Today Linkage & Recombination

1. TodayLinkage & Recombination

2. Effects of Recombination in a Test Cross P: AABB x aabb F1: AaBb Test Cross: AaBb x aabb Non-recombinants phenotypes: A-B- and aabb Recombinant phenotypes: A-bb and aaB-

3. Rate of recombination The further apart two genes are on a chromosome, the more likely recombination will separate them r = proportion of recombinant offspring in testcross r ranges from 0 (no recombination) to 0.5 (half recombinants).

4. Rate of recombination with a 2-point test cross P AABB x aabb ABab ABab F1 AaBb AB ab Test Cross: AaBb x aabb Non-recombinant gametes: ab, AB Recombinant gametes: Ab, aB Each type of gametes will correspond to a different phenotype in the progeny

6. Examples of how r is estimated from test cross

7. Genetic Mapping with Recombination • Genes that are far apart on a chromosome are more likely to be recombined • Rate of recombination increases with distance • If crossing-over occurs on one sister chromatid, it doesn’t occur on the other • Rate of recombination is never above 0.5 • 1 map unit = 1% recombination • map distance = r x 100

9. A Mapping Example • Three gene loci: E, F and G • Order of genes on chromosome unknown • Observed rates of recombination are: r(E/F) = 0.05, map distance = 5 r(E/G) = 0.10, map distance = 10 r(F/G) = 0.15, map distance = 15 Step 1: The greatest map distance must be between ends of map F and G are the ends Step 2: Put the third locus between them, spaced according to map distances F –– E ––––– G 5 10

10. Double Cross-Over

12. Double Cross-Overs • Double cross-overs have lower probabilities than single cross-overs • Pr(double)  Pr(single 1) x Pr(single 2) • Example Pr(double) = 0.2 x 0.1 = 0.02

13. Three-Point Mapping • Both single and double cross-overs used • All the loci that are being mapped must be heterozygous in one parent • Each possible gamete genotype must correspond to a distinct phenotype in the progeny • A large number of progeny must be counted to obtain accurate estimates

14. Crosses in which Gamete Genotypes Produce Distinct Phenotypes Test cross: ABC x abc abc x abc Hemizygous males: (X) ABC x (X)abc (X) abc x (Y)

15. Interpreting 3-Point Crosses • Nonrecombinant gametes are the most common • reveals the combination of alleles on the parental chromosome • Double recombinants are the least common • reveals which of the three genes is “in the middle” • Both single and double cross-overs count as cross-overs between particular pairs of genes • Map distances calculated by counting all crossovers between pairs of gene

16. Example from Textbook • Three gene loci with recessive mutant alleles in Drosophila: b Black body pr purple eyes c curved wings • Wild type alleles indicated by + • P generation: • one fly b b pr pr c c (all homozygous recessive mutants) • other fly b+ b+ pr+ pr+ c+ c+ (all homozygous dominant wild-type)

17. F1 progeny are heterozygous for all 3 loci

18. Test cross with homozygous recessive for all three loci In progeny of testcross: Non-recombinants: either all dominant or all recessive traits Recombinants: mixture of dominant and recessive traits

19. Reciprocal Classes • Progeny classes are reciprocal if between them they contain each mutant phenotype just once. Examples: • all wild type and black, purple, curved • purple, curved and black • black and purple, curved

20. Reciprocal classes are products of the same type of meiotic event Non-recombinants all wild type black, purple, curved Crossover between b and pr loci purple, curved and black

21. Progeny of testcross

22. Non-recombinants are always the most common classes

23. Double crossovers are always the rarest

24. Double crossovers tell you which gene is in the middle b + c + pr + 

25. Single crossovers

26. Crossovers between b and pr include both single crossovers and double crossovers 5.0 % + 0.9% = 5.9% total

27. Crossovers between pr and c include both single crossovers and double crossovers 18.6 % + 0.9% = 19.5% total

28. Map based on three-point cross vs. two point cross