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Topic 10.2 – Dihybrid Crosses & Gene Linkage
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Dihybrid crosses are crosses involving two separate traits (monohybrid is crossing one trait). For example: G = green g = yellow R = round r = wrinkled G g R r x G g R r X GGRR GGRr GGrr GgRr Ggrr ggRR ggRr ggrr Heterozygous Green Heterozygous Round Heterozygous Green Heterozygous Round Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.1
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Dihybrid Cross GgRr x GgRr
Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. GgRR GgRr ggRR ggRr GgRr Ggrr ggRr ggrr 10.2.1
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GR Gr gR gr GGRR GGRr GgRR GgRr GGRr GGrr GgRr Ggrr GgRR GgRr ggRR ggRr GgRr Ggrr ggRr ggrr Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. Phenotypic Ratio 9 Green/Round : 3 Green/Wrinkled : 3 Yellow/Round : 3 Yellow/Wrinkled 10.2.1
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Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.1
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Humans have 23 chromosome pairs
Humans have 23 chromosome pairs. Sex chromosomes are those that determine gender. Autosomes are all other chromosomes. Distinguish between autosomes and sex chromosomes. 10.2.2
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Crossing over during meiosis allows for alleles to be exchanged
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. Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles. 10.2.3
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Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles. 10.2.3
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The law of independent assortment does not apply to all situations
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. Define linkage group. 10.2.4
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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 F1? b. Give the expected numbers of the phenotypes. Define linkage group. 10.2.4
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GS Gs gS gs GgSs Ggss ggSs ggss 10.2.1 GgSs x ggss
Grey/Straight Grey/Curly Black/Straight Black/Curly 1:1:1:1 GS Gs gS gs GgSs Ggss ggSs ggss Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.1
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recombinant chromosomes
In dihybrid crosses, alleles are usually shown side by side like in the example below: GgLl x GgLl G = grey g = black L = long l = short X paternal chromosome maternal chromosome g l G L g l G L Explain an example of a cross between two linked genes. Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example G L g l G l g L recombinant chromosomes 10.2.5
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GL Gl gL gl GGLL GGLl GgLL GgLl GGll Ggll ggLL ggLl ggll 10.2.5
GgLl x GgLl G L g l G l g L GL Gl gL gl GGLL GGLl GgLL GgLl GGll Ggll ggLL ggLl ggll Explain an example of a cross between two linked genes. Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example 10.2.5
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By comparing the genotype of the offspring to that of the parent, you can see which are a product of recombination X g l G L g l G L parent chromosome offspring without recombination g l G L g l G L Identify which of the offspring are recombinants in a dihybrid cross involving linked genes. offspring with recombination G L G l g L g l G l g L 10.2.6
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10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.2 Distinguish between autosomes and sex chromosomes. 10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles. 10.2.4 Define linkage group. 10.2.5 Explain an example of a cross between two linked genes. 10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes. 10.2
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