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Dihybrid Crosses & Gene Linkage
Stephen Taylor
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
1
Mendel’s Law of Independent Assortment
“Can you remember it?”
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10.2 Dihybrid Crosses & Gene Linkage
2
Mendel’s Law of Independent Assortment
“The presence of an allele of one of the
genes in a gamete has no influence over
which allele of another gene is present.”
This only holds true for unlinked genes
(genes on different chromosomes).
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10.2 Dihybrid Crosses & Gene Linkage
3
Mendel’s Law of Independent Assortment
“The presence of an allele of one of the
genes in a gamete has no influence over
which allele of another gene is present.”
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
This only holds true for unlinked genes
(genes on different chromosomes).
SY
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meiosis
sY
Sy
sy
10.2 Dihybrid Crosses & Gene Linkage
4
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
What is the predicted phenotype ratio for a cross between
two pea plants which are heterozygous at both loci?
F0
Y = yellow
y = green
S = smooth
s = rough
Phenotype:
Heterozygous at both loci
Heterozygous at both loci
Genotype:
Punnet Grid:
gametes
F1
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10.2 Dihybrid Crosses & Gene Linkage
5
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
What is the predicted phenotype ratio for a cross between
two pea plants which are heterozygous at both loci?
F0
Phenotype:
Genotype:
Punnet Grid:
Smooth, yellow
Smooth, yellow
Heterozygous at both loci
Heterozygous at both loci
SsYy
SsYy
gametes
F1
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
6
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
What is the predicted phenotype ratio for a cross between
two pea plants which are heterozygous at both loci?
F0
Phenotype:
Genotype:
Punnet Grid:
Smooth, yellow
Smooth, yellow
Heterozygous at both loci
Heterozygous at both loci
SsYy
SsYy
gametes
SY
Sy
sY
sy
SY
SSYY
SSYy
SsYY
SsYy
Sy
SSYy
SSyy
SsYy
Ssyy
sY
SsYY
SsYy
ssYY
ssYy
sy
SsYy
Ssyy
ssYy
ssyy
F1
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
7
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
What is the predicted phenotype ratio for a cross between
two pea plants which are heterozygous at both loci?
F0
Phenotype:
Genotype:
Punnet Grid:
F1
Smooth, yellow
Smooth, yellow
Heterozygous at both loci
Heterozygous at both loci
SsYy
SsYy
gametes
SY
Sy
sY
sy
SY
SSYY
SSYy
SsYY
SsYy
Sy
SSYy
SSyy
SsYy
Ssyy
sY
SsYY
SsYy
ssYY
ssYy
sy
SsYy
Ssyy
ssYy
ssyy
Phenotypes: 9 Smooth, yellow : 3 Smooth, green : 3 Rough, yellow : 1 Rough, green
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
8
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Calculate the predicted phenotype ratio for:
F0
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Phenotype:
Heterozygous at both loci
Heterozygous for S, homozygous dominant for Y
Genotype:
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
9
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Calculate the predicted phenotype ratio for:
F0
Phenotype:
Smooth, yellow
Heterozygous at both loci
Genotype:
SsYy
Smooth, yellow
Heterozygous for S, homozygous dominant for Y
SsYY
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
10
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Calculate the predicted phenotype ratio for:
F0
Phenotype:
Smooth, yellow
Smooth, yellow
Heterozygous for S, homozygous dominant for Y
Heterozygous at both loci
Genotype:
Punnet Grid:
SsYY
SsYy
gametes
SY
sY
SY
Sy
sY
sy
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
11
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Calculate the predicted phenotype ratio for:
F0
Phenotype:
Smooth, yellow
Heterozygous for S, homozygous dominant for Y
Heterozygous at both loci
Genotype:
Punnet Grid:
F1
Smooth, yellow
SsYY
SsYy
gametes
SY
sY
SY
SSYY
SsYY
Sy
SSYy
SsYy
sY
SsYY
ssYY
sy
SsYy
ssYy
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
12
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Calculate the predicted phenotype ratio for:
F0
Phenotype:
Smooth, yellow
Smooth, yellow
Heterozygous for S, homozygous dominant for Y
Heterozygous at both loci
Genotype:
Punnet Grid:
SsYY
SsYy
gametes
SY
sY
SY
SSYY
SsYY
Sy
SSYy
SsYy
sY
SsYY
ssYY
sy
SsYy
ssYy
6 Smooth, yellow : 2 Rough, yellow
F1
Phenotypes: 3 Smooth, yellow : 1 Rough, yellow
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Present the ratio in the simplest
mathematical form.
