Key Concepts Of Section

Genetic Analysis of Mutations to Identify and Study Genes

■ Diploid organisms carry two copies (alleles) of each gene, whereas haploid organisms carry only one copy.

■ Recessive mutations lead to a loss of function, which is masked if a normal allele of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.

■ Dominant mutations lead to a mutant phenotype in the presence of a normal allele of the gene. The phenotypes associated with dominant mutations often represent a gain of function but in the case of some genes result from a loss of function.

■ In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four haploid cells in which maternal and paternal alleles are randomly assorted (see Figure 9-3).

■ Dominant and recessive mutations exhibit characteristic segregation patterns in genetic crosses (see Figure 9-4).

■ In haploid yeast, temperature-sensitive mutations are particularly useful for identifying and studying genes essential to survival.

■ The number of functionally related genes involved in a process can be defined by complementation analysis (see Figure 9-7).

■ The order in which genes function in either a biosyn-thetic or a signaling pathway can be deduced from the phe-notype of double mutants defective in two steps in the affected process.

(a) Suppression

Genotype AB aB

Phenotype Wild type Mutant

Ab ab

Mutant Suppressed mutant

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