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 lacks the SRY gene. The platypus has sex chromosomes . The males have , while females have . During meiosis, 5 of X form one chain, and 5 of Y form another chain. Thus, they behave effectively