10.2 Dihybrid Crosses & Gene Linkage
13
Dihybrid Crosses
Common expected ratios of dihybrid crosses.
SsYy
SsYy
SsYy
Heterozygous at
both loci
Heterozygous at
both loci
Heterozygous at
both loci
SsYy
Heterozygous at one locus,
homozygous dominant at the other
SY
sY
SY
SSYY
SsYY
Ssyy
Sy
SSYy
SsYy
ssYY
ssYy
sY
SsYY
ssYY
ssYy
ssyy
sy
SsYy
ssYy
SY
Sy
sY
sy
SY
SSYY
SSYy
SsYY
SsYy
Sy
SSYy
SSyy
SsYy
sY
SsYY
SsYy
sy
SsYy
Ssyy
3:2
9:3:3:1
Ssyy
SsYy
Heterozygous at
both loci
Heterozygous/
Homozygous recessive
Sy
sy
SY
SSYy
SsYy
Sy
SSyy
Ssyy
sY
SsYy
ssYy
sy
Ssyy
ssyy
SSyy
ssYY
= All SsYy
SSYY
ssyy
= all SyYy
Ssyy
ssYy
=1:1:1:1
4:3:1
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10.2 Dihybrid Crosses & Gene Linkage
14
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
F0
Phenotype:
Rough, yellow
Genotype:
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
15
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
F0
Phenotype:
Rough, yellow
ssYy
Genotype:
Punnet Grid:
F1
gametes
sY
sy
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
16
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
F0
Phenotype:
Rough, yellow
ssYy or ssYY
Genotype:
Punnet Grid:
F1
gametes
sY
sy
sY
sY
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
17
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
F0
Phenotype:
Rough, yellow
Punnet Grid:
ssYy or ssYY
ssyy
Genotype:
gametes
sY
sy
sY
sY
All sy
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
18
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
F0
Phenotype:
Rough, yellow
Punnet Grid:
F1
ssYy or ssYY
ssyy
Genotype:
gametes
sY
sy
sY
sY
All sy
ssYy
ssyy
ssYy
ssYy
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
19
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
F0
Phenotype:
Rough, yellow
Punnet Grid:
F1
ssYy or ssYY
ssyy
Genotype:
gametes
sY
sy
sY
sY
All sy
ssYy
ssyy
ssYy
ssYy
Phenotypes:
Some green peas will be present in
the offspring if the unknown parent
genotype is ssYy.
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No green peas will be present in the
offspring if the unknown parent
genotype is ssYY.
10.2 Dihybrid Crosses & Gene Linkage
20
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A smooth green pea is test crossed. Deduce the genotype.
Smooth green = nine offspring. Rough green = one offspring.
F0
Phenotype:
Smooth, green
Genotype:
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
21
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A smooth green pea is test crossed. Deduce the genotype.
Smooth green = nine offspring. Rough green = one offspring.
F0
Phenotype:
Smooth, green
ssyy
Genotype:
Punnet Grid:
gametes
All sy
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
22
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A smooth green pea is test crossed. Deduce the genotype.
Smooth green = nine offspring. Rough green = one offspring.
F0
Phenotype:
Smooth, green
Punnet Grid:
F1
SSyy
ssyy
Genotype:
gametes
Sy
Sy
All sy
Ssyy
Ssyy
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
23
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A smooth green pea is test crossed. Deduce the genotype.
Smooth green = nine offspring. Rough green = one offspring.
F0
Phenotype:
Smooth, green
Punnet Grid:
F1
SSyy or Ssyy
ssyy
Genotype:
gametes
Sy
Sy
Sy
sy
All sy
Ssyy
Ssyy
Ssyy
ssyy
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
24
Dihybrid Crosses
Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A smooth green pea is test crossed. Deduce the genotype.
Smooth green = nine offspring. Rough green = one offspring.
F0
Phenotype:
Smooth, green
Punnet Grid:
F1
SSyy or Ssyy
ssyy
Genotype:
gametes
Sy
Sy
Sy
sy
All sy
Ssyy
Ssyy
Ssyy
ssyy
Phenotypes:
No rough peas will be present in the
offspring if the unknown parent
genotype is SSyy.
The presence of rough green peas in the
offspring means that the unknown
genotype must be Ssyy.
The expected ratio in this cross is 3 smooth green : 1 rough green. This is not the same as the outcome.
