Heredity (II)
The botanist Hugo de Vries (1848-1935) maintained that 'pangenes' carried hereditary traits. He proposed in 1903 the theory that new species could be produced from one generation to the next one by alterations that he called 'mutations.' His theory was very popular as an alternative to Darwin's theory of natural selection, but later has to be modified. In 1902, Walter S. Sutton (1877-1916) studied in grasshoppers the behavior of some cell structures known as chromosomes. A chromosome is a little cell structure resembling a thread; chromosomes are present in all cells, but were found for the first time in the nucleus of the cells of higher plants and animals. In these cells, chromosomes have different forms and sizes, and present in pairs. The two members of a pair are called homologous chromosomes.
From his studies, Sutton concluded that the behavior of chromosomes during sexual reproduction was exactly the same that Mendel postulated for his hereditary units. Thomas Hunt Morgan (1866-1945) studied the chromosomes of the fruit fly and proposed that hereditary units were located in the chromosomes following a linear pattern. Morgan and his associates published in 1915 "The Mechanism of Mendelian Heredity." He was awarded the Nobel Prize in 1933.
Mendel's units were renamed as genes, and the existence of dominant and recessive genes led to the distinction between genotype and phenotype. Genotype is the set or pool of genes that exist in an individual, and phenotype is the set of those characteristics that actually manifest. Each gene is in a specific position of a chromosome.
Genes determine the cell's hereditary traits. The cells of the individuals of a species have all the same number of chromosomes. Somatic (or body) cells in humans have 23 pairs of chromosomes. Reproductive cells have half this number. During fertilization, the usual number of chromosomes is restored. The zygote, the cell resulting from the union of sperm and eggs, has half its chromosomes from the father and half from the mother.
After the identification of chromosomes as the localization of genes, research was conducted to accurately delimit their hereditary fraction. Chromosomes were found to be formed by nucleic acids and proteins. Nucleic acids are very complex molecules located for the first time in the cell nucleus, although some nucleic acids are in the cytoplasm of cells. It is believed that nucleic acids are as older as the life itself (3 million years). There are two classes of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), and both have helix form. It was discovered in 1944 by O. T. Avery (1877-1955) that DNA is the responsible for the transmission of hereditary traits.
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