Page "Sex-determination system" Paragraph 6

The XX / XY system is also found in most other mammals, as well as some insects.

Some fish also have variants of this, as well as the regular system.

For example, while it has an XY format, the Xiphophorus variatus also has a second Y chromosome, known as Y ', that creates XY ' females and YY ' males.

At least one monotreme, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and also lacks the SRY gene, whereas some rodents, such as several Arvicolinae ( voles and lemmings ), are also noted for their unusual sex determination systems.

The platypus has ten sex chromosomes ; males have an XYXYXYXYXY pattern while females have ten X chromosomes.

Although it is an XY system, the platypus ' sex chromosomes share no homologues with eutherian sex chromosomes.

Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were autosomes at the time that the monotremes diverged from the therian mammals ( marsupials and eutherian mammals ).

However, homologues to the avian DMRT1 gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination.

More research must be conducted in order to determine the exact sex determining gene of the platypus.