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染色体？

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，有是染色体，它决定了人的生物别；于女，它由两条X染色体组成，男的则由条X染色体和条Y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.

为了简单起见。我们来看看其中染色体发生了什么。

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

绝大多数人拥有20染色体，然而多出的染色体会导致该疾病的产生。

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，双方各占半。

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。

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

这46条染色体分为23，每染色体由分别来自于父、母的条染色体组成。

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

这染色体，即X染色体和Y染色体，决定了你的生物学别。

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

它们有X和Y染色体，而鸟类的雌有XX染色体。

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，别根本原因是染色体：女是XX，男是XY。

It's just got one chromosome pair.

它只有染色体。