How does a species with three "sexes", where only two needed are at a time, determine the sex of its offspring?

Here's another option:

There are three genes: The X gene, the Y gene, and the Z gene. And there are three sex chromosomes: The XY chromosome, the XZ chromosome and the YZ chromosome. Each sex chromosome has exactly the two genes found in its name. Just as with humans, each individual has two sex chromosomes, but the gametes have only one.

Now the following rules apply:

• Each individual must have all three of the genes.

• The one gene that appears twice determines the sex. For example, an individual that has an XY and an XZ chromosome is of sex X.

• Gametes of the same sex simply don't fuse, due to receptors on their gametes; that is, while 50% of same-sex fusions would give a working gene combination, this is irrelevant because the fusion simply doesn't take place.

So if e.g. an X (gene combination XY + XZ) and an Y (gene combination XY + YZ) get offspring, there are four possibilities:

1. XY + XY: The fertilized egg won't develop, as Z is missing. Since this happens before the egg even gets to the placenta, this will be indistinguishable from no fertilization happening in the first place.

2. XY + YZ: The child will have sex Y.

3. XZ + XY: The child will have sex X.

4. XZ + YZ: The child will have sex Z.

So independent of the sexes of the parents (as long as they are different), the children will have all three sexes with equal probability.

Note that while this has some similarity with Kyyshak's answer, it's not the same as it only needs standard chromosome pairs and does away with the dominant/recessive complexity by simply having the same-sex fertilization prevented by a different mechanism.

posted almost 7 years ago

celtschk‭

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