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In hamsters, as in all mammals, the gender of an individual depends on the chromosomes inherited from its parents. Females have a pair of X-chromosomes, (XX), one inherited from each of their parents. Males have one X-chromosome, (inherited from their mother), and a Y-chromosome, (inherited from their father). Since the Y-chromosome is a "cut down" version of the X, any genes present on the section of the X-chromosome that is absent in the Y may behave differently in males and females. The Yellow gene, To, is the currently the only "sex linked" gene in Syrian hamsters, and produces the Tortoiseshell.

The Yellow gene is "co-dominant"; animals will show the Yellow gene if they have a copy, but will also show the "non-Yellow" gene if they have a copy. Males, having only one X-chromosome, can have an X-chromosome with the Yellow gene, (XTo), or with the opposite, "non-yellow" gene, (Xto). They will show whatever genes they possess, so XTo will give a Yellow male and Xto will give a non-Yellow one. Since the Yellow gene combines with Cinnamon to give Honey and with Dark Grey to give Smoke Pearl, this also applies to these colours.

With females, the situation is slightly different. As with males, they show the genes that they have, but they have TWO copies of the X-chromosome. A Yellow female only shows Yellow - so her X-chromosomes are both XTo. A non-Yellow female shows no Yellow at all, so her X-chromosomes are both Xto. Some female hamsters, however, (Tortoiseshells), show a mosaic of Yellow and non-Yellow patches in their fur. This is because they have inherited a "Yellow" X-chromosome, ( XTo), from one parent and a "non-Yellow" one, (Xto), from the other.

Since Yellow combines with other genes to produce different colours, the "Yellow" patches on different coloured Tortoiseshell hamsters vary as well. For example, on a Golden Tortoiseshell, the patches of fur are Golden and Yellow, while on a Dark Grey Tortoiseshell they are Dark Grey and Smoke Pearl. The Yellow patches on a Cinnamon Tortoiseshell are Honey, but perhaps the most striking colour of Tortoiseshell of all is the Black Tortoiseshell, with patches of Black and Yellow.

NHC Standard

1. Overall Apperance: A tortoishell is a bi-coloured animal which consists of a balanced pattern of coloured and yellow patches. These patches shall be clear and distinct with no brindling.

2. The coloured areas shall conform to the recognised full colour variety whilst the yellow shall be a rich creamy yellow when in combination with the golden, but shall be present in a diluted form when combined with other recognised colours.e.g.

Main Colour Yellow
Golden Rich apricot yellow
Dark grey Smoke pearl
Cinnamon Honey
Lilac Lilac Pearl
Eye Colour As for the full coloured variety
Ear Colour As for the full coloured variety
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