Genetic Crosses And Linkage Analysis

Saccharomyces diploids undergo meiosis when placed in starvation conditions and form four haploid ascospores, or just spores for short, all contained in a single sack called an ascus. These four spores are referred to as a tetrad since each spore contains one chromatid from each of the 16 tetrads of chromatids found in prophase I of meiosis.

To fully understand the crosses outlined below, it would be helpful to first review the process of meiosis including chromatid segregation patterns and independent assortment, i.e. basic Mendelian genetics. The genetic cross is a powerful tool. In the initial stages of a genetic analysis the researcher must know whether a single mutation is producing the mutant phenotype under investigation. A simple genetic cross can demonstrate this. Crosses are used to construct heterozygous diploids to determine whether a mutation is dominant or recessive and can be used to demonstrate linkage. As can be seen from the literature, linkage analysis is used in a variety of ways and not simply to map genes on a chromosome.

SINGLE GENE CROSS

If two haploid strains carrying different alleles of the same gene are mated and the resulting diploid sporulated, the two alleles will segregate to different spores. The resulting four-spored tetrad will consist of two spores containing one allele of the gene and two spores containing the second allele of the gene. This is shown below in Cross 1 in which a strain containing a recessive mutation, genl-62, is crossed to a strain carrying the wild-type dominant allele GEN1.

Cross 1: GEN1 x genl-62 (genotypes of parental strains)

(wild-type) (mutant) (phenotype of parental strains)

Diploid: GEN1 (genotype of diploid) genl-62

(wild-type) (phenotype of diploid)

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