Genetics%20Terminology%20Illustrated:%20Genetic%20Crossover

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Genetics Terminology IllustratedGenetic
Crossover

• A Presentation for The Angelfish Society
• By Tamar Stephens
• For the October 19, 2008 TAS Meeting

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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.

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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.

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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
D
g
These two alleles are identical.
These two alleles are different.
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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.

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Crossover occurs before the chromosome pair
separates.
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.
1. Chromosomes are paired up before meiosis.
2. Crossover occurs when the two chromosomes swap
segments.
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How does crossover occur?

• The exact mechanism isnt 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.

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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.

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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.

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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
y
Chromosome A
Chromosome B
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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.
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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
X
X
x
X
x
z
z
Z
Z
z
Z
Chromosome A
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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.

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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.

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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
x
X
X
Z
z
Z
z
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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.

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Crossover black angelfish

• This pair of gold marble ghosts (Gm/g-S/)
  produced three black fry in one spawn.

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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.

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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.

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Disclaimer

• I dont 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.

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Thats all folks.

• I hope you enjoyed the presentation.
• Now back to the chat room for discussion!