10.2 Dihybrid Crosses and Gene Linkage

1. 10.2 Dihybrid Crosses and Gene Linkage Learning Targets: Calculate and predict offspring of dihybrid crosses. Define linkage group and identify recombinant offspring.

2. Gregor Mendel (1822-1884) Austrian monk who bred pea plants and studied inheritance He started with “true-breeding” plants (when self-fertilized, all offspring will have same traits = homozygous), and followed traits through three generations

3. P1 generation – parental F1 generation – first filial (kids) F2 generation – second filial (grandkids)

4. Mendel’s Hypothesis: Different alleles (versions of genes) produce different traits (*although Mendel didn’t actually know about genes) Each organism inherits two alleles for every trait; one from each parent

5. Mendel’s Hypothesis: If two alleles are different, the dominant allele will be expressed, the recessive allele will have no noticeable effect (*Dominance is NOT linked to frequency in a population) Alleles (on chromosomes) separate/segregate during meiosis – each parent only passes on one copy for each trait (This is Mendel’s “Law of Segregation”)

6. Homozygous – two identical alleles: TT, ttHeterozygous – two different alleles: Tt

7. Homozygous – two identical alleles: Tt, ttHeterozygous – two different alleles: Tt

8. Testcross – in order to determine the genotype of a dominant phenotype, cross a homozyogous recessive with the unknown genotype

9. Ex) If P = purple, and p = white, what are the possible genotypes of a purple flower? Testcross: If the offspring phenotype ratio is 1 purple : 1 white, what is the genotype of the purple flower? • Prove it with a Punnett square:

10. Genetics Problems Steps Make a key of dominant and recessive alleles and assign letters Determine parent genotypes and record Determine alleles for possible gametes Solve (often using a Punnett square) and report offspring genotypes and phenotypes.

11. Punnett Squares: Show all possible combinations of alleles that result from a genetic cross Follow probability rules Possible gametes go around outside of square Offspring genotypes are filled in inside squares Parent genotypes: TT x tt

12. Punnett Squares: Show all possible combinations of alleles that result from a genetic cross Follow probability rules Possible gametes go around outside of square Offspring genotypes are filled in inside squares Parent genotypes: TT x tt T T t t

13. Punnett Squares: Show all possible combinations of alleles that result from a genetic cross Follow probability rules Possible gametes go around outside of square Offspring genotypes are filled in inside squares Parent genotypes: TT x tt T T t t

14. Genotypes of offspring: All Tt Phenotypes of offspring: All Tall Parent genotypes: TT x tt T T t t

15. Mendel hypothesized that if two traits are followed, they will assort independently and offspring could get any possible combination (This is Mendel’s “Law of Independent Assortment”)

16. This is supported with a dihybrid cross: Ex) Two parents are heterozygous for hairy fingers (hairy fingers, H, is dominant) and widow’s peak (widow’s peak, W, is dominant) Parent genotypes: ________ x _______

17. Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: ___, ___, ___, ___ (Hint: FOIL)

18. HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

19. HW Hw hW hw HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

20. HW Hw hW hw HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

21. HW Hw hW hw HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

22. Genotypes of offspring: Phenotypes of offspring: HW Hw hW hw HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

23. Genotypes of offspring: 1HHWW: 2HHWw: HW Hw hW hw HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

24. Genotypes of offspring: 1HHWW: 2HHWw: 1HHww: 2HhWW: 4HhWw: 2Hhww: 1hhWW: 2hhWw: 1hhww HW Hw hW hw HW Hw hW hw Parent genotypes: HhWw x HhWw Possible gametes parents can pass on: HW, Hw, hW, hw

25. Genotypes of offspring: 1HHWW: 2HHWw: 1HHww: 2HhWW: 4HhWw: 2Hhww: 1hhWW: 2hhWw: 1hhww Phenotypes of offspring: 9 hairy fingers, widow’s peak: HW Hw hW hw HW Hw hW hw

26. Genotypes of offspring: 1HHWW: 2HHWw: 1HHww: 2HhWW: 4HhWw: 2Hhww: 1hhWW: 2hhWw: 1hhww Phenotypes of offspring: 9 hairy fingers, widow’s peak: 3 hairy fingers, no widow’s peak: 3 hairless fingers, widow’s peak: 1 hairless fingers, no widow’s peak HW Hw hW hw HW Hw hW hw

27. Now we understand, however, that while chromosomes assort independently, genes may be linked if they are on the same chromosome.

28. Linkage group – any two genes on the same chromosome. These are often passed on together in gametes and do not show predictable (Mendelian) ratios. The recombinant chromatids show new combinations. Circle the recombinants. Record the IB notation below: ABABAbaBab ab

29. Linked gene example: If the genes for traits X and Y are linked, identify the recombinants if a heterozygote is crossed with a homozygous recessive. Parent genotypes: XY x xy xy xy Possible gametes: XxYy  XY, Xy, xY, xy xyxy  xy Recombinant offspring: Xy and xY xy xy