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Expanded article and added section on history and on mutation.
The gene is the fundamental unit of heredity.
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|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 fly|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 [[mutation|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 [[Darwin’s theory of evolution|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 [[Africa|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.
==References==
==References==