Genetics Terminology Illustrated:Genetic Crossover

A Presentation for The Angelfish SocietyA Presentation for The Angelfish SocietyBy Tamar StephensBy Tamar Stephens

For the October 19, 2008 TAS MeetingFor the October 19, 2008 TAS Meeting

Chromosomal Crossover

• Chromosomal crossover Chromosomal crossover is an exchange of is an exchange of matching segments of matching segments of two paired two paired chromosomes, as shown chromosomes, as shown in this figure. in this figure.

• This presentation will This presentation will talk about when and talk about when and how this process occurs, how this process occurs, and why it is important. and why it is important.

A gene is a section of code on a chromosome

• As you know, chromosomes As you know, chromosomes occur in pairs.occur in pairs.

• A gene is a section of a A gene is a section of a chromosome that codes for a chromosome that codes for a specific trait or function. specific trait or function.

• The pink sections on the The pink sections on the figure represent a given figure represent a given gene on each chromosome. gene on each chromosome.

Alleles• Two paired genes are called Two paired genes are called

“alleles.”“alleles.”• The alleles might be identical, The alleles might be identical,

meaning they code for the meaning they code for the same expression of the trait. same expression of the trait.

• For example, both could code For example, both could code for black (D) color. for black (D) color.

• The alleles might code for The alleles might code for different forms of the trait. different forms of the trait. Black (D) and gold (g) code Black (D) and gold (g) code for different color expression.for different color expression.

D D D g

These two alleles are identical.

These two alleles are different.

When does crossover occur?• Crossover occurs during a Crossover occurs during a

process called “meiosis.” process called “meiosis.” • Meiosis is when eggs or sperm Meiosis is when eggs or sperm

are formed. are formed. • The cells that will become eggs The cells that will become eggs

or sperm will divide into two or sperm will divide into two cells (called “gametes”). Each cells (called “gametes”). Each chromosome pair will split up, chromosome pair will split up, with one chromosome going to with one chromosome going to one gamete and the other to the one gamete and the other to the other. other.

• In this figure, no crossover has In this figure, no crossover has occurred. 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 The exact mechanism isn’t fully known, but in simplistic terms, here is what happens. simplistic terms, here is what happens.

• Remember that chromosomes are really very long Remember that chromosomes are really very long strands of coiled up DNA. strands of coiled up DNA.

• Paired chromosomes spend most of their time Paired chromosomes spend most of their time twisted around each other.twisted around each other.

• When one chromosome “nicks” the other one, a When one chromosome “nicks” the other one, a break may occur, and the two chromosomes may break may occur, and the two chromosomes may swap segments as the break is restored.swap segments as the break is restored.

Why is crossover important?

• Chromosomal crossover is important in Chromosomal crossover is important in maintaining genetic diversity. maintaining genetic diversity.

• The next slides will explain this. First we The next slides will explain this. First we need to explain the concept of “independent need to explain the concept of “independent assortment.” Then we will see how assortment.” Then we will see how crossover contributes to independent crossover contributes to independent assortment. assortment.

Independent assortment

• Independent assortment means that when Independent assortment means that when two or more traits are inherited, each is two or more traits are inherited, each is inherited randomly. One trait does not inherited randomly. One trait does not affect how another one is inherited. affect how another one is inherited.

• Independent assortment holds true for Independent assortment holds true for many, many traits. many, many traits.

If two traits are on separate chromosomes…

• When each chromosome pair separates, it is When each chromosome pair separates, it is completely random which one of each pair will end completely random which one of each pair will end up in a given gamete (sperm or egg). So four types up in a given gamete (sperm or egg). So four types of gametes can be made from these two of gametes can be made from these two chromosomes. (next page)chromosomes. (next page)

Chromosome “A” Chromosome “B”

X xY y

Four possible combinations in gametes (egg or sperm)

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.

1

23

4

If two traits are on the same chromosome…

• When two genes are on the same chromosome, they still When two genes are on the same chromosome, they still independently assort because of chromosomal crossover. independently assort because of chromosomal crossover.

• Two genes on one chromosome pair can produce four kinds Two genes on one chromosome pair can produce four kinds of gametes. of gametes.

Chromosome “A”

xX

Z z

X X x

Z z Z

1 2 3 4

x

z

Chromosomal crossover occurs frequently

• Chromosomal Chromosomal crossover may crossover may occur two or more occur two or more times between two times between two chromosomes in a chromosomes in a pair. pair.