Remember that each reproduction event is chance and the sample size is very small. With a much larger sample
size, the outcome would be closer to the expected ratio, simply due to probability.
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10.2 Dihybrid Crosses & Gene Linkage
25
Sooty the Guinea Pig
Key to alleles*:
C = colour
A = agouti
c = albino
a = black
R = round ears
L = long whiskers
r = pointy ears
l = short whiskers
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
Sooty news story from the BBC:
http://news.bbc.co.uk/2/hi/uk_news/wales/1048327.stm
* C and A genes are real. The rest are made up for this story.
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10.2 Dihybrid Crosses & Gene Linkage
26
Sooty the Guinea Pig
Sooty has soft fur and sharp nails.
In one of his matings with a rough-furred, smooth-nailed
female, the following guinea piglets are produced:
6 x rough fur, sharp nails; 3 x soft fur sharp nails.
F0
Phenotype:
Rough fur, smooth nails
Key to alleles:
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
Deduce Sooty’s genotype.
Soft fur, sharp nails
Genotype:
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
27
Sooty the Guinea Pig
Sooty has soft fur and sharp nails.
In one of his matings with a rough-furred, smooth-nailed
female, the following guinea piglets are produced:
6 x rough fur, sharp nails; 3 x soft fur sharp nails.
F0
Phenotype:
Rough fur, smooth nails
Genotype:
Punnet Grid:
Key to alleles:
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
Deduce Sooty’s genotype.
Soft fur, sharp nails
ssnn
Possible
Gametes
All sn
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
28
Key to alleles:
Sooty the Guinea Pig
Sooty has soft fur and sharp nails.
In one of his matings with a rough-furred, smooth-nailed
female, the following guinea piglets are produced:
6 x rough fur, sharp nails; 3 x soft fur sharp nails.
F0
Phenotype:
Rough fur, smooth nails
Genotype:
Punnet Grid:
Deduce Sooty’s genotype.
Soft fur, sharp nails
ssnn
Possible
Gametes
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
SSNN or SsNN
SN
Sn
sN
or SsNn
sn
All sn
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
29
Key to alleles:
Sooty the Guinea Pig
Sooty has soft fur and sharp nails.
In one of his matings with a rough-furred, smooth-nailed
female, the following guinea piglets are produced:
6 x rough fur, sharp nails; 3 x soft fur sharp nails.
F0
Phenotype:
F1
Deduce Sooty’s genotype.
Rough fur, smooth nails
Genotype:
Punnet Grid:
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
Soft fur, sharp nails
ssnn
SSNN or SsNN
or SsNn
Possible
Gametes
SN
Sn
sN
sn
All sn
SsNn
Ssnn
ssNn
ssnn
Soft fur
Smooth nails
Rough fur
Sharp nails
Phenotypes:
http://sciencevideos.wordpress.com
Soft fur
Sharp nails
Rough fur
Smooth nails
10.2 Dihybrid Crosses & Gene Linkage
30
Key to alleles:
Sooty the Guinea Pig
Sooty has soft fur and sharp nails.
In one of his matings with a rough-furred, smooth-nailed
female, the following guinea piglets are produced:
6 x rough fur, sharp nails; 3 x soft fur sharp nails.
F0
Phenotype:
F1
Deduce Sooty’s genotype.
Rough fur, smooth nails
Genotype:
Punnet Grid:
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
Soft fur, sharp nails
ssnn
SSNN or SsNN
or SsNn
Possible
Gametes
SN
Sn
sN
sn
All sn
SsNn
Ssnn
ssNn
ssnn
Soft fur
Smooth nails
Rough fur
Sharp nails
Phenotypes:
Soft fur
Sharp nails
Rough fur
Smooth nails
Only these two phenotypes have been produced.
Sooty has only produced SN and sN gametes.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
31
Key to alleles:
Sooty the Guinea Pig
Sooty has soft fur and sharp nails.
In one of his matings with a rough-furred, smooth-nailed
female, the following guinea piglets are produced:
6 x rough fur, sharp nails; 3 x soft fur sharp nails.
F0
Phenotype:
F1
Deduce Sooty’s genotype.
Rough fur, smooth nails
Genotype:
Punnet Grid:
S = soft fur
s = rough fur
N = sharp nails n = smooth nails
Soft fur, sharp nails
ssnn
SSNN or SsNN
or SsNn
Possible
Gametes
SN
Sn
sN
sn
All sn
SsNn
Ssnn
ssNn
ssnn
Soft fur
Smooth nails
Rough fur
Sharp nails
Phenotypes:
Soft fur
Sharp nails
Rough fur
Smooth nails
Only these two phenotypes have been produced.
