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

通常，每父母各贡献条染色体形成染色体，双方各。

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

如果我们在进化上有关联，什么我们人类却少了染色体？

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

绝多数人拥有20染色体，然而多出的另染色体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条染色体23，每染色体由别来自于父、母的条染色体组成。

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

了简单起见。我们来看看其中染色体发生了什么。

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

于每染色体，个来自每个亲生父母。

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，性别的根本原因是染色体：女性 XX，男性 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，我们仅展示染色体，但是请记住所有23染色体都是这样的。

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染色体中，有是性染色体，它决定了人的生物性别；于女性，它由两条X染色体组成，男性的则由条X染色体和条Y染色体组成。