Page:Popular Science Monthly Volume 74.djvu/546

542 by Gregor Mendel, abbot of Brünn, in 1866. Since then our knowledge of Mendelian inheritance has been greatly extended through the researches notably of Professor de Vries, of Amsterdam; Professor Bateson and Professor Punnett, of Cambridge University, England; Professor Cuénot, of Paris; Professor Castle, of Harvard University; Dr. Davenport, of the Station of Experimental Evolution of the Carnegie Institution at Cold Spring Harbor, Long Island, and Professor Morgan, of Columbia University. For the purpose of elucidating this subject in respect to its essential principles and its peculiar nomenclature we will take the simple case of two varieties of pea, one with seed colored yellow and the other with seed colored green. These are said to be "pure" in the sense that from green seed-bearing individuals when interbred only green seed-bearing plants will arise, and from yellow peas only yellow seed-bearing plants. In order that plants or animals may be successfully crossed it is requisite that they be closely related species or varieties.

Mendel found at the beginning of his work that when a cross is made between two such varieties of peas, the first succeeding generation yields peas that are all colored yellow. When flowers from plants of the latter are self-pollinated, i. e., crossed with their own kind, there result peas of two kinds, yellow and green, and the proportion of such seed is three of the former to one of the latter. This means that the yellow color is "dominant" over the green color, which is said to be "recessive." Mendel further discovered that when the green recessives were self-pollinated their seed always produced only green seed-bearing plants. Such were called "pure recessives" or "extracted recessives."

He found also that among the 75 per cent, of yellow seed 25 per cent, always bred true, i. e., they gave rise to only yellow seed-bearing plants. Such were designated "pure" or "extracted dominants." But the remaining 50 per cent, on self-fertilization again yielded peas in the proportion of 75 per cent, yellows to 25 per cent, pure recessive greens. The Mendelian proportion then in the case where a single set of contrasting characters ("unit characters" or pair of "allelomorphs") are crossed is 25 per cent, pure dominants (yellow in color); 50 per cent, color hybrids (also yellow in color, since yellow dominates over green); and 25 per cent, pure recessives (green in color).

Mendel explained the foregoing facts on the assumption that during the process of maturation, when the unripe cells divide each into four mature germ-cells, eggs or pollen grains (ova or spermatozoa in animals), the carriers of the qualities yellow and green are segregated into different cells. In other words, no eggs or pollen grains (sperms) carry both characters, but only one or the other character. Accordingly, among the eggs 50 per cent, carry the quality yellow and 50 per cent, carry the quality green; among the pollen grains 50 per cent, again