Info

Wild-type

Gametes

Second filial generation, F2: 3/4 of offspring have mutant phenotype

Normal

Mutant

Normal this is done, we need first to review the type of cell division that gives rise to gametes (sperm and egg cells in higher plants and animals). Whereas the body (somatic) cells of most multicellular organisms divide by mitosis, the germ cells that give rise to gametes undergo meiosis. Like somatic cells, premeiotic germ cells are diploid, containing two homologs of each morphologic type of chromosome. The two homologs constituting each pair of homologous chromosomes are descended from different parents, and thus their genes may exist in different allelic forms. Figure 9-3 depicts the major events in mitotic and meiotic cell division. In mitosis DNA replication is always followed by cell division, yielding two diploid daughter cells. In meiosis one round of DNA replication is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of the replicated homologous chromosomes to daughter cells during the first meiotic division is random; that is, maternally and paternally derived homologs segregate independently, yielding daughter cells with different mixes of paternal and maternal chromosomes.

As a way to avoid unwanted complexity, geneticists usually strive to begin breeding experiments with strains that are homozygous for the genes under examination. In such true-breeding strains, every individual will receive the same allele from each parent and therefore the composition of alleles will not change from one generation to the next. When a true-breeding mutant strain is mated to a true-breeding wildtype strain, all the first filial (F1) progeny will be heterozygous (Figure 9-4). If the F1 progeny exhibit the mutant trait, then the mutant allele is dominant; if the F1 progeny exhibit the wild-type trait, then the mutant is recessive. Further crossing between F1 individuals will also reveal different patterns of inheritance according to whether the mutation is dominant or recessive. When F1 individuals that are heterozygous for a dominant allele are crossed among themselves, three-fourths of the resulting F2 progeny will exhibit the mutant trait. In contrast, when F1 individuals that are heterozygous for a recessive allele are crossed among themselves, only one-fourth of the resulting F2 progeny will exhibit the mutant trait.

As noted earlier, the yeast Saccharomyces, an important experimental organism, can exist in either a haploid or a diploid state. In these unicellular eukaryotes, crosses between haploid cells can determine whether a mutant allele is dominant or recessive. Haploid yeast cells, which carry one copy of each chromosome, can be of two different mating types known as a and a. Haploid cells of opposite mating type can mate to produce a/a diploids, which carry two copies of each chromosome. If a new mutation with an observable pheno-type is isolated in a haploid strain, the mutant strain can be mated to a wild-type strain of the opposite mating type to produce a/a diploids that are heterozygous for the mutant allele. If these diploids exhibit the mutant trait, then the mutant allele is dominant, but if the diploids appear as wild-type, then the mutant allele is recessive. When a/a diploids are placed under starvation conditions, the cells

(b) Segregation of recessive mutation

Gametes

First filial generation, Fr no offspring have mutant phenotype

Gametes

Second filial generation, F2: 1/4 of offspring have mutant phenotype

▲ FIGURE 9-4 Segregation patterns of dominant and recessive mutations in crosses between true-breeding strains of diploid organisms. All the offspring in the first (Ft) generation are heterozygous. If the mutant allele is dominant, the Ft offspring will exhibit the mutant phenotype, as in part (a). If the mutant allele is recessive, the Ft offspring will exhibit the wild-type phenotype, as in part (b). Crossing of the Ft heterozygotes among themselves also produces different segregation ratios for dominant and recessive mutant alleles in the F2 generation.

Wild type (type a)

Mutant (type a)

Sporulation

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