Sooty has only produced SN and sN gametes.
It is most likely that his genotype is SsNN.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
32
Sooty the Guinea Pig
Deduce Sooty’s genotype.
Offspring = five with pointy ears and long whiskers
F0
Phenotype:
Pointy ears, short whiskers
Key to alleles:
R = round ears
r = pointy ears
L = long whiskers l = short whiskers
Pointy ears, long whiskers
Genotype:
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
33
Key to alleles:
Sooty the Guinea Pig
Deduce Sooty’s genotype.
Offspring = five with pointy ears and long whiskers
F0
Phenotype:
Pointy ears, short whiskers
Genotype:
Punnet Grid:
R = round ears
r = pointy ears
L = long whiskers l = short whiskers
Pointy ears, long whiskers
rrll
Possible
Gametes
rrLL or
rL
rrLl
rl
All rl
F1
Phenotypes:
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
34
Key to alleles:
Sooty the Guinea Pig
Deduce Sooty’s genotype.
Offspring = five with pointy ears and long whiskers
F0
Phenotype:
Pointy ears, short whiskers
Genotype:
Punnet Grid:
F1
Phenotypes:
http://sciencevideos.wordpress.com
R = round ears
r = pointy ears
L = long whiskers l = short whiskers
Pointy ears, long whiskers
rrll
rrLL or
Possible
Gametes
rL
rl
All rl
rrLl
rrll
Pointy ears
Long whiskers
rrLl
Pointy ears
Short whiskers
10.2 Dihybrid Crosses & Gene Linkage
35
Key to alleles:
Sooty the Guinea Pig
Deduce Sooty’s genotype.
Offspring = five with pointy ears and long whiskers
F0
Phenotype:
Pointy ears, short whiskers
Genotype:
Punnet Grid:
F1
Phenotypes:
R = round ears
r = pointy ears
L = long whiskers l = short whiskers
Pointy ears, long whiskers
rrll
rrLL or
Possible
Gametes
rL
rl
All rl
rrLl
rrll
Pointy ears
Long whiskers
rrLl
Pointy ears
Short whiskers
Only this phenotype has been produced.
Sooty has only produced rL gametes.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
36
Key to alleles:
Sooty the Guinea Pig
Deduce Sooty’s genotype.
Offspring = five with pointy ears and long whiskers
F0
Phenotype:
Pointy ears, short whiskers
Genotype:
Punnet Grid:
F1
Phenotypes:
R = round ears
r = pointy ears
L = long whiskers l = short whiskers
Pointy ears, long whiskers
rrll
rrLL or
Possible
Gametes
rL
rl
All rl
rrLl
rrll
Pointy ears
Long whiskers
rrLl
Pointy ears
Short whiskers
Only this phenotype has been produced.
Sooty has only produced rL gametes.
It is most likely that his genotype is rrLL.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
37
Gene Interaction
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The expression of one gene is dependent
upon the prior expression of another.
10.2 Dihybrid Crosses & Gene Linkage
38
Gene Interaction
The expression of one gene is dependent
upon the prior expression of another.
In the case of guinea pigs, there is
gene interaction for fur colour.
Key to alleles:
C = colour
A = agouti
c = albino
a = black
The first gene, C, determines whether colour is present.
The second gene, A, is only expressed if C is first expressed.
It determines which colour will be produced.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
39
Gene Interaction
The expression of one gene is dependent
upon the prior expression of another.
In the case of guinea pigs, there is
gene interaction for fur colour.
Key to alleles:
C = colour
A = agouti
c = albino
a = black
The first gene, C, determines whether colour is present.
The second gene, A, is only expressed if C is first expressed.
It determines which colour will be produced.
Genotypes
ccAA
ccAa
ccaa
CCAA
CcAa
CCaa
Ccaa
If the genotype ‘cc’ is present, there
will be no expression of colour.
A will also not be expresssed.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
40
Gene Interaction
The expression of one gene is dependent
upon the prior expression of another.
In the case of guinea pigs, there is
gene interaction for fur colour.
Key to alleles:
C = colour
A = agouti
c = albino
a = black
The first gene, C, determines whether colour is present.
The second gene, A, is only expressed if C is first expressed.
It determines which colour will be produced.
