Genetics Terminology Illustrated: Genetic Crossover

1. Genetics Terminology Illustrated:Genetic Crossover A Presentation for The Angelfish Society By Tamar Stephens For the October 19, 2008 TAS Meeting

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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”

11. 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.

12. 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”

13. 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.

14. 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.

15. 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

16. 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.

17. “Crossover” black angelfish • This pair of gold marble ghosts (Gm/g-S/+) produced three black fry in one spawn.

18. 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.

19. 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.

20. 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.

21. That’s all folks. I hope you enjoyed the presentation. Now back to the chat room for discussion!