GAMETOGENESIS 477 



determination in line with our knowledge of inheritance in general, and 

 more particularly with the principles of Mendelian inheritance. This ac- 

 cordingly would seem to be the logical place for a brief presentation of the 

 main facts of Mendelism, and the hypothesis that attributes to the chromo- 

 somes the function of vehicles, or determiners for hereditary qualities. 

 Barring the sexual numerical differences above discussed, the specific num- 

 ber of chromosomes is constant; that is, all cells of the white human male 

 have 47, of the female 48 chromosomes. The more variable, superficial, or 

 more recently acquired characters are believed by many to depend upon the 

 presence in the developing egg consequently the cells of the organism of 

 certain chromosomes, the so-called 'determiners.' The more fundamental 

 characters are believed to inhere in the cytoplasm of the egg. For exam- 

 ple, the vertebrate condition of a man is thus believed to result from the 

 constitution of the egg; the color of his eyes from the presence or absence 

 of some determiner or factor in a particular chromosome. 



To elucidate the matter further, we must regard an organism as a 

 congeries of almost numberless characters; of these some are like those of 

 the mother and her ancestors, others like those of the father and his an- 

 cestors, others apparent blends. In Mendelian phraseology these characters 

 are called unit characters, and their material basis is located in particular 

 chromosomes. We may select a single pair of unit characters in order 

 further to present the principles of Mendelian inheritance. Suppose we 

 consider the coat color of gray and white guinea pigs. When individuals 

 characterized by these coat colors are crossed, the resulting offspring, first 

 generation, are all gray in color. The gray color is said to be dominant, 

 the white recessive. However, the potentialities or determiners for white 

 coat color are still present in the gray hybrid, for if such hybrids are in- 

 bred, there result gray and white offspring in the proportion of 3 of the 

 former to 1 of the latter. The latter if inbred produces only white off- 

 spring, hence called pure recessives, but of the 3 gray only one if bred with 

 its own type (pure dominants) will produce gray; two out of the three if 

 inbred produce again 3 gray to 1 white. The proportion of 1 pure domi- 

 nant to 2 hybrid dominants to 1 pure recessive is the well-known Mendelian 

 formula for one pair of unit characters in cross-breeding of specific varie- 

 ties. The character of this formula depends upon the fact that no germ 

 cell carries both determiners of a pair of unit characters, but only one or 

 the other. It thus follows that from the standpoint of a single pair of 

 unit characters (e. g., color of iris) there are two categories of eggs and 

 likewise of sperm. According to the laws of chance there will then be one 

 chance for the determiner of either character (e. g., gray and white coat) 

 to meet its like, to two chances that it may meet its opposite, h?nce the 

 formula 1 :2 :1 : 



The central concepts of Mendelian inheritance thus are (1) unit char- 

 acters; (2) the phenomenon of dominance; and (3) the principle of seg- 