Phenotype ratios do not fit the normal 9 : 3 : 3 : 1 ratio.
Genotypes
ccAA
ccAa
ccaa
CCAA
CcAa
CCaa
Ccaa
If the genotype ‘cc’ is present, there
will be no expression of colour.
A will also not be expresssed.
http://sciencevideos.wordpress.com
gametes
CA
Ca
cA
ca
CA
CCAA
CCAa
CcAA
CcAa
Ca
CCAa
Ccaa
CcAa
Ccaa
cA
CcAA
CcAa
ccAA
ccAa
ca
CcAa
Ccaa
ccAa
ccaa
9 agouti : 3 black : 4 albino
10.2 Dihybrid Crosses & Gene Linkage
41
Autosomes and Sex Chromosomes
Humans have 23 pairs of chromosomes in
diploid somatic cells (n=2).
22 pairs of these are autosomes, which are
homologous pairs.
One pair is the sex chromosomes.
XX gives the female gender, XY gives male.
Karyotype of a human male, showing X and Y chromosomes:
http://en.wikipedia.org/wiki/Karyotype
SRY
The X chromosome is much larger than the Y.
X carries many genes in the non-homologous
region which are not present on Y.
The presence and expression of the SRY
gene on Y leads to male development.
Chromosome images from Wikipedia:
http://en.wikipedia.org/wiki/Y_chromosome
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
42
Autosomal Gene Linkage vs Sex-Linked Disorders
Sex-linked disorders are carried
on the non-homologous regions
of the X chromosome.
SCN5A
(voltage-gated sodium channel)
Linked genes are pairs or
groups of genes which are
inherited together, carried
on the same chromosome.
A
B
a
b
Locus 1
Locus 2
PDCD10
Alleles are expressed whether
they are dominant or recessive,
as there is no alternate allele
carried on the Y chromosome.
Gene-related disorders which are
sex-linked include red-green
colour blindness and hemophilia.
Males are more frequently
affected by sex-linked disorders.
Y
X
(programmed cell death)
SOX2
(transcription factor - promoter region)
Chromosome 3 from:
http://en.wikipedia.org/wiki/Chromosome_3_%28human%29
http://sciencevideos.wordpress.com
There are about 2000 genes on X and 86 on Y.
Gene linkage is therefore also common on X and Y.
10.2 Dihybrid Crosses & Gene Linkage
43
Autosomal Gene Linkage
http://sciencevideos.wordpress.com
Linked genes are pairs or groups of genes which are
inherited together, carried on the same chromosome.
10.2 Dihybrid Crosses & Gene Linkage
44
Autosomal Gene Linkage
Linked genes are pairs or groups of genes which are
inherited together, carried on the same chromosome.
SCN5A
(voltage-gated sodium channel)
The SCN5A, PDCD10 and SOX2 genes
are all linked by being on chromosome 3.
They are a linkage group, and alleles of
each will therefore be inherited together.
Independent assortment does not occur
between linked genes.
PDCD10
(programmed cell death)
SOX2
(transcription factor - promoter region)
Chromosome 3 from:
http://en.wikipedia.org/wiki/Chromosome_3_%28human%29
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
45
Autosomal Gene Linkage
Linked genes are pairs or groups of genes which are
inherited together, carried on the same chromosome.
Standard notation for linked genes:
A
B
“heterozygous at
both loci”
a
b
SCN5A
(voltage-gated sodium channel)
Locus 1
The SCN5A, PDCD10 and SOX2 genes
are all linked by being on chromosome 3.
Locus 2
The line denotes the
chromosome, or the
fact that the two genes
are linked.
They are a linkage group, and alleles of
each will therefore be inherited together.
Independent assortment does not occur
between linked genes.
Syllabus examples of Linkage Groups:
PDCD10
(programmed cell death)
SOX2
Sweet peas (Lathyrus odoratus):
flower colour
(P/p)
linked with pollen grain shape (L/l)
(transcription factor - promoter region)
Chromosome 3 from:
http://en.wikipedia.org/wiki/Chromosome_3_%28human%29
http://sciencevideos.wordpress.com
Corn (Zea mays):
Kernel colour
(C/c)
linked with Waxiness of kernels (W/w)
10.2 Dihybrid Crosses & Gene Linkage
46
Linkage Groups
Are carried on the same chromosome and are inherited together.
They do not assort independently.
In sweet peas (Lathyrus odoratus), the genes for flower colour
and pollen grain shape are carried on the same chromosome.
