1. Di-hybrid Inheritance with linkage and crossing over

2. Linkage
Alters the inheritance pattern.

3. Linkage Alters the inheritance pattern.
Two genes on the same chromosome
D D d d
E E e e
**Crossing over unlikely to occur**

4. Linkage Alters the inheritance pattern.
Two genes on the same chromosome
When two linked genes are close together (like previous diagram), crossing over with recombination is unlikely to occur

5. Linkage Alters the inheritance pattern.
Two genes on the same chromosome
When two linked genes are close together, crossing over with recombination is unlikely to occur
If the two genes are far apart on the chromosome, crossing over is likely to occur.
D D d d
E E e e

6. Linkage
Linkage and crossing over have no effect on the inheritance pattern if the genotypes are homozygous.

7. Linkage
Linkage and crossing over have no effect on the inheritance pattern if the genotypes are homozygous.
Eg – if both parents are DDEE then all offspring will be DDEE

8. Linkage
Linkage and crossing over do affect the inheritance pattern if the genotypes are heterozygous (Eg – DdEe)

9. Linkage
Linkage and crossing over do affect the inheritance pattern if the genotypes are heterozygous (Eg – DdEe)

10. Linkage
Linkage and crossing over do affect the inheritance pattern if the genotypes are heterozygous (Eg – DdEe)
Crossing over occurs between the non-sister chromatids during prophase 1 in meiosis so than linked alleles D and E; d and e are crossed over to give the new combinations D and e; d and E.

11. Linkage
Linkage and crossing over do affect the inheritance pattern if the genotypes are heterozygous (Eg – DdEe)
Crossing over occurs between the non-sister chromatids during prophase 1 in meiosis so than linked alleles D and E; d and e are crossed over to give the new combinations D and e; d and E.
The parental combinations for the genotypes of these gametes are DE and de while the recombinants have the genotype De and dE

12. Linkage
The standard way to see the effect of crossing over is to compare the results of a test cross WITHOUT linkage of the genes and a test cross WITH linkage of the genes.

13. Linkage
The standard way to see the effect of crossing over is to compare the results of a test cross WITHOUT linkage of the genes and a test cross WITH linkage of the genes.
Without – cross = TtYy x ttyy ty TY
TtYy Ty
Ttyy tY
ttYy
ttyy

14. Question
In the fruit fly, normal wing (N) is dominant to short wing (n) and grey body (G) is dominant to ebony wing (g).
A test cross was set up between a heterozygous fly (NnGg) and a fly recessive for both traits (nngg).
The following phenotypes resulted:
Discuss the type of inheritance pattern shown by these results.
Phenotype
Number of flies
Normal wings, grey body
106
Normal wings, ebony body 32
Short wings, ebony body
114
Short wings, grey body 23