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

通常，母各贡献一条染色形一染色，双方各占一半。

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一染色？

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

绝多数人拥有20染色，然而多出的另一染色会导致该疾病的产生。

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

这46条染色分为23，一染色由分别来自于、母的一条染色。

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。我们来看看其中一染色发生了什么。

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

于一染色，一个来自个亲生母。

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，性别的根本原因是一染色：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.

假设我们正在制造一个精子细胞，简单起见，我们仅展示一染色，但是请记住所有23染色都是这样的。

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染色中，有一是性染色，它决定了人的生物性别；于女性，它由两条X染色，男性的则由一条X染色和一条Y染色。