Complementation Tests Determine Whether Different Recessive Mutations Are in the Same Gene

In the genetic approach to studying a particular cellular process, researchers often isolate multiple recessive mutations that produce the same phenotype. A common test for determining whether these mutations are in the same gene or in different genes exploits the phenomenon of genetic complementation, that is, the restoration of the wild-type phenotype by mating of two different mutants. If two recessive mutations, a and b, are in the same gene, then a diploid organism heterozygous for both mutations (i.e., carrying one a allele and one b allele) will exhibit the mutant phenotype because neither allele provides a functional copy of the gene. In contrast, if mutation a and b are in separate genes, then heterozygotes carrying a single copy of each mutant allele

Mate haploids of opposite mating types and carrying different recessive temperature-sensitive cdc mutations

Test resulting diploids for a temperature-sensitive cdc phenotype

Mutant (type a)

Mutant (type a)

Mutant (type a)

cdcX

Mutant (type a)

cdcZ

cdcXcdcY (type a/a)

cdcX/cdcZ (type a/a)

Plate and incubate at permissive temperature

Plate and incubate at permissive temperature

23 °C

Growth

Replica-plate and incubate at nonpermissive temperature

Growth

No growth

PHENOTYPE:

fcdcxl\ \cdcy )

cdcX/ \cdczJ

Growth indicates that mutations cdcX and cdcy are in different genes

Mutant

Absence of growth indicates that mutations cdcx and cdcz are in the same gene

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