Sex Linked Traits
by C. Williams

The X and Y chromosomes are named because of their shape. The Y chromosome, which is responsible for determining male gender, is shorter and contains less genetic code than the female Xchromosome. Due to this, the X Chromosome may contain alleles that have no "partner" on it's matching Y chromosome. When this is the case, whatever allele is present on the X chromosome, will express itself in the male.

Loci and alleles that are known to be found on one of the Sex Chromosomes (X or Y) but not the other, are considered to be Sex Linked Traits.

While none of the genes causing color seem to be Sex Linked (according to studies made to date), this genetic mechanism is still important to understand.

Hemophilia, or the inability of the blood to effectively clot and stop bleeding is the most well known sex-linked trait. It is a recessive carried on the X gene, but there is no corresponding locus "slot" for it on the "Y" gene. It should be noted that in the horse, most animals that would be hemophilic, do not live long enough (or are not allowed) to reproduce. Thus, the condition of hemophilia is usually only found among males, and is very rare to be found in females.

Normal Female HH, XX		Carrier Female Hh, XX		Normal Male H-, XY

Here's a chart to illustrate what happens when you cross a hemophilic carrier mare to a normal stallion.

Offspring		Sire
		XH	Y-
Dam	XH	XX HH Normal Female	XY H- Normal Male
	Xh	XX Hh Carrier Female	XY h- Hemophilic Male

In this example, we can see the following percentages:

• 25% Normal Male
• 25% Normal Female
• 25% Carrier Female
• 25% Hemophilic Male

Since the carrier female has a dominant "normal" allele that overrides the recessive allele for hemophilia, she won't be affected herself... but she will pass the gene onto half of her offspring. Half of her female offspring will be carriers. Half of her male offspring will be hemophilic.

The only way for a female to be hemophilic, is to have a hemophilic sire, and a dam who carries the hemophilic recessive allele. In livestock, it is unlikely that a hemophilic male would be kept for breeding purposes to cause this, however... it is more likely to occur in people.

Another, more amusing example of a sex-linked trait, involves the case of "tortoise-shell" cats. These cats show both black and orange in their coats simultaneously, often in a mish-mash variegated pattern.

The alleles for black or orange color are carried on the X chromosome in the cat, but there is no corresponding spot on the Y chromosome.

Female cats have two X chromosomes, so they may receive one allele apiece for each color. Since a male cat only carries one X chromosome, he can only be black or orange, but not both at the same time. Therefore, all tortoise-shell colored cats are female. Likewise, calico cats (featuring three colors... black, orange and white) are also female--the locus for white is found elsewhere. Male cats can be spotted with white and one other color.

What? You've heard of a tale of a male tortoise-shell cat?

Well, yes, it is possible and it has occurred. But, such cats are genetically abnormal. This condition is known as"Klinefelter's Syndrome", and such individuals carry two X chromosomes, plus a Y (XXY).

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Views expressed herein are those of the writers and compilers of the various information. Reference sources are cited where applicable. Copyrights are the property of the respective authors.