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

如果们在进化上有关联，为们人类却少了一对？

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

通常，每对父母各贡献一条形成一对，双方各占一半。

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，然而多出的另一对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条分为23对，每一对由分别来自于父、母的一条组成。

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。们来看看其中一对发生了。

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

这一对，即X和Y，决定了你的生物学性别。

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

它们有X和Y，而鸟类的雌性有一对XX。

It's just got one chromosome pair.

它只有一对。

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，性别根本原因是一对：女性是XX，男性是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.

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

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对中，有一对是性，它决定了人的生物性别；对于女性，它由两条X组成，男性的则由一条X和一条Y组成。