Wild-type S. cerevisiae form white colonies. You have isolated two mutants in a

Question

Wild-type S. cerevisiae form white colonies. You have isolated two mutants in a genetic screen for genes that influence the color of the colonies. Both mutants have pink colonies. You call these mutants pnk1 and pnk2. A. What is one cross you can perform to determine whether pnk1 confers a recessive or dominant phenotype? (1 point) ii. What color will the diploids be if the phenotype is recessive? (1 point) ili. What segregation pattern of pinkwhite spores would indicate that pnk1 is a mutation in a single gene? (1 point) B. You find that both pnk1 and pnk2 are mutations in single genes and confer recessive phenotypes. What one cross can you perform to determine whether pnk1 and pnk2 are in the same or different genes? What color will diploid colonies be if the pnk1 and pnk2 mutations are in different genes? (2 points) C. You determine that pnk1 and pnk2 are in different genes, and want to determine their genetic position. You cross MATa pnk1 x MATalpha pnk2, and sporulate 100 diploids. You find three classes of tetrads with the following spore color pattern: Class 1: 3 pink: 1 white Class 2: 4 pink Class 3: 2 pink: 2 white i. Which class represents the parental ditype? (2 points) ii. Assume that pnk1 is inseparable from its centromere. What is the map distance between pnk2 and its centromere if you count 46 PD, 44 NPD, and 10 TT? (3 points)

Explanation / Answer

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Answer 1:

Part i) In order to ascertain whether pnk1 confers recessive or dominant mode of inheritance, a cross between strains having heterozygosity for pnk1 and pnk2 must be performed. In this way, the presence of heterozygous as well as homozygous conditions with respect to both genes would be observed in the offsprings and their respective phenotype will decipher their mode of inheritance.

Part ii) If the mode of inheritance is recessive, the diploids will have pink color. On the other hand, the homozygous dominants as well as the heterozygotes will have white color.

Part iii) If the mutation takes place in a single location, the segregation of the offsprings will take place as a simple phenotype of 3:1 having 3/4 white and 1/4 pink colonies. This is because a single mutation will behave as an alternative allele and follow the regular Mendelian pattern of inheritance.

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