Example 4: Recombination intermezzo

What kind of gametes will this darkgreen/blue type II produce? Since both genes lie on the same chromosome, you would expect from the explenation on meiosis that both genes stay linked:
darkgreen/blue type II = bl⁺ D/bl D⁺ produces the gametes:

bl⁺ D

bl D⁺

But the explenation of meiosis wasn't accurate enough in order not to complicate things from the beginning. During meiosis recombination can occur.

What is this recombination or crossing-over as it is often called among budgerigar breeders?
It all has to do with meiosis. When recombination occurs, meiosis starts of as usual (only one chromosome pair is drawn for simplicity), the chromosomes are doubled:

In the next picture only the chromosomes are drawn. You can see that two arms of the two homologous (but not identical!) chromosomes come into close contact and finaly exchange a piece of themselves. Recombination during meiosis

This representation is a simplification of a very complex process. After this recombination meiosis just continues Meiosis continues

We see that recombination results in the exchange of material between homologues chromosomes. In our example of the darkgreen/blue type II bird, the following gametes are also formed:

bl⁺ D⁺

bl D

The chance that recombination splits two genes and recombines them is called the recombination frequency. For the bl and D this recombination frequency is about 15%. Notice that a recombination frequency can never be over 50%. A recombination frequency of 50% indicates that the genes are not linked and are therefore not on the same chromosome. This results in the following frequencies of gametes for a dargreen/blue type II:

85%	42.5%	*bl⁺ D*
85%	42.5%	*bl D⁺*
15%	7.5%	*bl⁺ D⁺*
15%	7.5%	*bl D*

This recombination frequency is an estimate obtained from real pairings where one observed recombination. I'm working on a part about LOD-scores where this estimation is explained extensivly.

Recombination, just a fluke of nature?
It is such a very complex and therefore highly regulated process. The smallest error during the exchage can lead to very serious defects in the chromosomes and the zygote. Recombination is not an accident, but a highly regulated process which allows more variation. Variation is the engine of evolution and selection (Darwin already knew this). Scientist believe that during each meiosis almost all chromosome pairs go though recombination. Furthermore, they have reason to believe that mostly recombination only occurs once for each pair of homologous strands. The place where recombination takes place determins if two genes are seperated and reunited with the alternetive allele of the other gene. The further away two genes are on a chromosome, the higher the chance of recombination, and hence the higher the recombination frequency will be (I'm working on a part about LOD-scores & genetic maps that will go deeper into it)

Let's put an end to this theory and let us continue with pairings here!