Gene Linkage In Budgies

So you are ready to explore gene linkage, but lets check a couple of things first. Before reading this page you should have read of the Budgie Genetics page and have a good grasp of basic genetics. Once you have achieved that, read on!

What is Gene Linkage?

As you know genes are found on chromosomes, like beads along a string. While we have been exploring the different types of inheritance we have assumed that all the genes are passed on independently of each other, which is exactly what happens when the genes in question are on different chromosomes. However, if we are talking about two genes that are on the same chromosome, then every time that chromosome is passed on it will take both the genes together! These genes are said to be linked - hence gene linkage.

How does this effect things? Firstly, it changes the results of some matings so that you get more of some colours, and less of others, than the punnett squares predict. Secondly, it can make it difficult to get certain varieties present together in a single bird (eg Cinnamon and Ino to create Lacewing). We will look at the first issue first and the second issue... second!

The first example we will use is the gene for 'dark factor' and the gene for body colour.

Dark Factor and Body Colour

It turns out that the dark gene (which determines what shade the body colour will be) and the body colour gene (blue or green) are on the same chromosome. They are a good example of gene linkage. Many years ago breeders found something odd happening with matings of a dark green/blue budgie and a skyblue budgie.

Lets take a look at this mating:
(this list of genetic symbols may help if they do not make sense to you.)

- dark green/blue = D/d Bl/bl1
- skyblue = d/d bl1/bl1

Expected results:
- 25% light green/blue
- 25% dark green/blue
- 25% skyblue
- 25% cobalt

However, many breeders found that the actual results from this mating were more like:

- 36% dark green/blue
- 14% light green/blue
- 36% skyblue
- 14% cobalt

So in reality they were more than twice as likely to get dark green than light green, and skyblue rather than cobalt. How does gene linkage explain this?

Firstly, we are going to add to the way we write the genotypes. For unlinked genes you place each gene on its own. So for light green we would write Bl/Bl, d/d. But if the genes are linked then we would write it as; Bl-d/Bl-d, to show that the green gene and the dark gene are on the same chromosome (linked) and passed on together.

Dark Green/Blue As a Special Case

Now we need to look at how the dark green/blue is bred. If we do a mating between an olive and a skyblue we get the following results:

- Olive = Bl-D/Bl-D x skyblue = bl1-d/bl1-d

= 100% dark green/blue - Bl-D/bl1-d

So with this mating the dark gene is linked to the gene for green body colour, so if this bird is then mated to a skyblue we get the following:

- dark green/blue - Bl-D/bl1-d x skyblue = bl1-d/bl1-d

= 50% dark green/blue - Bl-D/bl1-d
= 50% skyblue = bl1-d/bl1-d

If you breed a mauve with a light green however you get:

- light green = Bl-d/Bl-d
- mauve = bl1-D/bl1-D

= 100% dark Green/blue - Bl-d/bl1-D

The dark gene is linked to the blue gene rather than the green gene this time, so when mated to a skyblue you get:

- dark green/blue - Bl-d/bl1-D x skyblue = bl1-d/bl1-d

= 50% Light Green/Blue - Bl-d/bl1-d
= 50% Cobalt = bl1-D/bl1-d

So this time you are getting light greens and cobalts from the exact same colour parents!

The difference is caused by which body colour gene is linked to the dark gene, which is determined by its parents. This gene linkage is recorded by labelling dark green/blues as either Type 1 or Type 2. Which is which I hear you ask? If a bird has both dominant linked genes (D and Bl in this case) on one chromosome and both the recessive genes on the other (Bl-D/bl-d) then it is called Type 1. If it has one dominant and one recessive on each chromosome (Bl-d/bl-D) then it is Type 2.

So, whenever you are breeding from a dark green/blue bird you really need to look at its breeding to determine if it is Type 1 or 2, if you are after specific colours. If you can't work out which type it is then a look at the chicks colours should tell you. If you do not care which colours you get then it is not really relevant!

Now, the above information on gene linkage explains why you get more of one colour than another, but it does not explain why you still get small percentages of the other colours. From my explanations so far it would seem to be impossible, which is where an event called 'crossing over' comes into it. So hang in there a bit longer and all will be revealed. If you have plans to breed Lacewings then this information is exactly what you have been looking for...

