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→History: Expanded the section on what genes are.
==History==
==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|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|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|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.
One strand of [[DNA]] contains many genes. DNA is made of four chemicals, [[guanine]], [[adenine]], [[thymine]] and [[cytosine]]. G always pairs with C, and A with T. Different combinations of GC and AT join together in different orders along the strands of DNA coiled up along our [[chromosome|chromosomes]] to give our cells instructions for making the different kind of protein of which our bodies are made. These groups of instructions are called genes. Our bodies are made up of about 100 trillion cells, each of which is responsible for a specific function. The thousands of different proteins in the cells work together like a machine to make the [[cell]] function as it should. If the genes are normal, the body part will function well. If a change (mutation) has happened in a gene, the protein may be faulty, as for example in sickle cell anemia where the instructions for making [[blood]] cells are abnormal.
==Mutation==
==Mutation==