Question

Why is it that two people can have several children and the children look so different? I mean, why don't all kids from the same parents look exactly alike, since the parents just have one set of chromosomes and they don't change?

Answer

Nature is absolutely amazing when it comes to reproduction. There is a system specifically designed to make children different.

You have probably seen pictures of chromosomes, and you know that a person has a set of chromosomes. Each chromosome contains 2 halves that join in the middle so they look like an "X". All that a chromosome is, by the way, is 2 very tightly coiled strands of DNA.

The two halves of each chromosome come from the two parents. So your DNA came from your mother and your father. One half of the "X" in each chromosome comes from the mother, and the other half 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 has two copies of every gene - the "dominant" gene of the two is the one that is expressed.

A sperm and an egg meet to create a new person. The sperm carries one half of the new baby'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?" Every cell in the father contains a complete set of X-shaped chromosomes that are exactly alike. Same for the mother. The father's chromosomes came from his mother and father, and the mother's from her mother and father. To make a sperm cell, only one half of the X is contributed. But which half? This is where nature does the amazing part. When forming sperm cells, the father's body randomly chooses genes from the two halves of the father's chromosomes. This means that every sperm cell contains a random mix of the father's parents' genes. The same thing happens when forming eggs. Therefore, each child that a couple produces is a random mix of the four grandparents' genes.

How Stuff Works Power Panel

Table of Contents A complete list of all HSW articles About Marshall Brain Information about the creator of HSW Search Search for any word or concept in HSW Citing HSW Information about citing HSW in articles and papers Question of the Day Ask questions or browse hundreds in the archive Books A variety of books for people interested in how stuff works HSW Newsletter Sign up to receive the monthly HSW newsletter Contacting HSW Contact HSW directly with email What's New Review newsletters to chart HSW's development Awards Check out HSW's awards and reviews! Disclaimer The disclaimer required by the lawyer Author Information Learn how you can write for HSW HSW Fan Buttons Link to HSW with these snazzy fan buttons! Suggestions Leave a suggestion in the suggestion box

How Stuff Works (http://www.howstuffworks.com) is a production of BYG Publishing, Inc. http://www.bygpub.com - questions@bygpub.com P.O. Box 40492 - Raleigh, NC 27629

© 1998 BYG Publishing, Inc. All rights reserved.