Topic 10 pt 1: Mapping Flashcards
What is segregation
1st law
Alleles separate during meiosis
What is independent assortment
2nd law
Alleles at one locus sort independently from alleles at another locus
What is recombination
Alleles sort into new combinations (diff from parents)
In a test cross, what phenotypic ratios would we expect if the organism assorts independently? If it doesn’t?
Yes = 1:1:1:1 (50% parental, 50% recombinant)
No (linked) = >50% of offspring show parental, <50% show recombinant
How do linked genes segregate? Is there recombination
Segregate together (dependently)
Crossing over produces recombination between them
Genes that are linked are… (3)
- on the same chromosome
- physically close together ie same arm
- will only recombine in gametes if crossing over occurs between the two loci
What is crossing over? Leads to…
Exchange of DNA between non-sister chromatids
Leads to the recombination of alleles
Crossover events occur…
frequently and randomly along the length of a chromatid
During prophase 1 in meiosis 1
When is recombination frequency (RF) rare? Frequent?
Proportional to distance btw linked genes
Very close = recombination is rare
More distant = recombination is frequent
Recombination frequency formula
RF = (number of recombinant progeny/total number of progeny) x 100%
Steps for detecting recombinants and determining if genes are linked
- True breeding diploid parents that produce a one gamete genotype dihybrid
- Testcross the F1 hybrid
- Determine the number of parental and recombinant individuals in the progeny
- Calculate the recombination frequency
Why do we testcross the dihybrid to find recombination frequency
Progeny genotypes/phenotypes will reflect recombination that has occurred only in the dihybrid
