Sex Chromosomes and Sex Linkage

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Sex Chromosomes, Sex Linkage and Inheritance

Lets start with a little recap from the Articles on Genetics page.

- most chromosomes come in matching pairs, therefore there are two copies of the genes carried on those chromosomes.
- the pair of chromosomes that determine the gender of the budgie are called the sex chromosomes, and are referred to as the X and the Y chromosomes.
- X/X = male budgie
- X/Y = female budgie
- the X and Y chromosomes are not a matching pair. The variety genes that are carried on the sex chromosomes are found only on the X chromsome, so there is only one copy of these genes in and X/Y (hen) budgie.
- genes carried on the X sex chromosome are referred to as sex linked for reasons we will see soon

How does Sex Linkage Work?

Firstly, this is a list of the sex linked genes:

- cinnamon = Xcin
- opaline = Xop
- ino = Xino
- texas clearbody = Xino^cb
- slate - Xsl

All these varieties are both sex linked and recessive.

Possible genotypes for birds with a sex linked gene (we will use cinnamon as the example):
- XCin/XCin = 'normal' cock, no cinnamon gene
- XCin/Xcin = 'normal' cock, heterozygous for cinnamon
- Xcin/Xcin = cinnamon cock, homozygous for cinnamon

- XCin/Y = 'normal' hen, no cinnamon gene
- Xcin/Y = cinnamon hen

(In this case 'normal' refers to non-cinnamon)

As you can see there are only two types of genotype for a hen, she either has the sex linked gene or she doesn't. There is no way a hen can be split (heterozygous) for a sex linked gene as the gene is only carried on the single X chromosome.

Examples of sex linkage

Lets look at some matings to get an idea of how sex linkage works.

Cinnamon cock x cinnamon hen:
= Xcin/Xcin x Xcin/Y

If you do a punnett square you will see that the only outcome of this mating is cinnamon hens and cinnamon cocks.

What about this then:

Normal/cinnamon cock x cinnamon hen:
= XCin/Xcin x Xcin/Y

punnett square, budgie genetics, sex linkage

Results:
- 25% cinnamon cocks
- 25% normal/cinnamon cocks
- 25% cinnamon hens
- 25% normal hens

So far so good, if you got that result from a mating it wouldn't seem any different from a dominant/recessive interaction. But how about this one:

Cinnamon cock x normal hen
= Xcin/Xcin x XCin/Y

Results:
- 50% normal/cinnamon cocks
- 50% cinnamon hens

Now thats different! In this mating all the cocks will be non-cinnamon, and all the hens cinnamon. With this type of mating the outcome is linked to the sex of the chick - hence the label sex linked.

So, this means that if you want to breed a sex-linked recessive variety, the quickest way is to get hold of a male of that variety to breed from. If you can only get a hen the results will be:

Normal cock x Cinnamon hen:
= XCin/XCin x Xcin/Y

- 50% normal/cinnamon cocks
- 50% normal hens

To get cinnamons you would then mate the normal/cinnamon cocks with a normal or cinnamon hen, to get some cinnamon hen chicks.

These results apply to all the sex linked genes, just replace cinnamon with opaline etc. Budgies can also carry more than one sex linked genes at once, for instance an opaline cinnamon budgie. Let us do one final punnett square to show an example of this.

Cinnamon cock x opaline hen:
= XcinOp/XcinOp - cock bird with cinnamon and non-opaline genes
= XCinop/Y - hen with non-cinnamon and opaline gene

Results:
- normal/cinnamon opaline cocks
- cinnamon hens

Those are the basics of sex chromosomes and sex linkage.

Return from Sex Chromosomes to Article on Genetics

SBI!