Page:Popular Science Monthly Volume 85.djvu/439

Rh be color-blind. Consequently we have the curious phenomenon of simplex color-blindness appearing only in males and being transmitted to them only through apparently normal females.

 A color-blind male (here black) transmits his defect to his grandsons only. The corresponding distribution of the sex chromosomes is shown on the right, the one carrying the factor for color-blindness being black. (After Morgan.)

On the other hand, if a female is color-blind she has inherited it from both father and mother, i. e., the character in her is duplex, and in all of her children by a normal male the character will be simplex: accordingly, all of her sons will be color-blind and all of her daughters will be normal, though carrying the simplex determiner for color-blindness.

In all cases dominance means merely the development in offspring of certain characters of one parent, while contrasting characters of the other parent remain undeveloped. The appearance of any developed character in an organism depends upon many complicated reactions of germinal units to one another and to the environment. Under certain conditions of the germ or of the environment some characters may develop in hybrids to the exclusion of their opposites, whereas under other condition-these results may be reversed or the characters may be intermediate. The principle of dominance is not a fundamental part of Mendelian inheritance. Even when the characters of hybrids are intermediate between those of their parents, if the parental types reappear in the F2 generation we may be certain that we are dealing with cases of Mendelian inheritance.

3. The Principle of Segregation.—The individuality of inheritance units, and their segregation or separation in the sex cells and recombination in the zygote are fundamental principles of the Mendelian doctrine.