Chapter 5 | Chromosome Mapping in Eukaryotes

Linkage

Alleles on the same chromosome segregate as a unit during gamete formation

Recombination

Reshuffling of alleles between homologs during the tetrad stage

Chromosome maps

Indicate the relative locations of genes on the chromosomes

Complete linkage

Produces only parental/noncrossover, gametes that are formed in equal proportions

50% recombination

Results in a 1:1:1:1 ratio

Chiasmata

Points of genetic exchange

Single crossover (SCO)

Occurs between two nonsister chromatids, but not between the two loci being studied; therefore, the crossover is undetected because no recombinant gametes are produced for the two traits of interest

Double crossover (DCO)

Double exchanges of genetic material

Incomplete linkage

Genes located on the same chromosome are not always inherited together, as they are separated by crossing-over events during meiosis

Mapping cross

Crossing a heterozygote with a homozygous recessive

Interference

Formation of one single crossover event interferes with the formation of another single crossover event

(+) = yes

(-) = assistance

0 = none

Interference hypothesis

Interference is due to the synaptonemal complex forming around each recombination module, meaning the proteins are interfering with each other. When the proteins on each synaptonemal complex are the same, they assist each other

Determining the gene sequence

1. Simulate a double crossover event by putting each gene in the middle

2. From the DCO simulation, pick which matches the combination in the NCO → this cross tells which gene is in the middle

3. Simulate SCO (s) by using the NCO sequence

4. Match the correct sequence to the SCO (s)

5. Determine the total # of progeny

6. Add the total # from one SCO including the #s from the DCO x100% to get the recombination between gene 1-2. Do this again if 3 genes to find the recombination for 2-3

Interference formula

1 - coefficient of coincidence (observed DCO / expected DCO)

Expected DCO frequency

Multiply the decimals of the recombination frequencies between genes

Observed DCO frequency

Total # of DCO progeny / total # of progeny