If we're supposed to be related, why are we missing a chromosome?

如果我们在进化上有关联，什么我们人类却少一对染？

One of these pairs, the sex chromosomes, determines person's biological sex and it can be composed of either two X chromosomes for females or an X and a Y chromosome for males.

在23对染中，有一对是染，决定人的生物别；对于女，由两条X染组成，男的则由一条X染和一条Y染组成。

But to keep this simple. We are just going to look at what happens to one of these pairs of chromosomes.

单起见。我们来看看其中一对染发生什么。

Most people have 20 chromosomes, but the presence of an extra 21st chromosome creates the disorder.

绝大多数人拥有20对染，然而多出的另一对染会导致该疾病的产生。

Usually, each parent contributes one chromosome to each pair, fifty-fifty.

通常，每对父母各贡献一条染形成一对染，双方各占一半。

Let's just say we're making a sperm cell - for simplicity, we're only going to show one pair of chromosomes, but remember that all 23 pairs do this.

假设我们正在制造一个精子细胞，单起见，我们仅展示一对染，但是请记住所有23对染都是这样的。

These 46 chromosomes come in 23 pairs - and each pair has one chromosome from each parent.

这46条染分23对，每一对染由分别来自于父、母的一条染组成。

This pair, called the X and Y chromosomes, influences your biological sex.

这一对染，即X染和Y染，决定你的生物学别。

They have x and y chromosomes whereas females have a pair of exes In birds.

们有X和Y染，而鸟类的雌有一对XX染。

And biologically speaking, the root cause of sex is a pair of chromosomes: XX for females and XY for males.

从生物学角度来看，别根本原因是一对染：女是XX，男是XY。

It's just got one chromosome pair.

只有一对染。