The platypus (Ornithorhynchus anatinus) has 26 pairs of chromosomes in total, compared with the 23 pairs present in humans. But researchers had long been confused about which ones are autosomal (inherited equally by males and females), and which ones determine sex. Most of the time for humans two full copies (XX) of the arbitrarily numbered chromosome 23 is a female and one full and one partial copy (XY) is a male. One serious consequence for males of this arrangement is that any genes with challenges on the the unmatched regions of the X chromosome cannot be compensated for – there’s nothing there. This can be a very big deal – the X chromosome contains about 155 million base pairs (5% of the human genome), while the Y chromosome contains about 59 million base pairs. In addition there are what are called pseudo-autosomal regions on both the X and the Y. Most of the time in females one X chromosome in each cell is randomly deactivated so what’s available might either be DNA from the father or the mother. The current best guess is that the human Y chromosome contains between 50 and 60 protein-coding genes, and the human X chromosome contains between 800 and 900 protein-coding genes.
Most birds, some fish, some crustaceans (notably the very complicated Macrobrachium rosenbergii, known as the giant river prawn), some insects (butterflies and moths) and some reptiles (the Komodo dragon – Varanus komodoensis) use a ZW system. ZZ is a male and ZW is a female so the female’s contribution determines the sex of the offspring. In our present context the Komodo dragon is interesting because, while it can see objects as far away as 300 meters, its retinas only contain cones, so it is thought to have poor night vision, reasonable ability to distinguish colors, and poor visual discrimination of stationary objects. A tough laboratory subject.