Plants which are heterozygous at both loci are test-crossed.
What ratio of phenotypes is expected?
Key to alleles:
P = purple p = white
L = long
l = short
Genotype:
Phenotype:
Image: 'Sweet Pea'
http://www.flickr.com/photos/69166981@N00/3600419425
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
47
Linkage Groups
Are carried on the same chromosome and are inherited together.
They do not assort independently.
In sweet peas (Lathyrus odoratus), the genes for flower colour
and pollen grain shape are carried on the same chromosome.
Plants which are heterozygous at both loci are test-crossed.
What ratio of phenotypes is expected?
p
l
p
l
Locus 1
Locus 2
Key to alleles:
P = purple p = white
L = long
l = short
Genotype:
Phenotype:
White; Short
Image: 'Sweet Pea'
http://www.flickr.com/photos/69166981@N00/3600419425
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
48
Linkage Groups
Are carried on the same chromosome and are inherited together.
They do not assort independently.
In sweet peas (Lathyrus odoratus), the genes for flower colour
and pollen grain shape are carried on the same chromosome.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
What ratio of phenotypes is expected?
p
l
P
L
p
l
p
l
Locus 1
Locus 2
Locus 1
Locus 2
Genotype:
Phenotype:
White; Short
Purple; Long
Image: 'Sweet Pea'
http://www.flickr.com/photos/69166981@N00/3600419425
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
49
Linkage Groups
Are carried on the same chromosome and are inherited together.
They do not assort independently.
In sweet peas (Lathyrus odoratus), the genes for flower colour
and pollen grain shape are carried on the same chromosome.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
What ratio of phenotypes is expected?
p
l
P
L
p
l
p
l
Locus 1
Locus 2
Locus 1
Locus 2
Genotype:
Phenotype:
White; Short
Punnet Grid:
Possible
Gametes
Purple; Long
PL
pl
All pl
Phenotypes:
Ratio:
Image: 'Sweet Pea'
http://www.flickr.com/photos/69166981@N00/3600419425
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
50
Linkage Groups
Are carried on the same chromosome and are inherited together.
They do not assort independently.
In sweet peas (Lathyrus odoratus), the genes for flower colour
and pollen grain shape are carried on the same chromosome.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
What ratio of phenotypes is expected?
p
l
P
L
p
l
p
l
Locus 1
Locus 2
Locus 1
Locus 2
Genotype:
Purple; Long
Phenotype:
White; Short
Punnet Grid:
Possible
Gametes
PL
pl
All pl
PpLl
ppll
Purple; Long
White; Short
Phenotypes:
Ratio:
1 :
1
Image: 'Sweet Pea'
http://www.flickr.com/photos/69166981@N00/3600419425
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
51
Linkage Groups
Are carried on the same chromosome and are inherited together.
They do not assort independently.
In sweet peas (Lathyrus odoratus), the genes for flower colour
and pollen grain shape are carried on the same chromosome.
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals have
appeared in the offspring. Explain what has happened.
Key to alleles:
P = purple p = white
L = long
l = short
Image: 'Sweet Pea'
http://www.flickr.com/photos/69166981@N00/3600419425
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
52
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
Key to alleles:
P = purple p = white
L = long
l = short
Diploid cell
Heterozygous at both loci
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
53
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
The test cross individual is homozygous
recessive at both loci, so only one type of
gamete is produced.
Possible gametes:
Test individual:
p
l
Heterozygous individual:
Diploid cell
Heterozygous at both loci
Chromosomes replicate
in Synthesis phase
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
54
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
The test cross individual is homozygous
recessive at both loci, so only one type of
gamete is produced.
Alleles segregate in meiosis,
giving two possible gametes:
P
p
L
Possible gametes:
Test individual:
p
l
Heterozygous individual:
P
L
p
l
l
Diploid cell
Heterozygous at both loci
Chromosomes replicate
in Synthesis phase
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
55
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
Possible gametes:
Test individual:
p
l
Heterozygous individual:
Diploid cell
Heterozygous at both loci
Chromosomes replicate
in Synthesis phase
P
L
p
l
Crossing Over
Prophase I
Alleles are exchanged
Crossing-over occurs occasionally. It is more likely to happen
between linked genes which are further apart.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
56
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
Possible gametes:
Test individual:
p
l
Heterozygous individual:
Diploid cell
Heterozygous at both loci
Chromosomes replicate
in Synthesis phase
L
p
l
Recombinants:
Crossing Over
Sister chromatids are
Prophase I
separated in anaphase II.
