HEREDITY AND MUTATION AS CELL PHENOMENA* 
R. RuGGLES Gates 
Heredity is frequently defined as the tendency of like to beget 
like; and the degree of resemblance between parent and offspring is 
often considered to be a measure of inheritance. The modern Men- 
delian work with dominant and recessive characters has, however, 
rendered such definitions incomplete and therefore untenable, since a 
plant or an animal may inherit in a predictable manner characters 
which its immediate ancestors did not exhibit at all. Two white 
races of sweet pea may on crossing give rise to a purple race, a re- 
version to an ancestral type which in the absence of definite knowledge 
might have been looked upon as an unexpected variation. But 
breeding experiments with the two white races will show that their 
germinal constitution is different. The interpretation then follows 
that the purple character is not a variation but is a result of inheritance. 
What has been inherited, however, is not a similarity but a difference. 
In similar fashion the French zoologist Cuenot found that in 
certain cases where a wild gray mouse is crossed with an albino the 
second generation of offspring contain not only the two original types 
but black as well, the frequency being 9 gray: 3 black: 4 albino. 
Breeding experiments with the albino parent disclosed the fact that 
some of the germ cells of the albino carried the potentiality of pro- 
ducing black under certain conditions, i. e., when meeting a germ cell 
containing the capacity or factor for developing color. Again we are 
dealing with phenomena of inheritance not of similarities but of differ- 
ences. In the same way, in all sexually reproduced organisms it 
is not the similarities but the differences between the ancestors, or 
between the offspring, that we remark upon as being inherited. We 
can only speak of a boy inheriting his father's shape of nose or his 
mother's color of eyes when his parents differ in these attributes. 
It has therefore become necessary to reverse in a sense the usual 
point of view with regard to heredity. Since it is inaccurate to say 
that heredity is measured by the degree of resemblance between 
* Presented by invitation at the Genetics Conference, San Francisco, August 4, 
1915- 
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