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

通常，每对各贡献色体形成对色体，双方各占半。

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

如果我们在进化上有关联，为什么我们人类却少了对色体？

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

绝多数人拥有20对色体，然而多出另对色体会导致该疾病产生。

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

这46色体分为23对，每对色体由分别来自于、色体组成。

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

为了简单起见。我们来看看其中对色体发生了什么。

For every chromosome pair, one comes from each biological parent.

对于每对色体，个来自每个亲生。

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

从生物学上讲，性别根本原因是对色体：女性为 XX，男性为 XY。

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.

假设我们正在制造个精子细胞，简单起见，我们仅展示对色体，但是请记住所有23对色体都是这样。

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对色体中，有对是性色体，它决定了人生物性别；对于女性，它由两X色体组成，男性则由X色体和Y色体组成。