The first person to discover the basic laws of heredity and suggest the existence of genes was an Austrian friar, Gregor Mendel, b. July 22, 1822, d. Jan. 6, 1884. The importance of his work was not realized until 1900, at which time his findings laid the foundation for the science of genetics.Born Johann Mendel in Heinzendorf, Austrian Silesia (now Hyncice, Czech Republic), he changed his name to Gregor in 1843 when he entered the Augustinian friary at Brunn (now Brno). He was ordained a priest in 1847 and in 1851 was sent to the University of Vienna for training as a teacher of mathematics and natural sciences. He returned to Brunn in 1854, where he taught until 1868.In a monastery garden Mendel began (1856) the breeding experiments that led him to discover the laws of heredity. Working with garden peas, he studied seven characteristics that occur in alternative forms: plant height (tallness vs. shortness), seed color (green vs. yellow), seed shape (smooth vs. wrinkled), seed-coat color (gray vs. white), pod shape (full vs. constricted), pod color (green vs. yellow), and flower distribution (along length vs. at end of stem). Mendel made hundreds of crosses by means of artificial pollination. He kept careful records of the plants that were crossed and of the offspring. In 1865, Mendel reported his findings at a meeting of the Brunn Natural History Society. The following year his results were published as “Experiments with Plant Hybrids” in the society’s journal.Mendel summarized his findings in three theories. He asserted that during the formation of the sex cells--the egg and the sperm--paired factors segregated, or separated. Thus a sperm or egg may contain either a tallness factor or a shortness factor, not both. This theory is called Mendel’s first law, or the principle of segregation.Mendel’s second law, called the principle of independent assortment, stated that characteristics are inherited independently of one another. That is, the tallness factor may be inherited with any other factor, dominant or recessive. This law later was modified when Thomas Hunt Morgan discovered linkage, or the inheritance of two or more genes situated close to each other on the same chromosome.The third theory stated that each inherited characteristic is determined by the interaction of two hereditary factors (now called genes), one from each parent. In the characteristics that he studied, Mendel found that one factor of the pair always predominated over the other. For example, tallness was always dominant over shortness. This theory became known as the law of dominance.With his promotion to abbot of the monastery in 1868, Mendel gave up his experiments. Although respected by his fellow monks as well as by his students, Mendel, at the time of his death, was still not recognized as a great scientist. Sixteen years later, three European scientists--Hugo De Vries, Carl Correns, and Erich Tschermak--working independently discovered Mendel’s writings as they were conducting experiments similar to his and credited him as the discoverer of the laws of heredity.Bibliography: Bowler, P. J., Mendelian Revolution (1989); Corcos, A. F., and Monaghan, F. V., Gregor Mendel’s Experiments on Plant Hybrids (1993); Iltis, Hugo, Life of Mendel, trans. by Eden and Cedar Paul (1932; repr. 1966); Olby, Robert C., The Origins of Mendelism, 2d ed. (1985); Orel, Vitezlav, Mendel (1984).

Mendel’s Laws

Genetics is the area of biology concerned with the study of inheritance, or heredity, the process by which certain characteristics of organisms are handed down from parent to offspring. Modern genetics began in 1865, when the Austrian monk Gregor Mendel explained the inheritance patterns of variants of the garden pea, Pisum sativum, by postulating the existence of hereditary determinants now called genes.It is now known that genes determine the fundamental characteristics of organisms from bacteria to elephants. Parents pass their genes to offspring, and the basic characteristics of a species are passed down through the generations. Variations in genes cause a large proportion of the variation within a species. Genetics is the study of all aspects of genes: structure, action, inheritance, change, and patterns in populations.

Mendel’s Law of Equal Segregation

In a heterozygote such as Tt, the pulling apart of chromosomes during the first division of meiosis causes separation of the pair of alleles into an equal number of gametes (half T and half t). Hence, if a pea plant of genotype Tt is allowed to self-pollinate, then half of the pollen grains and ovules will have the T allele, and half will have the t allele. When these combine randomly during fertilization, the genotypes of the next generation can be represented in the following way: 25 percent TT, 50 percent Tt, and 25 percent tt. Seventy-five percent of the plants will have the tall phenotype, and 25 percent will be dwarf. This is often written as the ratio 3 tall: 1 dwarf.Recessive diseases in humans often show this type of inheritance pattern. phenylketonuria (PKU) is a recessive nutritional processing disease that if untreated leads to mental retardation. Most children with PKU (genotype pp) are born to unaffected parents, so the parents must both have been Pp. In a mating of individuals that are both Pp, the progeny are expected to be 75 percent PP or Pp (unaffected) and 25 percent pp, (with PKU), just as in the example from pea plants.Mendel’s Law of Independent AssortmentConsider a pea plant that has the heterozygous gene pairs Tt (tall and dwarf) and Pp (P for purple petals, and p for white petals). If Tt and Pp are on different chromosomes, then at meiosis they will sort independently. Individually they will produce phenotypic ratios of 3 tall: 1 dwarf and 3 purple: 1 white in the progeny. However, when these ratios are combined randomly, an overall ratio is produced of 9 tall purple: 3 tall white: 3 dwarf purple: 1 dwarf white. If this ratio is observed, the genes can be inferred to be on different chromosomes.Human disease is clearly an important type of discontinuous variation. The affected and unaffected phenotypes of such diseases are caused by an allele difference at one single gene. However, a good deal of apparently neutral discontinuous variation also has a similar basis. For example, red hair is caused by a simple recessive allele r, whereas the non-red phenotype is R. Furthermore, dark eyes (shades of brown) and light eyes (blue, green, gray) are generally determined by the allelic pair B (dark) and b (light), although there are sometimes exceptions to this rule.