Microscopic bodies in the nuclei of cells, called chromosomes, had been discovered by the end of the nineteenth century, and in 1901 it was proposed that chromosomes are the physical structures that contain Mendel's hereditary factors, or genes. Chromosomes were a likely structure for the location of genes because chromosomes occur in pairs, duplicate when the cell divides, and segregate into sperm and egg cells such that only one of the two chromosomes in each pair is passed on to any single offspring by each parent. The chromosomal theory of heredity made it easier for biologists to think of genes as physical objects of analysis, and studies of Mendelian patterns of inheritance and their chromosomal basis progressed rapidly.
Ageneticist named Thomas Hunt Morgan at Columbia University made several key discoveries using fruit flies between 1910 and 1920. He and his colleagues discovered mutations in flies that showed different patterns of inheritance in males and females, which led to association of these genes with the sex-determining X and Y chromosomes. Traits affected by genes on these chromosomes show a sex-linked pattern of inheritance in which recessive traits appear more often in males than in females. Human sex-linked traits, for example, include hemophilia, color-blindness, and baldness.
Fruit flies have three pairs of chromosomes besides the sex chromosomes, and Morgan's laboratory team showed that traits could be grouped together in "linkage groups" corresponding to their four pairs of chromosomes. They realized that Mendel's second law describing the principle of independent assortment corresponded to the assortment of chromosomes being passed from parent to offspring. Any genes on different chromosomes would be passed on independently, while genes linked together on the same chromosome would be passed on together as a unit. The discovery of linkage groups supported the idea that chromosomes were made up of collections of a large number of genes linked together.
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