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

通常，每对父母各贡献条染色体对染色体，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对染色体，然而多出的另对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条染色体为23对，每对染色体由自于父、母的条染色体组。

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。我们看看其中对染色体发生了什么。

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

对于每对染色体，个自每个亲生父母。

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，性的根本原因是对染色体：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，我们仅展示对染色体，但是请记住所有23对染色体都是这样的。

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