Changes

By 1900 scientists knew that [[cell|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.

By 1900 scientists knew that [[cell|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 [[gene|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.

Another American biologist, [[Thomas Hunt Morgan]], developed the idea that chromosomes were made up of linked groups of factors called [[gene|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.

In 1991 a project called the [[Human Genome Project]] began to use computers to map the three billion base pairs which make up the 46 human chromosomes.

In 1991 a project called the [[Human Genome Project]] began to use computers to map the three billion base pairs which make up the 46 human chromosomes.