Topic 10.2 – Dihybrid Crosses & Gene Linkage
Dihybrid crosses are crosses involving two separate traits
(monohybrid is crossing one trait). For example:
G = green
g = yellow
R = round
r = wrinkled
GgRr x GgRr
Heterozygous Green
Heterozygous Round
X
Heterozygous Green
Heterozygous Round
GGRR GGRr GGrr GgRr Ggrr ggRR ggRr ggrr
10.2.1
Dihybrid Cross
GgRr x GgRr
GR
Gr
gR
gr
GR
GGRR
GGRr
GgRR
GgRr
Gr
GGRr
GGrr
GgRr
Ggrr
gR
GgRR
GgRr
ggRR
ggRr
gr
GgRr
Ggrr
ggRr
ggrr
10.2.1
GR
Gr
gR
gr
GR
GGRR
GGRr
GgRR
GgRr
Gr
GGRr
GGrr
GgRr
Ggrr
gR
GgRR
GgRr
ggRR
ggRr
gr
GgRr
Ggrr
ggRr
ggrr
Phenotypic Ratio
9 Green/Round : 3 Green/Wrinkled :
3 Yellow/Round : 3 Yellow/Wrinkled
10.2.1
10.2.1
Humans have 23 chromosome pairs. Sex chromosomes are
those that determine gender. Autosomes are all other
chromosomes.
10.2.2
Crossing over during meiosis allows for alleles to be
exchanged. This allows for the allele from either
chromosome pair to be passed to any given gamete.
Recall that allele pairs are exchanged independently.
10.2.3
10.2.3
The law of independent assortment does not apply to all
situations. Linked genes are located on the same
chromosome and so are usually passed on together.
A linkage group consists of genes on a chromosome that are
passed on together. This applies to autosomes as well as the
sex chromosomes. For genetics problems, assume that genes
are not linked unless stated.
10.2.4
Practice Problem
In Drosphila (fruit flies) the allele for grey color is
dominant over black. Straight wings are dominant
over curly wings.
A heterozygous grey-straight winged fly was crossed
with a black-curly winged fly.
a. If these genes are not linked, which phenotypes
would you expect in F1?
b. Give the expected numbers of the phenotypes.
10.2.4
GgSs x ggss
GS
Gs
gS
gs
gs GgSs
Ggss
ggSs
ggss
gs GgSs
Ggss
ggSs
ggss
gs GgSs
Ggss
ggSs
ggss
gs GgSs
Ggss
ggSs
ggss
a. Grey/Straight Grey/Curly
b. 1:1:1:1
Black/Straight Black/Curly
10.2.1
In dihybrid crosses, alleles are usually shown side by side like
in the example below:
GgLl x GgLl
G = grey
g = black
L = long
l = short
paternal
chromosome
maternal
chromosome
G L
G
L
g
l
g l
X
G
L
g
l
G l
g L
recombinant chromosomes
10.2.5
GgLl x GgLl
G
L
g
GL
l
G
Gl
l
gL
g
L
gl
GL GGLL GGLl GgLL GgLl
Gl GGLl
GGll
GgLl
Ggll
gL GgLL
GgLl
ggLL
ggLl
gl GgLl
Ggll
ggLl
ggll
10.2.5
By comparing the genotype of the offspring to that of the
parent, you can see which are a product of recombination
parent
chromosome
G
L
g
l
X
G
L
g
l
offspring
without
recombination
G
L
g
l
G
L
g
l
g
l
G
L
offspring
with
recombination
G
l
g
l
G
l
g
L
G
L
g
L
G
l
g
L
10.2.6
10.2.1
10.2.2
10.2.3
10.2.4
10.2.5
10.2.6
Calculate and predict the genotypic and phenotypic
ratio of offspring of dihybrid crosses involving unlinked
autosomal genes.
Distinguish between autosomes and sex chromosomes.
Explain how crossing over between non-sister
chromatids of a homologous pair in prophase I can
result in an exchange of alleles.
Define linkage group.
Explain an example of a cross between two linked
genes.
Identify which of the offspring are recombinants in a
dihybrid cross involving linked genes.
10.2