Introduction of genetic material (by interbreeding) from one population of a species to another, thereby changing the composition of the gene pool of the receiving population. The introduction of new characteristics through gene flow increases variability within the population and makes possible new combinations of traits. In humans, gene flow usually comes about through human migration.

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Unit of heredity that occupies a fixed position on a chromosome. Genes achieve their effects by directing protein synthesis. They are composed of DNA, except in some viruses that contain RNA instead. The sequence of nitrogenous bases along a strand of DNA determines the genetic code. When the product of a particular gene is needed, the portion of the DNA molecule that contains that gene splits, and a complementary strand of RNA, called messenger RNA (mRNA), forms and then passes to ribosomes, where proteins are synthesized. A second type of RNA, transfer RNA (tRNA), matches up the mRNA with specific amino acids, which combine in series to form polypeptide chains, the building blocks of proteins. Experiments have shown that many of the genes within a cell are inactive much or even all of the time, but they can be switched on and off. Mutations occur when the number or order of bases in a gene is disrupted. Seealso genetic engineering, genetics, Hardy-Weinberg law, Human Genome Project, linkage group.

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Gene that determines whether or not a trait determined by another gene will be expressed. For example, when the gene responsible for albinism occurs, the genes that determine skin color are present but not expressed; the gene for albinism is therefore called an epistatic gene.

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(born May 25, 1898, New York, N.Y., U.S.—died Nov. 7, 1978, Greenwich, Conn.) U.S. boxer. Tunney boxed in the Marine Corps, earning the nickname “the Fighting Marine.” He defeated Jack Dempsey in 1926 to become the world heavyweight champion. In a controversial rematch in Chicago in 1927, Dempsey knocked Tunney to the canvas in the seventh round but failed to retire immediately to a neutral corner, thus delaying the count; the “long count” allowed Tunney to rise and win the 10-round fight. He retired the next year with a record of 65 wins in 77 bouts.

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(born Feb. 27, 1902, Harrison, N.Y., U.S.—died May 13, 1999, Naples, Fla.) U.S. golfer, prominent in the 1920s and '30s. Born to a poor Italian immigrant family, Sarazen began caddying when he was eight. He became a professional golfer in 1920. Sarazen was the first to win the four tournaments that comprise golf's grand slam in modern times. Sarazen won the U.S. Open (1922, 1932), the British Open (1932), and the PGA Championship (1922–23, 1933) and completed the grand slam with his Masters victory in 1935. That Masters victory was highlighted by his famous double eagle, two strokes (three under par) on the par-five 15th hole.

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(born Aug. 19, 1921, El Paso, Texas, U.S.—died Oct. 24, 1991, Santa Monica, Calif.) U.S. television and film producer. He worked as a pilot (1945–49) and police officer (1949–53) before becoming a writer for television series such as Dragnet and Dr. Kildare. He created the idea for the Star Trek series and produced the show from 1966 until it ended in 1969; later rerun in syndication, it developed a durable cult following among fans known as “Trekkies.” He produced six Star Trek movies, and from 1987 to 1991 he produced the television series Star Trek: The Next Generation.

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(born Aug. 23, 1912, Pittsburgh, Pa., U.S.—died Feb. 2, 1996, Beverly Hills, Calif.) U.S. dancer, choreographer, actor, and movie director. After training at his mother's dance school in Pittsburgh, he moved to New York in 1938 and danced in Broadway musicals, creating the h1 role in Pal Joey in 1940. Beginning in 1942, his athletic style and carefree acting—exemplified in the popular Anchors Aweigh (1945), On the Town (1949), An American in Paris (1951), and Singin' in the Rain (1952), which he also helped choreograph and direct—became hallmarks of the movie musical. His achievements earned him a special Academy Award in 1951. He later choreographed and directed numerous other movies and created a ballet for the Paris Opéra (1960).

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(born May 25, 1898, New York, N.Y., U.S.—died Nov. 7, 1978, Greenwich, Conn.) U.S. boxer. Tunney boxed in the Marine Corps, earning the nickname “the Fighting Marine.” He defeated Jack Dempsey in 1926 to become the world heavyweight champion. In a controversial rematch in Chicago in 1927, Dempsey knocked Tunney to the canvas in the seventh round but failed to retire immediately to a neutral corner, thus delaying the count; the “long count” allowed Tunney to rise and win the 10-round fight. He retired the next year with a record of 65 wins in 77 bouts.

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(born Feb. 27, 1902, Harrison, N.Y., U.S.—died May 13, 1999, Naples, Fla.) U.S. golfer, prominent in the 1920s and '30s. Born to a poor Italian immigrant family, Sarazen began caddying when he was eight. He became a professional golfer in 1920. Sarazen was the first to win the four tournaments that comprise golf's grand slam in modern times. Sarazen won the U.S. Open (1922, 1932), the British Open (1932), and the PGA Championship (1922–23, 1933) and completed the grand slam with his Masters victory in 1935. That Masters victory was highlighted by his famous double eagle, two strokes (three under par) on the par-five 15th hole.

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(born Aug. 19, 1921, El Paso, Texas, U.S.—died Oct. 24, 1991, Santa Monica, Calif.) U.S. television and film producer. He worked as a pilot (1945–49) and police officer (1949–53) before becoming a writer for television series such as Dragnet and Dr. Kildare. He created the idea for the Star Trek series and produced the show from 1966 until it ended in 1969; later rerun in syndication, it developed a durable cult following among fans known as “Trekkies.” He produced six Star Trek movies, and from 1987 to 1991 he produced the television series Star Trek: The Next Generation.

Learn more about Roddenberry, Gene with a free trial on Britannica.com.

(born Aug. 23, 1912, Pittsburgh, Pa., U.S.—died Feb. 2, 1996, Beverly Hills, Calif.) U.S. dancer, choreographer, actor, and movie director. After training at his mother's dance school in Pittsburgh, he moved to New York in 1938 and danced in Broadway musicals, creating the h1 role in Pal Joey in 1940. Beginning in 1942, his athletic style and carefree acting—exemplified in the popular Anchors Aweigh (1945), On the Town (1949), An American in Paris (1951), and Singin' in the Rain (1952), which he also helped choreograph and direct—became hallmarks of the movie musical. His achievements earned him a special Academy Award in 1951. He later choreographed and directed numerous other movies and created a ballet for the Paris Opéra (1960).

Learn more about Kelly, Gene with a free trial on Britannica.com.