contain a less chromosomal set. Because in meiosis II nondisjunction of sister chromatids occurs with both chromatids going with the same pole.