RECOMBINATION & CHROMOSOME MAPPING

Chiasma formation

It is the visible manifestations of crossover

occurs during pachytene/zygotene of meiotic prophase 1

brings our the recombination (Morgan)

Chromosomal recombination

Inter

Unlinked genes

follows the Independent assortment

Testcross progeny 1:1:1:1 ratio

produces 50% of recombination

Intra

Linked genes

Testcross progeny 1:1:1:1 ratio

produces less than 50% of recombination

bc the genes are close together ==> less chance of getting crossing over

Recombination frequency

Key of chromosome mapping

The further the distance between genes ➡ The higher the frequency

gives a clue to the distance between them

Ranges from 0-50%

0%: No crossover => genes are close together ▶ Complete Linkage

50%: crossover is likely to occur => genes are further away/ at different chromosomes

⚠ cannot exceed 50% bc the 2 outside chromatids don't recombine (even in the unlinked genes)

the frequency with which a single chromosomal crossover will take place between two genes during meiosis

Linkage maps

a table for a species or experimental population that shows the position of its known genes relative to each other in terms of recombination frequency

based on recombination frequency

developed by Morgan & Sturtevant

⛔ relative position and physical position are not necessarily the same

Map units

centimorgans(cM)

1% of recombination frequency = 1cM

Linear map ➡ corresponds to the linear arrangement of genes ➡ makes up the chromosome (suggestion)

Three point testcross

a cross of a trihybrid (triple heterozygote) to a triply recessive tester

produces 2 parental gametes BUT 6 recombinants

crossover

single

double

gives the rarest frequency of recombination

only the middle genes are altered

produces double recombinant chromatids that have the parental allele combination.

for recombination frequency => should count these as twice

Interference

crossover in one region inhibits crossovers in other region

Double cross is the exception of Interference

quantified by coefficient of coincidence = observed no. of double crossovers/ expected no. of double crossover

Interference = 1 - coefficient of coincidence

I=0 ➡ no interference ➡ observed and expected are equal

I=1 ➡ complete interference ➡ no observed crossover

I<1 (usually) ➡ observed < expected