Linkage
 Alters the inheritance pattern.
Linkage
 Alters the inheritance pattern.
 Two genes on the same chromosome
D
E
D
E
d
e
**Crossing over unlikely to occur**
d
e
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
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
Linkage
 Linkage and crossing over have no effect on the
inheritance pattern if the genotypes are homozygous.
Linkage
 Linkage and crossing over have no effect on the
inheritance pattern if the genotypes are homozygous.
 Eg – is both parents are DDEE then all offspring will
be DDEE
Linkage
 Linkage and crossing over do affect the inheritance
pattern if the genotypes are heterozygous (Eg – DdEe)
Linkage
 Linkage and crossing over do affect the inheritance
pattern if the genotypes are heterozygous (Eg – DdEe)
**See picture on board**
Linkage
 Linkage and crossing over do affect the inheritance
pattern if the genotypes are heterozygous (Eg – DdEe)
**See picture on board**
 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.
Linkage
 Linkage and crossing over do affect the inheritance
pattern if the genotypes are heterozygous (Eg – DdEe)
**See picture on board**
 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
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.
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
ty
ttyy
Linkage
 See handout.
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:
Phenotype
Number of flies
Normal wings, grey body
106
Normal wings, ebony body
32
Short wings, ebony body
114
Short wings, grey body
23
 Discuss the type of inheritance pattern shown by these
results.