Question

Advanced Genetics
Problem:
Yeast chromosome 3 has these genes in this order: HIS4 LYS1 URA3 ADE2. You have created a diploid strain of this genotype:

HIS4 LYS1 URA3 ADE2 or hiss lys1 ura3 ade2

You sporulate the diploid and analyze tetrads. Most tetrads show 2:2 segregation for all markers and recombination frequencies that are consistent with the established genetic map. However, you notice the one strange tetrad indicated below:

Your goal is to explain how the genotypes of this tetrad arose during meiosis:

a. Do these spores indicate that there was a recombination event? Briefly explain your answer. (1 pt)

b. Do these spores indicate that a crossover occurred? Briefly explain your answer. (1 pt)

c. Do any of these spores represent a gene conversion event or events? Briefly explain your answer. (1 pt)

d. Diagram a pair of homologous chromosomes containing these markers lined up at meiosis M1. Label each chromatid by genotype (+ or -) and circle the chromatids that are relevant to the segregation pattern in the table above. (2 pts)

e. Use a diagram of the two relevant chromatids in meiosis to indicate (a) where a DS break may have occurred to initiate a crossover, (b) the extent of heteroduplex formation, (c) how mismatch repair or DNA replication may have affected marker segregation, and (d) how Holiday junction resolution may have occurred. Although there are multiple explanations, you need only one explanation that is consistent with the principles described in lecture and the segregation of markers in the tetrad under consideration. (5 pts)

Answer 1

A. Yes. Spore B and C shows the presence of recombination.

B. Yes. There is a presence of crossover events in these spores. As out of 4 spores 2 shows parental genes while 2 spores shows recombinants.

C. spore B and c shows gene conversion events.