The platypus decides sex with ten chromosomes, not two
Humans rely on a single X and Y pairing, but the platypus carries five X chromosomes and five Y chromosomes that link into chains during cell division. Its system shares nothing with other mammals' sex chromosomes and looks more like a bird's, one of many surprising ways living things settle whether offspring become male or female.
Most sexually reproducing organisms come in two sexes, though hermaphrodites can function as either or both, and some species persist with only one sex through reproduction without fertilisation. In many creatures sex is genetic, usually tied to chromosome combinations labelled XY, ZW, XO or ZO, or to haplodiploidy. Typically one master gene flips the switch and a cascade of others follows. Elsewhere temperature or other surroundings decide, and some fish and plants change sex partway through life.
In humans and most mammals, females usually carry two X chromosomes and males an X and a Y. A gene on the Y called SRY triggers testosterone and anti-Müllerian hormone during fetal development, steering the embryo toward male anatomy; embryos without a working SRY usually develop as female. SRY needs helpers, though. In mice, an extra copy of a gene named DAX1 overrides it, while boosting a gene called SOX9 can produce testes even without a Y. Fruit flies work differently, counting X chromosomes instead of looking for a Y. Grasshoppers, crickets and cockroaches use an XO scheme, where males simply lack a second sex chromosome, and a few mammals, including two spiny rats from Japanese islands, have lost the Y entirely.
Understanding built up over centuries. In 1694 the German botanist Rudolf Jakob Camerarius identified separate male and female structures in plants such as maize. Gregor Mendel's pea experiments, published in 1866, supplied the basics of inheritance. C.E. McClung suggested in 1902 that chromosomes set sex, and in 1903 Nettie Stevens, studying mealworms, gave the first solid evidence for what became known as the X and Y. Charles Elmer Allen found sex chromosomes in plants in 1917, Calvin Bridges argued in 1922 that the ratio of X chromosomes to other chromosome sets mattered in fruit flies, and Fritz Baltzer first described environmental sex determination in 1928.
The platypus remains partly unsolved. Genes matching the human sex chromosomes sit on an ordinary platypus chromosome, implying our X and Y were once ordinary too. A bird sex gene appears on two platypus X chromosomes, hinting at a shared trigger.
Source: Sex-determination system
