Chapter 5: Genetic Linkage and Mapping in Eukaryotes

Linkage

• are always synthetic and always located near one another

  • leads to the production of significantly more gametes with parental allele combination than non-parental combination

  • recognized by comparing observed frequencies of gamete genotypes or progeny phenotypes with those expected under independent assortment

Crossing over

• less likely to occur between closely linked genes than between those farther apart on a chromosome

• occurs at a higher rate between genes that are farther apart

Recombination fraction

a measure of distance

• the farther away two genes are from one another, the more likely they are to have a crossover occur between them

• less distant genes are considered linked

Equation:

r = (# of recombinants) / (total number of progeny)

Parental allele combination

have higher frequencies than predicted by chance

Incomplete Genetic Linkage

more common

• a mixture of parental and non-parental gametes is produced

• two parental type are equal in frequency, same as two recombinant

Fail to reject the null hypothesis

means the genes are not linked

reject the null hypothesis

the genes are linked essentially that we reject the idea they are not linked

Why does distance between genes influence recombination frequency?

• In early prophase I, points of crossing over are established at recombination nodules along the synaptonemal complex

• Genes that are close together are less likely to have a recombination nodule between them than genes farther apart

Is there an upper limit to the frequency of recombinant gametes for a pair of linked genes?

• 6 possible double crossovers

• 8/16 result in recombinants  50%

• Additional crossovers yield the same pattern

Independent Assortment

• sort independently of one another because they’re on different chromosomes

Complete genetic linkage

observed when no crossing over occurs between linked genes

• only parental gametes are formed

Recombination frequency

• a reflection of the physical distance between two genes

Equation:

r = (# of recombinants) / (total number of progeny)

Recombinants

are individuals with phenotypes different from either parent

coupling

unknown mechanism that kept the two parental gamete combinations together

non-parental types describes as

repulsion of the parental alleles

Three point test crosses