GENE MAPPING

Linkage

genes (alleles) on the same chromosome are likely to be inherited together

Linkage group

set of linked genes

• ex: genes located on chromosome 4 are in linkage group IV

• ex: genes located on the X chromosome are X-linked

Genetic map

shows the order of linked genes and distances between genes on a chromosome

• constructed based the rate of recombination (recombination frequency) between genes of interests observed during test crosses

Distance

the distance between genes determines the likelihood of recombination

• the further apart two genes are, the more likely that there will be genetic recombination between them

Crossing over

happens every single cycle of meiosis

Genetic marker

DNA sequence, such as SNP or repeat

Physical marker

phenotype associated with a gene locus

Test cross

• cross F1 heterozygous with a homozygous recessive testes

• if we are interested in two genes, we do a two-point test cross

Recombinant progeny

progeny with a different combination of markers as the parent

• genes have undergone recombination

• result from crossover gametes

Parental progeny

progeny with the same combination of markers as the parent

• result from noncrossover gametes

Two-point test cross

Two extreme scenarios

1. genes are very far apart on same chromosome or are on different chromosomes (assort independently)

• 50% recombination between genes

2. genes are very close on same chromosome

• no recombination between genes

Map units

• mu (unit)

• = % recombination

Map distance formula

Double-crossover (DCO)

occurs when two chiasmata form and there is a double exchange between two nonsister chromatids

• DCO is less likely to happen than single crossover

From a three-point test cross…

• progeny in the highest abundance = parental

• progeny w/ intermediate abundance = single crossover recombinants

• progeny in the lowest abundance = double crossover recombinants

Four major steps

1. determine the order of the genes

2. reorganize the data table to reflect the gene order

3. calculate map units for the first two genes

4. calculate map units for the second two genes