542 TEE POPULAR SCIENCE MONTHLY 



by Gregor Mendel, abbot of Briinn, in 1866. Since then our knowl- 

 edge 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 Cuenot, of Paris; Professor Castle, of Harvard University; 

 Dr. Davenpoi-t, of the Station of Experimental Evolution of the Car- 

 negie 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 nomencla- 

 ture 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 con- 

 trasting 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. Accord- 

 ingly, 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 



