1. Extra Credit Question
Crossing over in Eukaryotes occurs during when there are _____ chromatids present at the metaphase plate in Meiosis I, but the crossover event involes _____ chromatids.

2. Announcements Homework #2 is due on Thursday before 9 am.
Midterm: Mar 8, 9:00 am.

3. Genetic Linkage and Mapping

6. Notation A Two homologous Aa chromosomes, Diploid
four chromatids total Aa
Diploid
Adult
—•—————
—•————— a
Haploid gametes
(single chromatid)
__•_______ A
__•_______ A
1/2 A : 1/2 a
__•_______ a
__•_______ a

7. Dihybrid Cross AA BB Female aa bb Male A B a b
—•————— ——•————— —•————— ——•—————
P: —•————— ——•————— —•————— ——•—————
A B a b
Gametes: Egg (A B) Sperm (a b)
A B a b
—•————— ——•———— —•————— ——•————

8. F1 adult after the S phase A B —•————— ——•————— F1: —•————— ——•—————
—•————— ——•—————
F1: —•————— ——•—————
a b

9. 4 gamete types, equally frequent
A B
—•————— ——•—————
F1: —•————— ——•—————
a b
A B
—•————— ——•————
A b
a B
a b
Parental
Recombinant
4 gamete types, equally frequent

10. F2 of dihybrid cross when genes not linked
What proportion of the gametes of the double heterozygote are recombinant?
50 %

11. But what if the genes are on the same chromosome?
(What if the genes are linked?)
They do not assort independently at meiosis

12. Two linked genes Female Male A B a b Gametes: Egg (A B) Sperm (a b)
—•————————— —•—————————
P: —•————————— —•—————————
Gametes: Egg (A B) Sperm (a b)
A B a b
—•————————— —•————————

13. No crossing over between markers
A B
—•—————————
F1: —•—————————
a b
A B
—•—————————
a b
Parental
No crossing over between markers
A B
—•—————————
a b
Parental

14. When there is no crossing over between markers, all gametes are parental

15. Crossing over in 4-strand stage
A B
—•—————————
F1: —•—————————
a b
A B
—•—————————
a b
Parental
Crossing over in 4-strand stage
A b
—•—————————
a B
Recombinant

16. When there is a crossover, 50% of gametes are parental and 50% are recombinant

17. No crossover Crossover
A B
—•—————————
a b
F1: —•—————————
A b
a B
Parental
Recom.

18. When genes are on the same chromosome, but far apart:
Crossing over between them will be common, approaching 100% of the time
In this case, what proportion of all F1 gametes will be recombinant (have a combination of alleles that was not present in the original true-breeding parents)?

19. Crossing over in 4-strand stage
A B
—•—————————
F1: —•—————————
a b
A B
—•—————————
a b
Parental
Crossing over in 4-strand stage
A b
—•—————————
a B
Recombinant

20. Genes that are so far apart on the same chromosome that crossing over occurs between them in 100% of meioses behave as if they are unlinked: 50% parental combinations 50% recombinants

21. What if genes are close together?
Say that crossing over occurs between the genes in 20% of meioses
What proportion of F1 gametes will be recombinant?

22. 80% 20% A B A B A B a b A B a b A B a b A b a B —•—————————
F1: —•—————————
a b
80%
A B
—•—————————
F1: —•—————————
a b
20%
A B
—•—————————
a b
A B
—•—————————
a b
A B
—•—————————
a b
A b
—•—————————
a B

23. If crossing over between genes occurs in 20% of meiosis
10% of gametes will be recombinant!

24. Genes with < 50% recombination are on the same chromosome
Genes that have 50% recombination (independent assortment) are either on non-homologous chromosomes or are far apart on the same chromosome

25. This is the basis of genetic mapping
The frequency of crossing over between two genes on the same chromosome is proportional to the distance between the genes
This is the basis of genetic mapping

26. At small distances crossing over is rare and few gametes are recombinant (r is near 0%)
As distance between genes increases, crossover frequency increases, and more gametes are recombinant (0<r<50%)
At greater distances, crossover nearly always occurs and r --> 50%

27. How would you tell if a gamete produced by a double heterozygote is recombinant or parental?
How would you determine the genotypes of gametes produced by a doubly-heterozygous individual?

28. Mate double heterozygote to an individual that is homozygous recessive for both traits

29. Two linked genes Female Male bw hv+ bw+ hv
—•————————— —•—————————
P: —•————————— —•—————————
Gametes: Egg (A B) Sperm (a b)
bw hv bw hv
—•———————— —•————————

30. X Par Rec bw hv+ bw hv bw hv+ bw+ hv bw hv bw hv bw+ hv+ —•—————————
F1: —•—————————
bw+ hv
bw hv
—•————————— X
bw hv+
—•—————————
bw+ hv
bw hv
—•—————————
Par
Rec
bw hv
—•—————————
bw+ hv+

31. If 15% of the offspring of the testcross are recombinant, what proportion of meioses had crossover between the brown and the heavy-veined genes?

32. If 15% of the gametes are recombinant, the genes are said to be 15 map units apart on a genetic map
bw hv
15 map units

33. Unit of distance in linkage map is a map unit: 1 map unit is equal to 1 percent recombination
A map unit is also equivalent to the physical distance along a chromosome which will experience 1 crossover event in every 50 meiotic divisions

35. Two X-linked genes Female Male w m w+ m+
—•————————— —•—————————
P: —•————————— —•————/
w m
Gametes: Egg (A B) Sperm (a b)
w m w m+
—•———————— —•————————
—•———/

36. X Par Rec w+ m+ w m w+ m+ w m w m w+ m w m+ —•—————————
F1: —•—————————
w m
w m
—•—————————
—•————/ X
w+ m+
—•—————————
w m
w m
—•—————————
Par
—•————/
Rec
w+ m
—•—————————
w m+

37. 62.8% non-recombinant offspring, 37.2% recombinant offspring
How far apart (in map units) are the genes?
37.2 map units

38. What would you conclude about these two genes?
If two genes are 50 map units apart, what proportion of recombinant offspring would you observe?
What would you conclude about these two genes?

40. Multiple crossovers can lead to inaccuracy
—•—————————
a c B b

41. Multiple crossovers can be useful for mapping three genes at a time!
A B C
—•—————————
a b c
A B C
a b c
A B C
—•—————————
a b c
A b C
a B c