DEALING WITH DRAGONS- GENE LINKAGE
Up until now, you have dealt with either a single gene with two alleles (a monohybrid cross)
or two genes on separate chromosomes, each with two alleles (a dihybrid cross).
First, let’s review what happens during meiosis in a cell with two chromosomes. On the
next page draw the chromosomes as they would appear in each stage of meiosis using the
cell below as a starting guide. Label all genes/alleles at each stage.
From your diagram, what are the four possible gametes that can be produced by an AaBb
heterozygote? What is the probability or ratio of each gamete occuring?
Possible
Gametes
AB
Ab
aB
ab
Probability
1 (or 25% etc)
1
1
1
Let’s assume that we are looking at the genotype of a rare East
Melbourne Dragon. These shy creatures live discarded supermarket
trollies and eat mostly rubbish. A heterozygous dragon had acid saliva
(used to digest old cans), represented by gene A, and blue wings
(represented by gene B). It mated with a recessive dragon with non-acid
saliva and red wings. Assume both genes are on different chromosomes.
Complete the punnet square below and state what the possible
genotypes and phenotypes are, along with their frequency.
Parent Genotypes: _________AaBb_____________x _________aabb_________________
ab
AB
Ab
aB
ab
1
1
1
1
Offspring Genotypes:
1 AaBb: 1 Aabb: 1 aaBb: 1 aabb
Offspring Phenotypes:
1 acid saliva, blue wings: 1 acid saliva, red wings: 1 non-acid saliva, blue wings: 1 non-acid
saliva, red wings
a
A
B
A
A a
b
a
B
B b
b
A
A
First Division
OR
A
a
A
a
b
B
B
b
a
b
a
b
B
B
Second
Division
A
a
B
A
B
b
b
a
A
a
a
A
b
B
b
B
So far, we have assumed that the chromosomes
line up neatly in metaphase and each
chromosome is pulled to one or the other side of
the dividing cell with no problems. In real life
however, things can be a bit more complicated.
As you can see from the photo to the left,
chromosomes are like long thin threads. These
threads can become tangled and break resulting
in an exchange in genetic material between sister
chromosomes.
Paired maize chromosomes during the first
metaphase of meiosis
From http://ls.berkeley.edu/node/313
Here’s what happens. When the chromosomes line up during metaphase 1, sister
chromatids can sometimes overlap or cross over. When the chromosomes try and separate
the two pieces of DNA break and the cell’s emergency repair systems join pieces back
together again. However, they don’t always join the same chromatid back to its original
chromosome, sometimes it joins to the sister chromosome and vice versa. This process is
called recombination.
Sister chromosomes
lined up during
metaphase
Chromatids cross over
The chromosome DNA
breaks and is rejoined by
the cellular machinery
resulting in an exchange in
genetic material
Recombination means that a much great variety of variation is possible. Not only will the
combination of chromosomes vary between gametes, genetic material can vary along each
chromosome. It is because of this siblings do not appear the same even though they have
the same parents.
Let’s do the meiosis diagram again, but this time with two genes on the same chromosome.
Fill in the chromosomes for meiosis (you may find using two colours helps). Remember to
label your genes and alleles for ALL chromatids at each step.
If recombination occurs between A
and B
If no recombination occurs
A
B
A
B
a
b
a
A
b
B
First division
a A
Bb
a
b
First division
A
A
B
B
a
b
Second division
a
b
Second division
A
a
A
a
B
b
B
b
A
a
a
A
B
b
B
b
If no recombination occurs, what are the possible gametes that can be formed?
AB and ab
If recombination does occur, what are the possible gametes that can be formed?
AB, ab, aB and Ab
The gametes that have formed due to a recombination event are known as recombinant
gametes (ie Ab and aB). The with chromosomes that are unchanged from the original
parental cell are known as parental gametes (ie AB and ab).
As you can see, when cross over occurs we get the same possible gamete combinations that
we saw when we had two genes on separate chromosomes. But wait it’s a bit more
complicated than that.
Whether or not recombination occurs between two genes will depend on how close the two
genes are on the chromosome. If they are far apart, the odds of a recombination event
happening will be high. When they are close together the odds are small. If two genes are
close together on a chromosome they are said to be linked. This means that because the
chance of recombination occurring is low, we see more of the two parental (original)
gametes than we do of the recombinant gametes. The way we determine if two genes are
linked or not is to look at the offspring and see if the gametes occur in a even 1:1:1:1
mendelian ratio or are lots: lots: little: little where the two ‘lots’ are the two parental
genotypes and the ‘littles’ are the recombinant genotypes.
