In mammals, sex is determined using chromosomes, where two X chromosomes (XX) generally results in the formation of an anatomical female, and one X and one Y chromosome (XY) generally results in the formation of an anatomical male.
There is, however, a family (as in family, genus, species) of rats called Echimyidae, or spiny rats, that have sex chromosomes that don't fit the normal mammal mold.
|Stuffed Echimyidae: Creative Commons access, image by ZeWrestler|
There is a lot to talk about with these rats, but I'll focus on the recent paper, detailed at the bottom, who study a smaller subset of the spiny rats, a genus called Proechimys, that lives in the Amazon forrest. Generally mammals have an even number of total chromosomes, one half coming from the mother, and one half coming from the father (for example, humans have 2n=46 chromosomes) In these rats, however, the researchers noticed that all of the females had full complement (2n) of 16 chromosomes, but that males had 17 chromosomes! After more investigation they realized that this incongruence happened because there was an addition to the sex chromosomes (X and Y) that had fused to the X chromosome, but had not fused to the Y. This means that the X chromosome in these rats is quite large, and that females have two of these large X chromosomes, but males have one large X chromosome and two Y chromosomes:
So, females have 14 autosomes (non-sex chromosomes) and two X chromosomes (for a total of 16) and males have 14 autosomes, one X, and two Y's (for a total of 17). This kind of addition to the sex chromosomes has happened, and will continue to happen in many species. It even happened in humans (the region was fused to both the X and the Y). Because the Y chromosome is constantly degenerating, these additions may serve to give more raw material to the Y, prolonging its life. I am excited to see what we'll find out as technology give us the opportunity to learn about the genetics of more and more species.
BMC Genet. 2013 Mar 11;14(1):21. [Epub ahead of print]
Proechimys (Rodentia, Echimyidae): characterization and taxonomic considerations of a form with a very low diploid number and a multiple sex chromosome system.
Amaral PJ, Nagamachi CY, Noronha RC, Costa MJ, Pereira AL, Rossi RV, Mendes-Oliveira AC, Pieczarka JC.
The full paper is in production, but will be open access (as far as I can tell). You can read the abstract, and the provisional pdf here.