BIOL 207 Lecture 8 - Gene Linkage (Questions)

How does the probability of two different genes crossing over (are unindependent/assort together) change with gene locus distance? At very large distance, how does independence change?

The probability of crossing over between two genes increases with greater distances between gene loci (If gene loci are close, crossover rate increases)

At large distance, it will appear as though they are independently assorting because all gametes occur at relatively equal frequencies

What are the three processes by which recombination occurs, and how?

1. Random fertilization: random fusion of gametes to create new genotypes in organisms (new combinations of alleles)

2. Independent assortment: different and random chromosome assortments in meiosis I and meiosis II, so chromosomes are no longer identical after (different allele combinations)

3. Crossing over: exchanging of alleles between homologous chromosomes results in different allele combinations

How is recombination frequency calculated? What is the maximum of this frequency and why?

Sum of all recombinants divided by total number of progeny

Recombination frequency does not exceed 50%. There is only two possible locations in recombination crossing over: either on homologous chromosome I or homologous chromosome II

There is no push or pull of the gene to either specific homolog, other than random crossover, and it is bidirectionally occurring, so there is equal chance for the allele to be on either homolog (50%)

How can a test cross reveal if genes are linked or not?

Genes that are linked → less crossing over → less recombinant offspring

Genes that are unlinked or act unlinked → more crossing over → more recombinant offspring

What is the difference between synteny and gene linkage?

Synteny: physically linked

Gene linkage: near one another

Genes that are linked are always assorted together, but syntentic chromosomes can be far enough apart that they appear independent

How many map units apart do two genes have to be in order to be considered independent?

Less than 50 map units apart: genes do not assort independently (are considered close together)

50 or more map units apart: genes appear to assort independently (are considered far apart enough to be separate)

If the recombination frequency for two genes is found to be 50%, what does this indicate for genes on the same chromosome? What possibilities of assortment could this gene have?

50% is the max recombination frequency → must be independently assorted or act independently assorted

On the same chromosome: must be 50 or more map units (far enough apart to act independent)

Otherwise, genes are found on different chromosomes

Why might recombination frequency/map units not reflect the actual distance between genes?

1. Cross overs are reduced near the centromere → percentage is actually not representative of difference

2. Once a cross over occurs in one area, it tends to interfere with additional cross overs in that area → crossover rate may not be only reflective of the distance

3. Genes that are far apart show a maximum of 50% recombination, may be further apart