REPRODUCTION 575 



acters so that any given germ-cell may contain either the deter- 

 miners for the dominant character or those for the recessive, but 

 not both. This principle of "the purity of the germ-cell" is the 

 corner stone of Mendelian inheritance. When such hybrids mate 

 it is evident that there are four possible combinations of germ-cells 

 that may occur. If we designate the dominant by D and the 

 recessive by R, the maternal germ-cell by m and the paternal by 

 p, we may represent the four possible combinations thus Dm + 

 Dp; Dm + Rp; Rm + Dp; Em + Rp. Of these four the first 

 and last are pure; the second and third are hybrid. Since the 

 dominant character is present in the hybrids they will have the 

 same appearance as number 1, which is pure dominant. Number 4, 

 however, which is pure recessive, will have the appearance char- 

 acteristic of the recessive trait. A simple illustration is furnished 

 by eye color. Brown eyes are dominant and blue eyes recessive. 

 According to the principles just stated brown-eyed persons may 

 be pure dominant or hybrid, but all blue-eyed persons are pure 

 recessive. If both parents are blue-eyed all the offspring must 

 therefore be blue-eyed also. If both parents are brown-eyed the 

 eye color of the offspring will depend on whether the parents are 

 pure dominant or hybrid. If one or both are pure dominant all off- 

 spring will have brown eyes. If both are hybrid one in four of the 

 offspring may have blue eyes. 



The actual situation is complicated by the numerous factors that 

 may be in conflict, but extension of the principle stated above is 

 believed to cover all forms of hereditary transmission that are 

 susceptible of modification by breeding. Whether the fundamental 

 features of inheritance; those that make the offspring of dogs dogs 

 and of roses roses, are also Mendelian; is at present a subject of 

 discussion. 



Sex Determination. An interesting application of the prin- 

 ciples of Mendel is in the determination of sex. It appears that 

 in general in the germ-cells of males there is one less chromosome 

 than in the cells of females of the same species. Human females, 

 for example, have 48 chromosomes and human males 47. In the 

 reduction division that occurs in connection with maturation 

 (p. 561) one of the daughter cells that is formed from the division 

 of the primary sperm cell has only 23 chromosomes, while the 

 other has 24. The subsequent division of the daughter cells to 