Let’s look at our cross from page one again:
A heterozygous dragon had acid saliva (used to digest old cans), represented by gene
A, and blue wings (represented by gene B). It mated with a recessive dragon with
non-acid saliva and red wings. The numbers of resultant offspring were:
17 acid saliva, blue wings
19 acid saliva, red wings
15 non-acid saliva, blue wings
18 non-acid saliva, red wings
Are the two genes linked? Justify your answer.
No because the ratios are 1:1:1:1
Now let’s take the same cross but change the offspring numbers:
32 acid saliva, blue wings
6 acid saliva, red wings
8 non-acid saliva, blue wings
29 non-acid saliva, red wings
Are the two genes linked? Justify your answer.
Yes because the offspring produced did not segregate in a 1:1:1:1 ratio.
What were the parental gametes in this cross? How do you know?
AB and ab because these two types occurred in the highest numbers ‘lots’
What were the recombinant gametes?
aB and Ab because these two types occurred in the smallest numbers ‘littles’
For the following problems identify if the two genes are linked or unlinked. You will need
to assign appropriate alleles and write out the cross for each question. If you find that the
genes are linked, identify the parental and the recombinant gametes.
The Urban Dragon (Draconus urban) has a mottled skin colour
that allows it to blend into its environment. This skin can either
be grey (dominant) or brown (recessive). It also can have two
sizes, the recessive large phenotype (to enable it to eat rubbish
from wheelie bins) and small (to allow it to prowl storm drains).
A heterozygous swamp dragon bred with a recessive dragon.
The resulting offspring were
25 large grey
30 small brown
24 small grey
26 large brown
Alleles selected: L- small and l- large; G- grey and g- brown
Parent genotypes: LlGg x llgg
Expected F1
genotypes
lg
LG
Lg
lG
lg
LlGg
Llgg
llGg
llgg
The observed offspring ratios are approximately 1:1:1:1 and so are not linked
What if the numbers of offspring in the cross on the previous page were instead:
25 large grey
30 small brown
4 small grey
6 large brown
Alleles selected: L- small and l- large; G- grey and g- brown
Parent genotypes: LlGg x llgg
Expected F1
genotypes
lg
LG
Lg
lG
lg
LlGg
Llgg
llGg
llgg
The observed offspring ratios are NOT 1:1:1:1 and so the two genes must be linked.
The parental gametes would be lG and Lg
The recombinant gametes would be LG and lg
BONUS QUESTION
The little known swamp dragon, Draconus lustrum has an
unfortunate habit of exploding when startled. As it has been
known to be surprised by it’s own shadow, very few exist in the
wild and the only way the species survives is by producing
hundreds of offspring from a single mating. . This species can
breathe fire (dominant) or not and can also have a forked tongue
(dominant) or a pointed tongue. Two heterozygous swamp
dragons mated and the resulting offspring are listed below. State whether the
forked tongue and fire breathing abilities are linked or not, justifying your answer.
917 fire breathing, forked tongue
313 fire breathing, pointed tongue
298 non-fire breathing, forked tongue
104 non-fire breathing, pointed tongue
Alleles selected: F- Fire breathing and f- non-fire breathing; T- Forked tongue and t- pointed
tongue
Parent genotypes: FfTt x FfTt
Expected F1
genotypes
FT
Ft
fT
ft
FT
Ft
fT
ft
This question is more complex than the previous questions but the method remains the
same. The expected phenotypic ratio would be 9 fire, forked: 3 fire, pointed: 3 non-fire,
forked: 1 non-fire, pointed. This matches the ratio of numbers we see in the offspring,
therefore the genes are not linked.
LINKAGE TOP TIPS
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Work out the expected genotypic and phenotypic ratio for a cross. Compare this to
your data. If the numbers are roughly the same, the genes are not linked. If the
numbers are NOT in roughly the same ratios as your predictions the genes are linked.
If genes are linked use ‘lots, lots, little, little’ to identify which classes are the
parentals and which are the recombinants (remember- recombination between two
genes that are close together is rare!)
Genes that are not linked may be on different chromosomes or may be really far
apart from each other on the same chromosome- there is no way to know which
without further experiments. But genes that are linked MUST be on the same
chromosome and fairly close together.