Key Concepts of Section 6.1

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

• Recessive mutations often lead to a loss of function, which is masked if a wild-type allele of the gene is present. For the mutant phenotype to be observed, both alleles must carry the mutation.

• Dominant mutations lead to a mutant phenotype in the presence of a wild-type allele of the gene. The phenotypes associated with dominant mutations often represent a gain of function.

• In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four haploid cells in which maternal and paternal chromosomes and their associated alleles are randomly assorted (see Figure 6-3).

• Dominant and recessive mutations exhibit characteristic segregation patterns in genetic crosses (see Figure 6-4).

• 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 6-7).

• The order in which genes function in a biosynthetic or signaling pathway can be deduced from the phenotype of double mutants defective in two steps in the affected process.

• Functionally significant interactions between proteins can be deduced from the phenotypic effects of allele-specific suppressor mutations or synthetic lethal mutations.

• Genetic mapping experiments make use of crossing over between homologous chromosomes during meiosis to measure the genetic distance between two different mutations on the same chromosome.