Alleles are exchanged Recombined gametes are
produced.
Crossing-over occurs occasionally. It is more likely to happen
between linked genes which are further apart.
http://sciencevideos.wordpress.com
P
P
l
p
L
10.2 Dihybrid Crosses & Gene Linkage
57
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
Normal gametes
(majority)
Possible gametes:
Test individual:
p
Possible
Gametes
P L
p l
l
Heterozygous individual:
All p l
P
L
p
l
Recombinants:
P
l
p
L
Crossing-over occurs occasionally. It is more likely to happen
between linked genes which are further apart.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
58
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
Normal gametes
(majority)
Possible gametes:
Test individual:
p
l
Possible
Gametes
P L
p l
All p l
PpLl
ppll
P
L
Purple; long
White, short
p
l
Heterozygous individual:
Recombinants:
P
l
p
L
Crossing-over occurs occasionally. It is more likely to happen
between linked genes which are further apart.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
59
Recombination
of alleles occurs as a result of crossing-over between non-sister
chromatids. Exchange of alleles gives new genotypes of gametes.
Key to alleles:
P = purple p = white
L = long
l = short
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals
have appeared in the offspring. Explain what has happened.
Normal gametes
(majority)
Recombinant gametes
(small number)
Possible gametes:
Test individual:
p
l
Possible
Gametes
P L
p l
P l
p L
All p l
PpLl
ppll
Ppll
ppLl
P
L
Purple; long
White, short
Purple; short
White, long
p
l
Heterozygous individual:
Recombinants:
P
l
p
L
Crossing-over occurs occasionally. It is more likely to happen
between linked genes which are further apart.
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
60
Crossing-Over
Synapsis
Homologous chromosomes
associate
http://sciencevideos.wordpress.com
Increases genetic variation through
recombination of linked alleles.
Chiasma Formation
Neighbouring non-sister chromatids
are cut at the same point.
A Holliday junction forms as the DNA
of the cut sections attach to the
open end of the opposite non-sister
chromatid.
Recombination
As a result, alleles are
swapped between nonsister chromatids.
10.2 Dihybrid Crosses & Gene Linkage
61
Crossing-Over
http://sciencevideos.wordpress.com
Increases genetic variation through
recombination of linked alleles.
10.2 Dihybrid Crosses & Gene Linkage
62
Gene Linkage & Recombination
SCN5A
The further apart a pair of alleles are on a
chromosome, the more likely it is that crossing over
may occur between them - leading to recombination.
(voltage-gated sodium channel)
Crossing-over is more
likely to occur
between SCN5A and
PDCD10 than
between PDCD10
and SOX2.
Knowing this, researchers can map the position of
genes on a chromosome based on the frequency of
recombination between gene pairs: the further apart
they are, the more often they cross over.
PDCD10
(programmed cell death)
SOX2
(transcription factor - promoter region)
Chromosome 3 from:
http://en.wikipedia.org/wiki/Chromosome_3_%28human%29
http://sciencevideos.wordpress.com
Animation and quiz from:
http://www.csuchico.edu/~jbell/Biol207/animations/recombination.html
10.2 Dihybrid Crosses & Gene Linkage
63
Gene Linkage & Recombination
Which description best fits this image?
a.
b.
c.
d.
Four chromosomes, four chiasmata
Four chromatids, two chiasmata, two centromeres
Two chromosomes, four chiasmata
A pair of sister chromatids
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
64
Gene Linkage & Recombination
Which description best fits this image?
a.
b.
c.
d.
Four chromosomes, four chiasmata
Four chromatids, two chiasmata, two centromeres
Two chromosomes, four chiasmata
A pair of sister chromatids
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
65
Gene Linkage & Recombination
Which description best fits this image?
chiasmata
Chromosome 1a
Sister chromatids
Chromosome 1b
Sister chromatids
centromeres
a.
b.
c.
d.
