lecture 7

Question 1

If dihybrid cross doesn’t result in 1:1:1:1 ratio what can be inferred?

define notations for: known and unknown linkage; known indep assort

Answer 1

• if genes are linked, the parental combination of gametes are overrepresetned
• genese close together on the same chr are linked
• known linked /
• unknown linked .
• indep assort ;

Question 2

Explaing crossing over

Answer 2

• the physical exchange of DNA b/w non-sister chromatids of homologs in prophase 1
• rare occurance
• expect that most meiosis to have parental combination of gametes
• c/o gives recombinant gametes
• freq recomb gametes are proportional to the genetic distance b/w 2 genes

Question 3

Explain the first proposed model: Holliday Model

Answer 3

1. nicking; occuring at the exact same spot on both homologs = dsDNA break
2. invasion; nicked strand invades the other nicked homolog; must have seq similar to continue
3. resolution; covalent linkage to form Holliday Junction

Question 4

How do you know if genes are linked?

what are the expected ratios for a test cross and a dihybrid

Answer 4

• over-representation of parental phenotypes in F2 of test cross
• progeny doesnt look like 1:1:1:1
• parental alleles were more likely to be inherited tgt than expected from independent assortment
• dihybrid cross not expecting 9:3:3:1

Question 5

How do you measure genetic distance?

define RF value

Answer 5

• NOT = to physical distance
• recombinantion frequence proportional to the distance b/w linked genes
• the further apart 2 loci are, the higher likelihood of a C/O occuring
• if 2 loci are too far apart (>50 cM or mu) then they are unlinked but syntenic = located on the same chr
• appear to assort independently

Question 6

How do you calculate RF value and what does >50%

Answer 6

(# of recombinants/total) x100%
%RF = map units
- either genes are broken in linkage by a centromere (unlinked bu syntenic)
- or high mu probability of a c/o is so high its as if they sort independently