Rachael French @DrRachaelF 
Mike Pence literally does not believe in science.
It is utterly irresponsible to put him in charge of US coronavirus response as the world sits on the cusp of a pandemic.
This decision could cost people their lives. Pence’s past decisions already have.
As you are speaking as the oracle of science, tell us, what exactly is a Y chromosome?
So, hey, I'm an actual expert in this, so let me take this one. A "Y" chromosome is one of two what are called "sex chromosomes" in mammals. In humans, it's about 57 million base pairs in size, making it one of the smaller chromosomes. 1/n
Rachael French @DrRachaelF It has about 71 protein-encoding genes, which by that measure makes it the smallest human chromosome. Of these genes, most seem to be involved in male fertility - i.e. gamete formation. ONE of the genes on the Y chromosome plays a fairly important role in sex determination. 2/n
Rachael French @DrRachaelF This gene, called "SRY" for "sex determining region on the Y" is called that because, if you delete it, it results in XY individuals developing as physically female. So, you know, to a crude approximation, it's sex-determining. 3/n
Rachael French @DrRachaelF Thing is, though, that's not all there is to it. SRY encodes a protein of a type called a "transcription factor". Transcription factors regulate other genes. In this case, those genes are involved in sex determination, and they're all over the genome. 4/n.
Rachael French @DrRachaelF So the Y chromosome itself doesn't actually determine sex. It carries one gene (which still does its thing if you move it to another chromosome) that is important in the process, yes. HOWEVER... 5/n
Rachael French @DrRachaelF What Ted REALLY means is "XY people are all men". And here he is wrong. Because:
SEX AND GENDER ARE NOT THE SAME THING.
They are co-regulated, to a degree. They usually correspond. But they are different things. 6/n
Rachael French @DrRachaelF "Sex" - by which I mean the physical manifestation of "male" vs. "female" anatomy is...well, it's anatomy. "Gender" is more...the person's relationship with gender roles, for want of a longer explanation. Which gender does their brain say they are? 7/n
Rachael French @DrRachaelF For completeness, too - sexual orientation is NOT the same thing as gender identity, nor is it the same thing as sex. 8/n
Rachael French @DrRachaelF So when Ted asked "what is a Y chromosome," he was really trying to suggest a complex biological and psychological phenomenon was under the control of something simple.
He'd like it to be that way. Makes his argument easier. But he's wrong. 9/n
Rachael French @DrRachaelF One last thing: I think Ted KNOWS he's wrong. I think Ted is playing to his base of bigoted supporters. Because, yes, if you want to put people in boxes based on their karyotype, you're a bigot. Sorry.
Except, not sorry.
10/10
Rachael French @DrRachaelF Oh, sorry, right: By the way, "sex chromosome" actually means "different between the sexes". It wasn't meant to suggest it is the sole arbiter of sex.
11/10
Rachael French @DrRachaelF @DavidJakubiak Fun fact: the mammalian Y is, in fact, a degenerate X.
Looks like we started out all having the same chromosomes, and over time, the Y diverged from the X.