Crossing Over

Chromosomes can do some very odd things. One of these is called crossing over, or recombination. Each chromosome is made from many units joined together rather than being a single object. Sometimes the two halves of a pair of chromosomes get tangled up with each other. When this happens a break can occur where they are tangled and the two pieces can reconnect to the opposite chromosome. This means the chromosome now has some genes that it started with and some that were previously on the other member of the pair!

These very simplified images may help visualise this process. The first is a pair for chromosomes lined up beside each other, note the different coloured gene near the bottom of the right hand one. The next image is the chromosomes lying 'tangled' over each other. And the last is after they have broken and recombined with the coloured gene now on the left chromosome.

So, that light blue coloured gene near the bottom has switched from one chromosome to the other. Crossing over happens fairly commonly, but is only noticable when it effects something physical, like colour, that we can count or measure. The breaks and rejoining occur randomly along the chromosome so it is statistically more likely to happen between genes that are far apart than between those that are close together. You can actually tell how far apart two linked genes are by recording how often a cross over happens between them.

Back to Our Dark Green/Blue Example.

Our original mating was using a dark green/blue type 1 (meaning the dominant genes are on one chromosome, and the recessive genes on the other). Which we mated with a Skyblue to give:

- dark green/blue = Bl-D/bl1-d x skyblue = bl1-d/bl1-d

In theory this mating only produces two types of chromosomes to pass on:
- Bl-D
- bl1-d

Giving only:
= dark green/blue - Bl-D/bl1-d
= skyblue = bl1-d/bl1-d

However, a cross over event on the dark green parents chromosome also gives a small number of:
- Bl-d
- bl1-D

Which means we can now get a few light green and cobalt chicks:

= light green/blue - Bl-d/bl1-d
= cobalt = bl1-D/bl1-d

Now the dark gene and the body colour gene are far enough away for the cross over events to recombine them about 14% of the time, which is quite frequent, but still makes a difference if you are specifically using this mating to produce skyblues or light greens for example.

Make sense? Feel free to read it again, and follow along using a few punnett squares as you read! Then read on to how this is event creates lacewings.

Gene Linkage and Sex Linked Varieties

A little bit of thought will reveal that all the genes for sex linked varieties are linked, as they are all found on the X chromosome. This has relevance to the combination of ino and cinnamon, which creates a variety called lacewing. I often see people asking how to get a lacewing; the answer is to buy one or to breed one. It is cheaper and easier to buy one, but in some places they are very hard to find, so it may be preferable to breed one.

As a lacewing is a cinnamon-ino the obvious starting point is that you need birds with cinnamon and ino genes. Lets say you are starting from scratch and have a cinnamon hen and an ino cock.

- cinnamon hen (Xcin-Ino/Y) x ino cock (XCin-ino/XCin-ino)
(cin = cinnamon, Cin = non-cinnamon, ino = ino, Ino = non-ino)

A punnett square shows the possible chicks are:
- Xcin-Ino/XCin-ino = normal/cinnamon ino type 2 cocks
- XCin-ino/Y = ino hens

So know we have both cinnamon and ino genes in the cock chicks, however they are on separate chromosomes. This means that he should only ever pass one or the other onto his chicks, not both.

So if we mate him with an ino hen (cinnamon would work the same way):

- normal/cin ino type 2 (Xcin-Ino/XCin-ino) x ino hen (XCin-ino/Y)

- Xcin-Ino/XCin-ino = normal/cinnamon ino type 2 cocks
- XCin-ino/ XCino-ino = ino cocks
- Xcin-Ino/Y = cinnamon hens
- XCin-ino/Y = ino hens

So again we do not get chicks with both genes showing. However, as we now know about crossing over we can add some more possibilities. If a cross over event occurs between the ino and cinnamon genes there is also the potential of getting cinnamon and ino on a single chromosome. This would then be passed on to create:

- Xcin-ino/XCin-ino = ino/cinnamon cocks
- Xcin-ino/Y = cinnamon ino hens = lacewings!

The main difficulty with this is that the cinnamon and ino genes are quite close together, so you only get a cross over event between them about 3% of the time. This means you may have to breed quite a few before you get one unless you are very lucky. This also means you need to think responsibly and be sure you have a way to rehome all the extra chicks you may breed.

A cross over event is also necessary to create cinnamon opalines, but because opaline and cinnamon genes are quite spread out on the X chromosome it is fairly common to have a cross over event bring them together and create the combination.

Whew! We reached the end... I know that is a lot to digest, take your time, practice with a few punnett squares and let it settle for a bit.

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