We’ve just seen how reproduction sifts and selects among pairs of alleles, and gives rise to a creature’s phenotype. but what is it that actually makes one creature male and another female?

In the case of Phantasia, every creature has one sex chromosome pair, usually chromosome 4. The ‘left’ strand is entirely inherited from dad, and the right strand is entirely inherited from mom. There’s no randomness involved.

But male creatures can either contain two ‘male’ chromosomes or one male and one ‘female’. They will still be male, because the male version will be dominant over the female (who could have guessed???), but they are capable of passing on either of them to their offspring.

Female creatures, on the other hand, always contain two ‘female’ strands. We can stick to the terminology used for humans, and call female creatures XX, while male creatures are XY.

So, when a child is born to a male and female couple, they are certain to inherit an X chromosome from mom, since she has two. They probably contain different alleles, but they are both X.

Whereas, they could inherit dad’s Y chromosome, or his ‘silent’ X chromosome. Hence they will end up either XX, in which case they will be a girl, or XY, in which case they’ll be a boy. Girls and boys are equally probable, and so there will tend to be about as many boys born as girls.

If a child is born XY, the X chromosome is completely silent, and only the alleles on the Y chromosome are expressed. In female phantasia, on the other hand, the two versions of X compete with each other, just as they do for the other chromosome pairs.

Boys are therefore much more like their dad, while girls are only somewhat like their mom, depending on dominance.

Sex in real animals works in all sorts of different ways, but phantasia are quite like humans, in the sense that they are either XX or XY, on a single chromosome. But it’s not exactly identical to how humans work, since human Y chromosomes are basically X chromosomes with an arm missing!

Either way, the results can be quite complex. A male phantasian might produce the equivalent of testosterone, while females might produce estrogen from the same gene. There may be chemicals that both sexes produce, which do nothing at all in males. And of course, there is the strong possibility that male and female brains will be wired up slightly differently, especially in terms of their instincts. Hence, being male or female involves a lot more than just being blue versus pink, or having horns versus not!