Four chromosomes, four chiasmata
Four chromatids, two chiasmata, two centromeres
Two chromosomes, four chiasmata
A pair of sister chromatids
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
66
Gene Linkage & Recombination
The genes for kernel colour and waxiness are linked in the corn
plant (Zea mays). In a cross between a plant that is homozygous
dominant at both loci with a plant that is heterozygous at both loci,
identify the following genotypes as:
a: regular
b:recombinants
c: impossible
CcWw
CCWw
http://sciencevideos.wordpress.com
CcWW
CCWW
CCww
ccWW
10.2 Dihybrid Crosses & Gene Linkage
67
Gene Linkage & Recombination
The genes for kernel colour and waxiness are linked in the corn
plant (Zea mays). In a cross between a plant that is homozygous
dominant at both loci with a plant that is heterozygous at both loci,
identify the following genotypes as:
a: regular
b:recombinants
c: impossible
CcWw
C
C
CCWw
CcWW
CCWW
Key to alleles:
C = coloured c = no colour
W = waxy
w = not waxy
CCww
ccWW
W
W
Regular gametes
(majority)
Recombinant gametes
(small number)
Possible
Gametes
All C W
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
68
Gene Linkage & Recombination
The genes for kernel colour and waxiness are linked in the corn
plant (Zea mays). In a cross between a plant that is homozygous
dominant at both loci with a plant that is heterozygous at both loci,
identify the following genotypes as:
a: regular
b:recombinants
c: impossible
CcWw
C
C
CCWw
W
W
CcWW
C
W
c
w
Regular gametes
(majority)
Possible
Gametes
C W
CCWW
Key to alleles:
C = coloured c = no colour
W = waxy
w = not waxy
CCww
ccWW
Recombinant gametes
(small number)
c w
All C W
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
69
Gene Linkage & Recombination
The genes for kernel colour and waxiness are linked in the corn
plant (Zea mays). In a cross between a plant that is homozygous
dominant at both loci with a plant that is heterozygous at both loci,
identify the following genotypes as:
a: regular
b:recombinants
c: impossible
CcWw
C
C
CCWw
W
W
CcWW
C
W
c
w
Regular gametes
(majority)
Possible
Gametes
C W
c w
All C W
CCWW
CcWw
http://sciencevideos.wordpress.com
CCWW
Key to alleles:
C = coloured c = no colour
W = waxy
w = not waxy
CCww
ccWW
Recombinant gametes
(small number)
10.2 Dihybrid Crosses & Gene Linkage
70
Gene Linkage & Recombination
The genes for kernel colour and waxiness are linked in the corn
plant (Zea mays). In a cross between a plant that is homozygous
dominant at both loci with a plant that is heterozygous at both loci,
identify the following genotypes as:
a: regular
b:recombinants
c: impossible
CcWw
C
C
CCWw
W
W
CcWW
CCWW
Key to alleles:
C = coloured c = no colour
W = waxy
w = not waxy
CCww
C
W
C
w
c
w
c
W
Regular gametes
(majority)
Recombinant gametes
(small number)
Possible
Gametes
C W
c w
C w
c W
All C W
CCWW
CcWw
CCWw
CcWW
http://sciencevideos.wordpress.com
ccWW
10.2 Dihybrid Crosses & Gene Linkage
71
Gene Linkage & Recombination
Two genes are linked as shown here
E
m
e
M
The genes are far apart such that crossing-over between the alleles occurs
occasionally. Which statement is true of the gametes?
A. All of the gametes will be Em and eM
B. There will be equal numbers of EM, EM, eM and em
C. There will be approximately equal numbers of EM and eM gametes
D. There will be more Em gametes than em gametes
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
72
Gene Linkage & Recombination
Two genes are linked as shown here
E
m
e
M
The genes are far apart such that crossing-over between the alleles occurs
occasionally. Which statement is true of the gametes?
A. All of the gametes will be Em and eM
B. There will be equal numbers of EM, EM, eM and em
C. There will be approximately equal numbers of EM and eM gametes
D. There will be more Em gametes than em gametes
http://sciencevideos.wordpress.com
10.2 Dihybrid Crosses & Gene Linkage
73
Gene Linkage & Recombination
Two genes are linked as shown here
E
m
e
M
The genes are far apart such that crossing-over between the alleles occurs
occasionally. Which statement is true of the gametes?
A. All of the gametes will be Em and eM
B. There will be equal numbers of EM, EM, eM and em
C. There will be approximately equal numbers of EM and eM gametes
D. There will be more Em gametes than em gametes
E
m
E
M
e
M
e
m
Regular gametes
(majority)
E m
http://sciencevideos.wordpress.com
e M
Recombinant gametes
(small number)
E M
e m
10.2 Dihybrid Crosses & Gene Linkage
74
For more IB Biology resources:
http://sciencevideos.wordpress.com
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10.2 Dihybrid Crosses & Gene Linkage
75