L5: Linkage analysis and genetic mapping I

gametes after independent assortment

• parental and recombinant combinations occur with equal frequency (1:1)

• AB:ab:aB:Ab (1:1:1:1)

parental gametes

AABB and aabb

what happens when crossing over does not occur

• no formation of recombinant gametes

• genes remain together in parental combinations

• known as cosegregation

• after meiosis II, 100% of the gametes are identical to one of the parental gametes

what happens when crossing over occurs

• formation of both parental and recombinant gametes occur

• known as independent assortment

• 9:3:3:1 ratio observed

parental:recombinant ratio after crossing over

1:1

synapses

pairing of homologous chromosomes

linkage

• genes that are physically together on a chromosome show this phenomenon

• tend to cosegregate and do not assort independently

• less probability of forming a chiasmata, therefore are less likely to be seperated during cross over

biggest possible linkage group

chromosome

progeny of linked genes that cosegregate

parental gametic combinations > recombinant gametic combinations

chiasmata

region where crossing over happens

3 possible outcomes in gamete formation from a AaBb selfing

• independent assortment

• linkage without crossing over

• linkage with crossing over

AaBb - independent assortment gamete

• 2 genes on 2 diff homologous pairs

• dihybrid cross

• both parents and recombinant gametes seen

AaBb - linkage without crossover gamete

• 2 genes on a single pair of homologs

• only parental gametes seen

• co-segregation

AaBb - linkage with crossing over

• 2 genes on a single pair of homologs

• crossing over occurs between the 2 genes

• both parental and recombinant gametes seen

• independent assortment

incomplete linkage

• recombinant progeny appears but in much lower frequency

• complete linkage should only produce parental types

how to reveal linkage

testcross

parental types when conducting a testcross

• heterozygous x homozygous recessive

• AaBb x aabb

what does the test cross reveal when there is complete linkage

only parent progeny is produced

what does the testcross reveal when genes are unlinked

half the progeny is recombinant and half are parental

what does the testcross reveal when genes are linked with some crossing over

both parental and recombinant gametes observed but parental gametes dominate