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?
• How Can the Message Go Wrong?
• Play a Game: Which is a Genetic Disease?
• Sickle Cell Anemia: The First Molecular Look at a Genetic Disease

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. Collection of DeWitt Stetten, Jr., Museum of Medical Research.

Gregor Mendel and Pea Plants. Courtesy of Medical Arts and Photography Branch

The Czech 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.

A gene can exist in many different forms, called alleles. 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 recessive alleles 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

A person with two dominant or two recessive alleles for a trait is "homozygous." A person with one dominant and one recessive allele for a trait is "heterozygous."

Two 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.

Because a boy inherits only one X-chromosome, if there is a recessive gene on that X-chromosome, the gene is expressed. That is why some genetic diseases are found only in males -- there can be no dominant allele to correct the problem caused by the recessive allele. Such genetic diseases are called "X-linked diseases."

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.

Red blood cells are round when they are carrying oxygen, but become sickle-shaped without oxygen. In 1949, Dr. Linus Pauling’s group at the California Institute of Technology found that the mutant sickle hemoglobin molecule had an electrical charge different from normal hemoglobin. Dr. Vernon Ingram (Massachusetts Institute of Technology) discovered why in 1956. The amino acid valine had replaced glutamic acid in each of two places on the mutant hemoglobin molecule.