Thread by @zaelefty

Sex determination in mammals is binary. Every karyotype in humans results in a male or female.

Genetic errors, such as the unequal distributions of chromosomes (nondisjunction), do not create new sexes.

This shows us that karyotype (the collection of an individual’s chromosomes) is conceptually independent from what sexes are.

It is true that 46:XX and 46:XY are tightly linked to females and males, respectively, but it is not true that this is the defining feature of F and M.

In other words, you can have a karyotype outside of the norm, and you will still be a male or female.

Ex: XXY = male; XO = female

If I wake up tomorrow and find out that I have XX chromosomes, that would be quite the shocking discovery, but I’d still be a male.

This is because male and female are defined by the phenotypes that produce small or large gametes, respectively, not chromosomes.

This definition is the universal biological meaning of male and female.

And it applies to all species that reproduce through gametes of differing size and form (almost all species in the plant and animal kingdoms).

The fundamentals for why sex is binary can be condensed into four simple principles.

The details of how the binary is composed is different among species.

For example, humans are gonochoric: each individual is either male or female through their entire life.

And clownfish are sequential hermaphrodites: individuals can change from male to female.

Even in species that are incredibly different, like humans and clownfish, male and female are defined the same: the phenotypes that produce small or large gametes, respectively.

Q: Male seahorses gestate and birth the baby seahorses, a trait usually found in mammalian females.

What makes them male?