How do female and male characteristics evolve when each has a parent of each gender?
The first pre-requisite is there must be meiosis, i.e. sexual reproduction. Some animals such as some aphids and ants reproduce asexually by mitosis and they are all female – defined by the fact that they are egg producing. Mammalian sex is controlled by the Y chromosome which carries the sex-determining gene sry. This is passed on entirely through the male line, sons inherit Y and become male and daughters inherit X and are female. The sex determining gene causes male development, in its absence development continues along the female route. The platypus seems to be the odd one out when it comes to sex determination, as well as in lots of other ways, as it has no sry gene, the sex determination mechanism in this species is still unknown. In birds and butterflies the situation is reversed and females carry the sex determination gene on the W chromosome and males are ZZ. There are also all sorts of other chromosomal systems, the insects are particularly variable, especially social insect
