Linkage & Recombination

When two genes are involved, there are two possibilities:

The genes lie on different chromosomes. (see part on Two Genes)
Linkage: the genes lie on the same chromosome, the two genes belong to the same linkage group. (this part)

What happens when genes lie on the same chromosome? From the explanation on meiosis in one would say that these genes are linked. Indeed genes on the same chromosome are linked. A chromosomes is sometimes called a linkagegroup. But this linkage is not 100%, recombination does occure. I will come back to recombination or crossing-over as it is often called among budgerigar breeders, but I will first introduce an example of linked genes.

The most obvious example of linked genes are the sex-linked traits that lie on the X-chromosome. Because the sex chromosome is a special case it will be handled in a seperatly part. I'll first address linkage on a normal chromosome. As an example I'll use the genes bl and D.

I've already introduced the bl gene. The D stands for darkfactor and the gene is located on the same chromosome as bl. The D gene is partialy dominant, the possible phenotypes are:

in green birds in blue birds
D⁺ -- D⁺ lightgreen (normal) skyblue (normal blue)

D⁺ -- D darkgreen cobalt

D -- D olivgreen mauve

An important note on notation

When both alleles of a genepair are wild-type, they can be ommited. Hence, bl/bl Sp⁺/Sp⁺ is the same as bl/bl or as skyblue.

When two genes are not linked the paires are written seperatly: bl⁺/bl Sp⁺/Sp (the spangle and blue gene are not linked).

When two genes are linked, the linked alleles are written together: bl⁺ D/bl D⁺

This last one is very important. Genetically bl⁺ D/bl D⁺ and bl⁺ D⁺/bl D are not the same, although they have the same alleles. bl⁺ D/bl D⁺ is called darkgreen/blue type II and bl⁺ D⁺/bl D is called darkgreen/blue type I.

What happens with linked genes at meiosis? How are the gametes formed?

In general: linked genes stay linked during meiosis because they travel on the same chromosome.