Gene
Genes are the physical units of heredity and are located along each chromosome in the cells of the human body. For each physical trait--eye color, height, hair color--a person inherits two genes or two groups of genes, one from each parent. Because both of these genes cannot be expressed together, one usually overpowers the other. The more powerful gene is called the dominant gene and the weaker, the recessive gene.
All genes on the same chromosome are called linked genes because they are usually inherited together. Genes on the X and Y chromosomes are called sex-linked because the X and Y chromosomes are the ones that determine sex.
Sometimes genes on the same chromosome are not inherited together. When sex cells divide to form an egg or sperm cell (a process called meiosis), each chromosome pairs off with a partner. As the chromosomes lie side by side, groups of genes from one chromosome may change places with groups of genes from the partner chromosome. This is called crossing over and thus explains how families inherit different combinations of linked traits.
An Austrian monk, Gregor Mendel, introduced the world to hereditary factors—genes--that determine all hereditary traits. During his experiments with pea plants, Mendel noticed that the plants inherited traits in a predictable way. It was as if the pea plants had a pair of factors responsible for each trait. Even though he never actually saw them, Mendel was convinced that tiny independent units determined how an individual would develop. Before Mendel's findings, traits were thought to be passed on through a blending of the mother and father's characteristics, much like a blending of two liquids.
When Mendel's laws of heredity were rediscovered in 1900, they became vitally important to biologists. Among other things, Mendel's laws established heredity as a combining of independent units, not a blending of two liquids. Wilhelm Johannsen, a strong supporter of Mendel's theories, coined the term "gene" to replace the variety of terms used to describe hereditary factors. His definition of the gene led him to distinguish between genotype (an organism's genetic makeup) and phenotype (an organism's appearance).
In 1910, Thomas Hunt Morgan began to uncover the interesting relationship between genes and chromosomes. Morgan discovered that genes are located on chromosomes. He also found that the appearance of the characteristics associated with linked genes occur together in the offspring. In other words, genes located on the same chromosome are usually inherited together, or "linked." But as Morgan and his colleagues worked with more and more characteristics simultaneously, they discovered that genes on the same chromosome were not always linked. Sometimes crossing over occurred; paired chromosomes would break apart and rejoin during meiosis, causing many different gene combinations. Morgan and his colleagues went on to develop and perfect these and other gene concepts, which laid the groundwork for all genetically based medical research.
Currently, almost 4,000 genetic diseases are known to affect humans. In recent years, scientists have been identifying genes associated with various diseases and conditions at a furious pace. For example, in 1995, scientists found genes linked to at least some cases of male infertility, epilepsy, schizophrenia, Alzheimer's disease, and breast cancer. In 1996, they found genes associated with some cases of retinitis pigmentosa (a progressive form of blindness) and basal cell carcinoma (the most common form of skin cancer), and in 1997, with heroin addiction, Parkinson's disease, fetal alcohol syndrome, glaucoma, and obsessive-compulsive disorder. Given enough time and effort, scientists may one day learn how to prevent or treat many such diseases.
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