Gene

Template:StubA section of DNA that codes for the production of an RNA molecule. In most cases that RNA molecule codes for the production of a protein, but in some cases (most notably the transfer RNA (tRNA) and ribosomal RNA (rRNA) genes) the RNA molecule is the end product.

The gene is the fundamental unit of heredity.

History

By 1900 scientists knew that cells were the building blocks of living things. All living things start as a single fertilized cell which keeps dividing. Scientists identified tiny threads in the nucleus of the cells. Because they could stain the threads with colored dyes to study them under a microscope, they called the threads chromosomes, from the Greek words for colored bodies. They could see that chromosomes came in pairs, and that human cells all contained 23 matching pairs. American biologist Walter Sutton knew Mendel's principles of genetics work on peas, and suggested that chromosomes held the secret of inheritance.

Another American biologist, Thomas Hunt Morgan, developed the idea that chromosomes were made up of linked groups of factors called genes. He experimented with red-eyed fruit flies and found that sometimes a white-eyed fly appeared. When he mated them, he found that as he expected there were three red-eyed flies to every white-eyed fly, but that all the white-eyed flies were male. He concluded that the gene for white eyes must be on a chromosome that was related to being male. Later workers found that this is why some hereditary diseases such as hemophilia and muscular dystrophy only show in males, though women can carry the gene for the disease without showing it.

Mutation

Changes of a species from its normal state are called mutations. Sometimes they happen on their own, but they can also happen when a parent is exposed to something that will affect the way they reproduce. High doses of X-rays can produce mutations, and so can some chemicals. According to theory of evolution, most mutations will be harmful, but some will help a member of a species to survive and will pass on to its offspring. Some mutations can be both helpful and harmful. For example, people from some African countries carry a gene for an illness called sickle-cell anemia. This illness causes health problems, but people who have the gene for sickle-cell anemia are also less likely to catch malaria, a serious illness common in Africa.