Chapter 5: linkage, recombination, and the mapping of genes on chromosomes

syntenic genes

genes located on the same chromosome. have a higher probability of assorting together (if they are less than 50 map units apart) but can still be separated by recombination

what happens to the 9:3:3:1 ratio with linkage?

the ratio is altered. We can detect linkage by generating a heterozygote and crossing it with a homozygous recessive (test cross)

where is evidence of recombination found?

in the areas where reciprocal exchanges between the homologous chromosomes are found

H. Sturtevant

proposed that recombination frequencies could be used as a measure of physical distance between 2 genes

recombinant genes

are less frequent than parental gametes when genes are linked. The frequency of recombinants never exceeds 50% due to independent assortment

parental chromosomes

occur when meiosis produces no cross overs

single and double crossovers

produce a 1:1 parental to recombinant chromosome ratio on average

two-point cross

occurs when two points on a gene cross over.

limitations of two-point gene crosses and distance measurements

- difficult to determine gene order if two genes are close together

- actual distances between genes do not always add up

- pairwise crosses are time and labor consuming

three-point cross

when 3 points on homologous chromosomes cross over. We can testcross a triply-heterozygous F1 with a tester to produce 8 different phenotypes in the offspring. to find out if a crossover event occurred and in what location, compare parent genotype to offspring two genes at a time

what is the formula for determining genetic map distance on a cross?

(#of recombinant genotypes/total #of genotypes)*100

does estimated genetic distance correlate with physical reality?

the order of genes revealed by a genetic map is accurate, but the actual physical distance (amount of DNA) does not always show direct correspondence to genetic distance, especially with double, triple, or more crossovers. There are also recombination hotspots and a 50%limit on observable recombination frequency

linkage group

is synonymous with a chromosome when many genes per chromosome have been mapped

what does the Chi square test pinpoint?

the probability that ratios are evidence of linkage. In other words, how likely genes are to be linked. Deviations from 1:1:1:1 ratio can represent chance event or linkage.

null hypothesis

states that observed values are no different than expected values (AKA no significant results). i.e. there is no linkage in the case of linkage studies vs proof of linkage

how to use chi square test

• take (observed - expected)² /expected, then multiply by 100 to get x²

• expected is how many offspring you’d expect with a 1:1:1:1 ratio