GENE3200 - Linkage 1 (class 11)

genes located on the same chromosome

-do not assort independently

-if no crossing over occurs, alleles on the same chromosome will always be inherited together

crossing over during meiosis I results in genetic variation

-non-sister chromatids recombine (cross over) and create new combinations of alleles

-a dihybrid for two linked genes → four gamete genotypes produced: two nonrecombinants and two recombinants

-frequency of recombinants depends on the physical distance between the two genes

recombinants

don’t look like parental (aB, Ab)

nonrecombinants

look like parents (AB, ab)

slash

phased genotype for linked genes have a slash for alleles on separate homologs (AB/ab)

coupling (cis) configuration

dominant (or wild-type) alleles of two linked genes are present on the same homolog (AB/ab)

repulsion (trans) configuration

dominant (or wild-type) alleles of two linked genes are present on different homologs (Ab/aB)

recombination frequency is used to calculate genetic distance

-crossing over happens randomly across a chromosome

-the frequency of crossing over between genes that are farther apart is higher than the frequency of crossing over between genes that are close together

RF (recombination frequency)

relative measure of distance between two linked loci

a testcross can show when two genes are linked

if the A and B genes are unlinked, the phenotype ratios are in testcross progeny are expected to be 1:1:1:1