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A gene is a small piece of the genome. It's the genetic equivalent of the atom: As an atom is the fundamental unit of matter, a gene is the fundamental unit of heredity.

Genes are found on chromosomes and are made of DNA. Different genes determine the different characteristics, or traits, of an organism. In the simplest terms (which are actually too simple in many cases), one gene might determine the color of a bird's feathers, while another gene would determine the shape of its beak.

The number of genes in the genome varies from species to species. More complex organisms tend to have more genes. Bacteria have several hundred to several thousand genes. Estimates of the number of human genes, by contrast, range from 25,000 to 30,000.

How did people figure out that genes exist?

Heredity is an old idea. Since ancient times, people have known that offspring tend to resemble their parents. Physical appearance, temperament, and many other traits are passed down from generation to generation.

Gregor Mendel

But the concept of the gene dates back only to the late 19th century. It was first put forth by an Austrian monk named Gregor Mendel, who used his monastery garden for a now-famous series of breeding experiments with pea plants. In his carefully planned, painstakingly executed research, Mendel orchestrated various matings, placing pollen from one pea plant on the female flower parts of another to determine how different traits are inherited.

One of Mendel's most important observations was that traits are passed in a discrete form from generation to generation. For example, when he bred plants that had green pea pods with plants that had yellow pea pods, all of the offspring had green pods. These results contradicted the then-popular concept of heredity, known as blending inheritance, which predicted that when green-pod plants are bred with yellow-pod plants, the offspring should have greenish-yellow pods.

Even more surprising, when Mendel bred the "hybrid" plants with one another, some of their offspring had green pods, and some had yellow pods. A trait that disappeared in one generation could reappear, unchanged, in the next.

Mendel reasoned that since traits are inherited in a discrete form, then hereditary information must also come in discrete parcels that are passed down unchanged from generation to generation. He called these parcels of hereditary information "Elemente," and today we know them as genes.

How do genes determine characteristics?

All individuals in a species have the same set of genes: in peas there is a gene for pod color, a gene for plant height, a gene for pea shape, and so on. What makes individuals different is that a gene can have several different forms, or alleles. Thus, in peas, the pod color gene has green and yellow alleles, the plant height gene has tall and dwarf alleles, and so on.

Individuals have two copies of each gene, one inherited from each parent. How the two copies interact with each other determines an organism's characteristics. It doesn't matter whether a gene comes from the maternal or paternal side—a green-pod allele from a sperm cell is the same as a green-pod allele from an egg cell. What matters is how many of which type of allele an individual gets.

Mendel laid the foundation for our understanding of this business. He referred to the form of a trait that was visible in his first generation of hybrids (green pods, for example) as dominant. He called the other form of the trait (yellow pods, for example) recessive. Today, we know that the dominant and recessive forms of a trait are underlaid by dominant and recessive genes—or, more precisely, alleles. In pea plants, for example, the green-pod allele is dominant, while the yellow-pod allele is recessive.

Mendel's pea pods

Only individuals that inherit two recessive alleles show the recessive form of a trait. Those that inherit one recessive and one dominant allele show the dominant form of the trait (which is why all pea plants in Mendel's first generation of hybrids, carrying one green-pod allele and one yellow-pod allele, had green pods). But these individuals can still pass the recessive allele on to future generations, where it might link up with another copy of the recessive allele and allow the recessive trait to appear once again (which is why some of the pea plants in Mendel's second generation of hybrids had yellow pods).

Some human diseases, such as sickle cell anemia and Huntington's disease, are inherited in this simple, dominant/recessive fashion. Sickle cell is a recessive disease, while Huntington's is dominant.

But for the most part genetics, especially human genetics, is more complicated. One gene can affect several traits; one trait may be affected by several genes. And most traits are also influenced by an individual's environment, which includes hundreds of internal and external variables such as whether your diet includes enough vitamin D, whether you grew up at sea level or 8,000 feet, whether you live in Kenya, France, or New York City, and how your genes interact with each other inside your body.

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Updated on January 15, 2003