A yeast geneticist irradiates haploid cells of a strain that...

A yeast geneticist irradiates haploid cells of a strain that is an adenine-requiring auxotrophic mutant, caused by mutation of the gene ade1. Millions of the irradiated cells are plated on minimal medium, and a small number of cells divide and produce prototrophic colonies. These colonies are crossed individually with a wildtype strain. Two types of results are obtained: (1) prototroph $\times$ wild type : progeny all prototrophic (2) prototroph $\times$ wild type : progeny $75 \%$ prototrophic, $25 \%$ adenine-requiring auxotrophs a. Explain the difference between these two types of results. b. Write the genotypes of the prototrophs in each case. c. What progeny phenotypes and ratios do you predict from crossing a prototroph of type 2 by the original ade1 auxotroph?

A yeast geneticist irradiates haploid cells of a strain that is an adenine-requiring auxotrophic mutant, caused by mutation of the gene ade1. Millions of the irradiated cells are plated on minimal medium, and a small number of cells divide and produce prototrophic colonies. These colonies are crossed individually with a wildtype strain. Two types of results are obtained:
(1) prototroph $\times$ wild type : progeny all prototrophic
(2) prototroph $\times$ wild type : progeny $75 \%$ prototrophic, $25 \%$ adenine-requiring auxotrophs
a. Explain the difference between these two types of results.
b. Write the genotypes of the prototrophs in each case.
c. What progeny phenotypes and ratios do you predict from crossing a prototroph of type 2 by the original ade1 auxotroph?

Key Concepts

Genetic Reversion

Genetic reversion refers to the process by which a mutation causes a re?mutation that restores the original function of a gene. In reversion, the altered nucleotide(s) is changed back to the wild–type sequence (or an equivalent nucleotide change that permits normal function), thereby eliminating the mutant phenotype. This concept is crucial in understanding how a single gene can show recovery of function after an initial mutagenic event.

Second–Site Suppression

Second–site suppression occurs when a mutation in a different gene, or in another part of the same gene, compensates for the original defect caused by a primary mutation. The suppressor mutation alleviates the phenotypic consequences without restoring the wild–type sequence at the original locus. It illustrates the genetic interplay between different gene products or functional domains that converge on a similar phenotype.

Auxotrophy and Prototrophy

Auxotrophy is the inability of an organism to synthesize a compound required for its growth, usually due to a mutation in a biosynthetic pathway, whereas prototrophy refers to the organism's ability to grow on minimal medium because it can synthesize all essential compounds. This distinction is commonly used to screen for mutations and revertants, as changes in required nutrients reveal underlying genetic defects or restorations.

Complementation Testing in Yeast

Complementation testing is a genetic tool used to determine whether two mutations producing a similar phenotype are in the same or different genes. In yeast, mating haploid strains with different mutations and analyzing the phenotype of the diploid progeny helps to distinguish between allelic (same gene) reversions and mutations that are suppressed by second–site mutations in other genes.

Mendelian Segregation and Genetic Ratios

Mendelian segregation refers to the principle that alleles segregate during gamete formation and then recombine at fertilization, leading to predictable ratios in the progeny. Observing ratios such as a uniform phenotype or a 3:1 split in progeny informs geneticists about gene linkage, dominance relationships, and the involvement of one versus multiple genetic loci in producing a given phenotype.

Transcript

00:01 So if we are asked to explain the differences between these two phenomena, if we look at a part with the prototrope that is bred with a wild type that produces all prototrophic organisms, this phenomenon can be explained by a reversion.

00:31 So essentially what's happened is the original aid -1 mutant has reverted to a wild type.

00:37 So we have aid -1 mutant back to the wild type.

00:45 Now, if we look to explain the second phenomenon where you took a prototrope and you crossed it with a wild type, that you get three -quarters of your population being prototrophic and one quarter being oxytrophic.

01:09 This can be explained only with a new mutation.

01:12 And so the aid one mutant still carries that aid one gene, but now there's a second mutant in the second gene.

01:27 And that gene is unlinked, and we'll call it mute two.

01:36 And so that suppresses the aid one mutant phenotype, yielding the three to one ratio that we have here.

01:44 And so if we write the genotypes for this outcome, what you have is, aid one mutants with a wild type for the second mutation, cross with the aid one wild type with a mutation, the second, the new mutation, and that yields four outcomes.

02:11 Remember, two are parents and two are recombinants, assuming independent assortment.

02:17 And so the first one will make like this parent.

02:21 So aid one, mutant two.

02:25 And that is going to be a prototrope.

02:32 And this one will be the second parent.

02:34 So here, aid 1 plus mute 2 minus.

02:39 That is also a prototrope...