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Genetics Terminology Illustrated:
Genetic Crossover
A Presentation for The Angelfish Society
By Tamar Stephens
For the October 19, 2008 TAS Meeting
Chromosomal Crossover
• Chromosomal crossover
is an exchange of
matching segments of
two paired
chromosomes, as shown
in this figure.
• This presentation will
talk about when and
how this process occurs,
and why it is important.
A gene is a section of code on a
chromosome
• As you know,
chromosomes occur in
pairs.
• A gene is a section of a
chromosome that codes for
a specific trait or function.
• The pink sections on the
figure represent a given
gene on each chromosome.
Alleles
• Two paired genes are called
“alleles.”
• The alleles might be identical,
meaning they code for the
same expression of the trait.
• For example, both could code
for black (D) color.
• The alleles might code for
different forms of the trait.
Black (D) and gold (g) code
for different color expression.
D
D
These two
alleles are
identical.
D
g
These two
alleles are
different.
When does crossover occur?
• Crossover occurs during a
process called “meiosis.”
• Meiosis is when eggs or sperm
are formed.
• The cells that will become eggs
or sperm will divide into two
cells (called “gametes”). Each
chromosome pair will split up,
with one chromosome going to
one gamete and the other to the
other.
• In this figure, no crossover has
occurred.
Crossover occurs before the
chromosome pair separates.
1. Chromosomes
are paired up
before meiosis.
2. Crossover occurs
when the two
chromosomes swap
segments.
3. The pair of chromosomes
separates. When the cell
divides to form two egg or
sperm cells, each new cell gets
one of the chromosomes.
How does crossover occur?
• The exact mechanism isn’t fully known, but in
simplistic terms, here is what happens.
• Remember that chromosomes are really very long
strands of coiled up DNA.
• Paired chromosomes spend most of their time
twisted around each other.
• When one chromosome “nicks” the other one, a
break may occur, and the two chromosomes may
swap segments as the break is restored.
Why is crossover important?
• Chromosomal crossover is important in
maintaining genetic diversity.
• The next slides will explain this. First we
need to explain the concept of “independent
assortment.” Then we will see how
crossover contributes to independent
assortment.
Independent assortment
• Independent assortment means that when
two or more traits are inherited, each is
inherited randomly. One trait does not
affect how another one is inherited.
• Independent assortment holds true for many,
many traits.
If two traits are on separate
chromosomes…
• When each chromosome pair separates, it is
completely random which one of each pair will end
up in a given gamete (sperm or egg). So four types
of gametes can be made from these two
chromosomes. (next page)
X
x
Y
Chromosome “A”
y
Chromosome “B”
Four possible combinations in
gametes (egg or sperm)
2
3
1
4
Each sperm or egg has a single copy of each
chromosomes until the egg is fertilized and
receives matching chromosomes from the
sperm cell to form chromosome pairs again.
If two traits are on the same
chromosome…
• When two genes are on the same chromosome, they still
independently assort because of chromosomal crossover.
• Two genes on one chromosome pair can produce four kinds
of gametes.
2
1
3
4
X
Z
x
X
z
Z
Chromosome “A”
x
z
X
x
z
Z
Chromosomal crossover occurs
frequently
• Chromosomal
crossover may
occur two or more
times between two
chromosomes in a
pair.
• Here is a diagram
representing a
chromosome pair
with two
crossovers.
This gives wide genetic diversity
• Chromosomal crossover occurs so
often that usually two genes on a give
chromosome will independently assort,
as if they were on separate
chromosomes.
• An angelfish receives one chromosome
from each parent. But when a female
produces eggs (or a male produces
sperm) chromosomal crossover mixes
and matches this genetic material.
• When you multiply the possibilities by
many genes and multiple
chromosomes, you can see almost
endless ways to recombine genes.
Crossover and linkage
• When two genes are close to each
other on a chromosome, they will
be separated by a crossover less
often.
• Scientists can calculate how close
genes are to each other by looking
at offspring counts.
• If two gene are close enough, then
the chance of having a crossover
between them is very rare. In this
case we say the two genes are
“linked” because they almost
always occur together.
X
Z
x
z
X
Z
x
z
So why do we care about
chromosome crossovers?
• First of all, in our endless quest for knowledge,
this is interesting information.
• Second – most of you have heard of a cross
between two gold marbles producing an
occasional black offspring. Some of you have
probably seen this happen in one or more of your
spawns. You may have heard it called a
“crossover” black.
“Crossover” black angelfish
• This pair of
gold marble
ghosts
(Gm/g-S/+)
produced
three black
fry in one
spawn.
Gold marble may be the result of a
crossover
• There is a hypothesis that gold marble arose as a
result of a genetic crossover that modified black.
• One hypothesis is that black and gold are actually
on separate genes that are located very close to
each other. A crossover occurred that resulted in
both a black and a gold gene occurring on the
same chromosome.
• This hypothesis further says that the interaction
between them results in the patchy appearance of
black, giving the gold marble effect. Since they
are linked, they tend to act like one gene.
Black “crossovers”
• Enough people have found one or more black
fry in a spawn from gold marble parents that
we know it occurs.
• If the gold marble hypothesis is true the two
linked genes are very rarely separated.
• Occasionally a “reverse” crossover will occur
between these linked genes. When that
occurs, a black offspring may result from this
reverse crossover.
Disclaimer
• I don’t know whether the hypothesis about the
origin of gold marbles is true.
• Many of us have observed the occasional black fry
in a spawn from gold marble parents, and a
chromosomal crossover is the most likely
explanation.
• I have not seen any convincing explanations of the
mechanism behind this, though.
• If anyone knows more on this subject, please share
that information with the rest of TAS. This would
make a good discussion topic for the TAS forum.
That’s all folks.
I hope you enjoyed the presentation.
Now back to the chat room for discussion!