• Here is a diagram Here is a diagram representing a representing a chromosome pair chromosome pair with two with two crossovers.crossovers.

This gives wide genetic diversity• Chromosomal crossover occurs so Chromosomal crossover occurs so

often that usually two genes on a give often that usually two genes on a give chromosome will independently assort, chromosome will independently assort, as if they were on separate as if they were on separate chromosomes. chromosomes.

• An angelfish receives one An angelfish receives one chromosome from each parent. But chromosome from each parent. But when a female produces eggs (or a when a female produces eggs (or a male produces sperm) chromosomal male produces sperm) chromosomal crossover mixes and matches this crossover mixes and matches this genetic material.genetic material.

• When you multiply the possibilities by When you multiply the possibilities by many genes and multiple many genes and multiple chromosomes, you can see almost chromosomes, you can see almost endless ways to recombine genes. endless ways to recombine genes.

Crossover and linkage

• When two genes are close to each When two genes are close to each other on a chromosome, they will other on a chromosome, they will be separated by a crossover less be separated by a crossover less often. often.

• Scientists can calculate how close Scientists can calculate how close genes are to each other by looking genes are to each other by looking at offspring counts.at offspring counts.

• If two gene are close enough, then If two gene are close enough, then the chance of having a crossover the chance of having a crossover between them is very rare. In this between them is very rare. In this case we say the two genes are case we say the two genes are “linked” because they almost “linked” because they almost always occur together.always occur together.

xZ zX x

Z zX

So why do we care about chromosome crossovers?

• First of all, in our endless quest for knowledge, First of all, in our endless quest for knowledge, this is interesting information.this is interesting information.

• Second – most of you have heard of a cross Second – most of you have heard of a cross between two gold marbles producing an between two gold marbles producing an occasional black offspring. Some of you have occasional black offspring. Some of you have probably seen this happen in one or more of your probably seen this happen in one or more of your spawns. You may have heard it called a spawns. You may have heard it called a “crossover” black. “crossover” black.

“Crossover” black angelfish• This pair of This pair of

gold marble gold marble ghosts ghosts (Gm/g-S/+) (Gm/g-S/+) produced produced three black three black fry in one fry in one spawn. spawn.

Gold marble may be the result of a crossover

• There is a hypothesis that gold marble arose as a There is a hypothesis that gold marble arose as a result of a genetic crossover that modified black. result of a genetic crossover that modified black.

• One hypothesis is that black and gold are actually One hypothesis is that black and gold are actually on separate genes that are located very close to on separate genes that are located very close to each other. A crossover occurred that resulted in each other. A crossover occurred that resulted in both a black and a gold gene occurring on the both a black and a gold gene occurring on the same chromosome. same chromosome.

• This hypothesis further says that the interaction This hypothesis further says that the interaction between them results in the patchy appearance of between them results in the patchy appearance of black, giving the gold marble effect. Since they black, giving the gold marble effect. Since they are linked, they tend to act like one gene.are linked, they tend to act like one gene.

Black “crossovers”

• Enough people have found one or more black Enough people have found one or more black fry in a spawn from gold marble parents that fry in a spawn from gold marble parents that we know it occurs. we know it occurs.

• If the gold marble hypothesis is true the two If the gold marble hypothesis is true the two linked genes are very rarely separated.linked genes are very rarely separated.

• Occasionally a “reverse” crossover will Occasionally a “reverse” crossover will occur between these linked genes. When that occur between these linked genes. When that occurs, a black offspring may result from this occurs, a black offspring may result from this reverse crossover.reverse crossover.

Disclaimer• I don’t know whether the hypothesis about the I don’t know whether the hypothesis about the

origin of gold marbles is true.origin of gold marbles is true.• Many of us have observed the occasional black fry Many of us have observed the occasional black fry

in a spawn from gold marble parents, and a in a spawn from gold marble parents, and a chromosomal crossover is the most likely chromosomal crossover is the most likely explanation. explanation.

• I have not seen any convincing explanations of the I have not seen any convincing explanations of the mechanism behind this, though.mechanism behind this, though.

• If anyone knows more on this subject, please share If anyone knows more on this subject, please share that information with the rest of TAS. This would that information with the rest of TAS. This would make a good discussion topic for the TAS forum.make a good discussion topic for the TAS forum.

That’s all folks.

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