The Genetic Basics: What Are Genes and What Do They Do?
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In this section, you can investigate what genes are and what they do, and even play a game testing your knowledge of what causes disease. Just click on where you want to go.

What Are Genes?

Chromosomes contain the recipe for making a living thing. They are found in almost every cell’s nucleus and are made from strands of DNA (deoxyribonucleic acid). Segments of DNA called "genes" are the ingredients. Each gene adds a specific protein to the recipe. Proteins build, regulate and maintain your body. For instance, they build bones, enable muscles to move, control digestion, and keep your heart beating.


Courtesy of the National Human Genome Research Institute

Most of our cells contain 46 chromosomes (here is an early look at our chromosomes, taken in the 1950s by Dr. Joe Hin Tjio of the National Institute of Diabetes and Digestive and Kidney Diseases). Sperm and egg cells contain only 23 chromosomes. When the sperm and egg cells unite, the resulting fetus inherits half of its DNA recipe from its mother and half from its father.

Two of these 46 chromosomes determine the sex of a person. A girl inherits two X-chromosomes, one from her mother and one from her father. A boy inherits one X-chromosome from his mother and a small Y-chromosome from his father.

46 chromosomes photograph - Collection of DeWitt Stetten, Jr., Museum of Medical Research
46 Chromosomes. Collection of DeWitt Stetten, Jr., Museum of Medical Research
The Austrian monk Gregor Mendel (1822-1884) was the first person to describe how traits are inherited from generation to generation. He studied how pea plants inherited traits such as color and smoothness, and discovered that traits are inherited from parents in certain patterns. Not until the 20th century did other scientists take his ideas further. Mendel is considered to be the father of genetics, although his work was relatively unappreciated until the early 20th century.
Gregor Mendel and Pea Plants. Courtesy of Medical Arts and Photography Branch

A gene can exist in many different forms, calledalleles. For example, let’s say that there is one gene which determines the color of your hair. That one gene may have many forms, or alleles: black hair, brown hair, auburn hair, red hair, blond hair, etc. You inherit one allele for each gene from your mother and one from your father.

Each of the two alleles you inherit for a gene each may be strong ("dominant") or weak ("recessive"). When an allele is dominant, it means that the physical characteristic ("trait") it codes for usually is expressed, or shown, in the living organism. You need only one dominant allele to express a dominant trait. You need two recessivealleles to show a recessive form of a trait. See the heredity diagram for tongue rolling to see how dominant and recessive alleles work.

Tongue Rolling Heredity Diagram

For example, mild forms of red/green color blindness are very common, resulting only in the inability to tell apart shades of red and green. The gene for this trait is located on the X-chromosome. A mother who carries this recessive trait has normal red/green vision. Any of her sons who inherit the X-chromosome that carries this trait -- the allele for color blindedness -- will be mildly red/green color blind. In this chart used to test for color-blindedness, people with normal color vision can see the number seven. People with red/green color blindness cannot see the number seven.  

Red/Green "Seven" Chart.

Courtesy of the National Eye Institute

There are several ways the genetic code can be altered. Sometimes genes are deleted or in the wrong place on a chromosome, or pieces of genes are swapped between chromosomes. As a result, the gene may not work or may turn on in the wrong part of the body.

"Point mutations" alter the genetic code by changing the letters in the codons -- the three-symbol genetic words that specify which protein to make. This can change the protein.

Original message: SAM AND TOM ATE THE HAM

What it does
Kind of point mutation
Example of altered protein
Frameshift mutation Message starts in the wrong place AMA NDT OMA TET HEH AMS
Stop codon Prevents part of the protein from being made SAM AND TOM
Missense mutation Causes an amino acid substitution SAM AND TOM ATE THE DAM
mRNA splicing mutation Portion of message is left out, leading to a shortened protein SAM THE HAM












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Revolution in Progress: Human Genetics and Medical Research/
National Institutes of Health