Topic 10 2 Dihybrid Crosses Gene Linkage Dihybrid

Topic 10. 2 – Dihybrid Crosses & Gene Linkage

Dihybrid crosses are crosses involving two separate traits (monohybrid is crossing one trait). For example: G = green g = yellow R = round r = wrinkled Gg. Rr x Gg. Rr Heterozygous Green Heterozygous Round X Heterozygous Green Heterozygous Round GGRR GGRr GGrr Gg. Rr Ggrr gg. RR gg. Rr ggrr 10. 2. 1

Dihybrid Cross Gg. Rr x Gg. Rr GR Gr g. R gr GR GGRr Gg. RR Gg. Rr Gr GGRr GGrr Gg. Rr Ggrr g. R Gg. Rr gg. RR gg. Rr gr Gg. Rr Ggrr gg. Rr ggrr 10. 2. 1

GR Gr g. R gr GR GGRr Gg. RR Gg. Rr Gr GGRr GGrr Gg. Rr Ggrr g. R Gg. Rr gg. RR gg. Rr gr Gg. Rr Ggrr gg. Rr ggrr Phenotypic Ratio 9 Green/Round : 3 Green/Wrinkled : 3 Yellow/Round : 3 Yellow/Wrinkled 10. 2. 1

10. 2. 1

Humans have 23 chromosome pairs. Sex chromosomes are those that determine gender. Autosomes are all other chromosomes. 10. 2. 2

Crossing over during meiosis allows for alleles to be exchanged. This allows for the allele from either chromosome pair to be passed to any given gamete. Recall that allele pairs are exchanged independently. 10. 2. 3

10. 2. 3

The law of independent assortment does not apply to all situations. Linked genes are located on the same chromosome and so are usually passed on together. A linkage group consists of genes on a chromosome that are passed on together. This applies to autosomes as well as the sex chromosomes. For genetics problems, assume that genes are not linked unless stated. 10. 2. 4

Practice Problem In Drosphila (fruit flies) the allele for grey color is dominant over black. Straight wings are dominant over curly wings. A heterozygous grey-straight winged fly was crossed with a black-curly winged fly. a. If these genes are not linked, which phenotypes would you expect in F 1? b. Give the expected numbers of the phenotypes. 10. 2. 4

Gg. Ss x ggss GS Gs g. S gs gs Gg. Ss Ggss gg. Ss ggss a. Grey/Straight Grey/Curly Black/Straight Black/Curly b. 1: 1: 1: 1 10. 2. 1

In dihybrid crosses, alleles are usually shown side by side like in the example below: Gg. Ll x Gg. Ll G = grey g = black L = long l = short paternal chromosome maternal chromosome G L g l X G G L g l l g L recombinant chromosomes 10. 2. 5

Gg. Ll x Gg. Ll G L g l GL G Gl l g. L gl GL GGLl Gg. LL Gg. Ll Gl GGLl Ggll g. L Gg. Ll gg. LL gg. Ll gl Gg. Ll GGll Ggll gg. Ll ggll 10. 2. 5

By comparing the genotype of the offspring to that of the parent, you can see which are a product of recombination parent chromosome G L g l X offspring without recombination G L g l g g offspring with recombination G l G L g l g L l l G L g l G G L L G G l l g L 10. 2. 6

10. 2. 1 10. 2. 2 10. 2. 3 10. 2. 4 10. 2. 5 10. 2. 6 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. Distinguish between autosomes and sex chromosomes. Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles. Define linkage group. Explain an example of a cross between two linked genes. Identify which of the offspring are recombinants in a dihybrid cross involving linked genes. 10. 2