Could someone explain what the y and x chromosome are, what part they play in our biology and how they determine our sex? I hear this topic come up a lot and I have no idea what they are. Doesn’t an extra chromosome also mean a person might have down syndrome? Thanks guys.
(Not looking for a political debate btw, just straight biology, thank you)
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The sex of a child is determined by two chromosomes. You get one from each parent. A female has two X and a male has 1X and 1Y. Female give X and if male give X the child is a female. If male gives Y the child will be male. As for the extra chromosome question downs is just one of a multitude of outcomes. The outcome is determined by which chromosome is the extra one. Hope this helps.
The chromosomes are just certain settings in a big long string of DNA. Those settings determine what proteins are produced, in what order, and by a very complex process every time a cell splits it is determining how that cell operates, what type of cell is made, and what other proteins are produced.
Those settings literally determine almost everything that every cell in your body does, including how to make those cells and how each of those cells is different in your eye, your brain, your lung, your blood or wherever.
When that cell is an egg cell and a sperm cell and they mix their DNA, that determines the “sex chromosome” of the child.
However, the sex chromosomes are far from being the only determiner of sex, by the way:
[https://twitter.com/RebeccaRHelm/status/1207834357639139328](https://twitter.com/RebeccaRHelm/status/1207834357639139328)
There’s no such thing as “XX is always female”. That’s literally nonsense. And why 1% of people are medically “intersex” and many of those that aren’t intersex actually have entirely the “wrong” sex chromosome for their outward-facing sex. This is also the cause of some infertility – people who have every aspect outward-facing of being a female and have lived a perfectly ordinary female life for their entire existence actually have male sex chromosomes that just aren’t being acted upon, so they have trouble conceiving.
Also: It’s not true that there are “only two options” even when it comes down to pure chromosomes. The same way you describe Down’s as being an extra chromosome – that can happen just the same for sex with someone ending up with an XXY chromosome, for example.
Depending where such “errors” occur in the DNA, they can result in Down’s and significant developmental problems, or in the sex chromosomes and thus result in all kinds of effects on your body, genitals, hormones, fertility, etc.
DNA is a string-like chemical that exists in each and every one of your cells (except red blood cells). I can take a hair follicle or a bit of skin or some extra tissue from the inside of your cheek, and that small amount is enough for me to get a copy of all of your DNA.
Each of your cells as 46 strings of DNA. When it comes time for a cell to replicate, all that DNA is duplicated, then it wraps up tightly (imagine twisting a string until it folds on itself, then twisting the fold until *that* folds on itself, etc.) until you get these big X-shaped DNA knots called chromosomes. Then the cell splits in two, and the DNA unravels again.
Here’s a (https://www.youtube.com/watch?v=L0k-enzoeOM) of that happening.
So you have 23 unique chromosomes, but you have 2 copies of each. One from mom and one from dad. Chromosomes 1 through 22 are just named after numbers. If you have three chromosome 21s, that’s what Down’s syndrome is. If you have three chromosome 18s, that’s what Edward’s syndrome is. The 23rd chromosome comes in two varieties, X and Y, and we don’t call them chromosome 23.
So mom has two X chromosomes. That means, no matter what, she gives the child an X chromosome. Dad, on the other hand, has an X and a Y. So when he gives you half of his genes, there’s a 50% chance he gives you an X and a 50% chance he gives you a Y.
On the Y chromosome, there is a gene called the SRY gene. Occasionally the SRY gene gets broken off and put on the wrong chromosome, but for the most part it’s on the Y chromosome. If you get an SRY gene, you’ll be a dude (most of the time) and if you don’t, you’ll be a girl (most of the time). So that’s why XY is male and XX is female.
If you want to have a deeper understanding, just watch all the crash course biology episodes on youtube.
Others have answered the first part, but I just wanna address this
>Doesn’t an extra chromosome also mean a person might have down syndrome?
Down syndrome isn’t just an extra chromosome, it’s specifically an extra copy of chromosome 21. It’s also called trisomy 21, since that person will have 3 copies of chromosome 21 instead of the normal 2 copies.
The only other syndromes you can see with extra chromosomes are trisomy 13 (Patau syndrome) and trisomy 18 (Edward’s syndrome). These two are much more severe than down syndrome and most babies won’t live beyond a few days or weeks.
No other trisomies are seen (except with the sex chromosomes) because they aren’t compatible with life, and an embryo who has one will most likely result in a miscarriage.
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