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对染色？

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

通常，每对父母各贡献一条染色形成一对染色，双方各占一半。

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

数人拥有20对染色，出的另一对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对，每一对染色由分别来自于父、母的一条染色组成。

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。我们来看看其中一对染色发生了什么。

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

这一对染色，即X染色和Y染色，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染色，鸟类的雌性有一对XX染色。

It's just got one chromosome pair.

它只有一对染色。

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染色：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.

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

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