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Hi I'm Marshall Brain with today's question how can children from the same

parents look so different the DNA of the parents never changes so how can there be so much variation it turns out that

there's a system specifically designed to make children different and we can

explore that a little bit today. Every person has a set of chromosomes.

Each chromosome contains two halves that join in the middle,

so they look like an X.

All that a chromosome is, by the way, is two very tightly coiled strands of DNA.

The two halves of each chromosome come from the two parents.

One half of the X in each chromosome comes from the

mother and the other half comes from the father. The two halves are bonded

together at the middle of the X. Each half of the X contains a complete set of genes so each chromosome

actually has two copies of every gene. A sperm and an egg meat to create a new person.

The sperm carries one half of the new person's ultimate chromosomes and the egg contains

the other half.

The question then comes down to, where does the DNA in the sperm and the egg come from?

A person has two of every gene, but when a sperm gets formed or an egg gets formed, it will only contain one of those two genes, but which one?

This is where nature does an especially amazing thing.

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