MENDEL’S LAWS OF HEREDITY 387 
for tallness or for dwarfness, but not both. In other words, allelo- 
morphs cannot be represented in the same gamete. If the gamete 
having the character for tallness unites with one having the character 
for dwarfness, the resulting zygote will contain both, but will produce 
a tall individual because tallness is dominant over dwarfness. When 
this tall hybrid produces gametes, however, one-half of them will 
contain the character for dwarfness. Thus the alternative characters 
are ‘‘segregated” in gamete formation and no gamete will have both 
characters. 
These three theses, independent unit characters, dominance, and 
purity of gametes (better called segregation), make up the theoretical 
explanation of Mendel’s law. Independent unit characters was of 
course a necessary conception. It was original with Mendel, and has 
also been original with other investigators, but this conception does not 
represent the essential feature of Mendel’s law. The idea of domi- 
nance had been somewhat vaguely proposed before Mendel’s time. 
In the old literature on animal] breeding one meets theories of pre- 
potency, which were proposed again and again before the discovery 
of Mendel’s work in 1900. In any event Mendel was the first to 
formulate definitely the theory of dominance among unit characters. 
It should be realized also that dominance is not an essential feature of 
Mendel’s theory. Many cases are known in which dominance fails, 
but in other regards the Mendelian inheritance is strictly followed. 
The essential feature of Mendel’s theory is his conception of the 
purity of gametes, brought about by the segregation of alternative 
characters. The striking fact is that this conception, purely theoreti- 
cal with Mendel, has since been confirmed by cytology. In the 
mechanism of cell division each chromosome is divided into two equal 
parts and each daughter-cell receives one of these parts. It is a 
reasonable inference that chromosomes are bearers of hereditary 
characters. In the production of gametes the number of chromosomes, 
characteristic of the organism is reduced one-half. As a consequence 
each gamete carries only one-half the characters of the individual that 
produced it. An application of these statements to an explanation of 
Mendel’s 3:1 ratio will illustrate the situation. 
For convenience we will assume that the nuclei of Mendel’s peas 
have four chromosomes each (Fig. 67). In the case of a tall plant two 
of the four chromosomes carry the character for tallness, that is, some- 
thing that determines the production of the tall character in the 
somatoplasm, which is practically the body builder. This unknown 
