Multi-Trait Inheritance 4.6 – Dihybrid Crosses & The Law of Independent Assortment

1. Multi-Trait Inheritance • 4.6 – Dihybrid Crosses & The Law of Independent Assortment • Mendel also studied the inheritance of two separate traits, following the same procedure he had used for single traits • cross-pollinated “pure breeding” pea plants that produced round, yellow seeds with ones that produced wrinkled, green seeds •  F1 generation phenotype all round, yellow and genotype all YyRr (see Fig. 1, P. 150) •  cross of heterozygotes produced four different phenotypes in the F2 generation in the ratio of 9:3:3:1 (Fig. 4, P. 152)

2. Mendel’s Second Law: • The Law of Independent Assortment • Mendel found that crossing pure-breeding green round with yellow wrinkled showed that inheritance of the gene for seed colour was not affected by the inheritance of the gene for seed shape • (see Fig. 2, P. 152) • gametes formed by meiosis determined with “FOIL” because of independent assortment • as homologous chromosomes move to opposite poles during anaphase I of meiosis, the yellow allele wil segregate with round and wrinkled alleles in equal frequency • YR gametes = yR gametes • = Yr gametes = yr gametes • (see Fig. 3, P. 151) • Sample Problems 1 and 2

3. Gene Linkage • Thomas Hunt Morgan discovered a number of observable mutations in fruit flies (Table 1, P. 164) and he noted that some of them seemed to be linked to other traits • chromosomes located on the same gene tend to be inherited together • when two different traits are determined by linked genes, they do not assort independently • i.e. fruit flies G – grey body W – normal wings • Test cross: GgWw x ggww:  G and W, g and w are linked, tend to be inherited together (Ggww and ggWw a result of crossing over)

4. in any species, the number of linkage groups = the number of homologous pairs of chromosomes  humans have 23 linkage groups • the proximity of linked genes to each other determines the frequency of linkage (closer together, less chance of crossing-over, more likely to be transmitted together) • Gene Mapping  study of linkage and crossover frequencies enables the approximate location and order of genes on chromosomes •  Gene Mapping Problem