Changes

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 [[chromosome]]s, 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|Mendel's]] work on peas, and suggested that chromosomes held the secret of inheritance.

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 [[chromosome]]s, 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|Mendel's]] 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.

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.

Once Crick and Watson had unraveled the structure of [[DNA]], the function of genes became clearer.

Once Crick and Watson had unraveled the structure of [[DNA]], the function of genes became clearer.