Topic 10.2 Dihybrid crosses and gene linkage

Question 1

10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

Answer 1

G = 4:2:2:2:2:1:1:1:1

P = 9:3:3:1

Question 2

10.2.2 Distinguish between autosomes and sex chromosomes.

Answer 2

Autosomes: pairs of chromosomes that are identical in appearance (e.g. same size, same gene loci, etc.) and are not involved in sex determination

Sex chromosomes: pairs of chromosomes involved in sex determination and are not identical in appearance (e.g. X and Y chromosome in humans)

Question 3

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.

Answer 3

• During crossing over in prophase I, non-sister chromatids of a homologous pair may break and reform at points of attachment called chiasmata
• As these chromatids break at the same point, any gene loci below the point of the break will be exchanged as a result of recombination
• This means that maternal and paternal alleles may be exchanged between the maternal and paternal chromosomes, creating new gene combinations
• The further apart two gene loci are on a chromosome, the more likely they are to be exchanged

Question 4

10.2.4 Define linkage group.

Answer 4

Linkage group: all of the genes that have their loci on the same chromosome type

e.g. A B & a b

• do not follow law of independent assortment
• tend to be inherited together - only way to separate is through recombination

Question 5

10.2.5 Explain an example of a cross between two linked genes.

Answer 5

An example of a cross between two linked genes is the mating of a grey bodied, normal wing fruit fly with a black bodied, vestigial wing mutant.

• Linked genes don’t follow the expected phenotypic ratio for a dihybrid cross between heterozygous parents
• Instead the phenotypic ratio will follow that of a monohybrid cross (heterozygous single cross - complete dominance) as the two genes are inherited together (P = 3:1)
• This means that offspring will tend to produce the parental phenotypes
• Recombinant phenotypes will only be evident if crossing over occurs in prophase I and would thus be expected to appear in low numbers (if at all) (P = 3:1 + few recombinants)

Question 6

10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

Answer 6

Linked genes that have undergone recombination can be distinguished from unlinked genes via a test cross because the frequency of the recombinant genotypes will always be less than would occur for unlinked genes (crossing over does not happen every time)

e.g.

Heterozygous test cross of unlinked genes = 1 : 1 : 1 : 1 phenotypic ratio

Heterozygous test cross of linked genes = 1 : 1 : 0.1 : 01 phenotypic ratio (uncommon phenotypes are recombinants)