Page:Popular Science Monthly Volume 85.djvu/436

432 in the female and simplex in the male, and the chance anions of male and female gametes yield females (XX) and males (XO) in equal numbers.

In either sex many secondary sexual characters of the other sex are present during development, and traces of these may persist in the adult; but one set of these characters develops in the male and another in the female, so that they may be called sex-limited. The development of the secondary sex characters is usually determined by the ovaries or testes, which are the primary sex characters, though in some instances they may develop in animals which have lost their ovaries or testes, but in the last analysis both primary and secondary sex characters are dependent upon the sex determiner. Sex and sex-limited inheritance are only special cases of Mendelian inheritance in which conditions of dominance differ in the two sexes, depending upon whether the factor for sex is duplex or simplex.

In this connection we may consider another class of characters, which are linked with sex but are in no wise connected with sexual reproduction. Such characters are not necessarily limited to one sex or the other, as are many primary and secondary sexual characters, but they may appear in either sex, though they are usually transmitted from fathers to daughters, or from mothers to sons ("criss-cross" inheritance) in exactly the way in which the sex chromosomes (X) are transmitted. Morgan has therefore concluded that the factors for these characters are carried by the sex chromosomes and has named them sex-linked characters. In the fruit fly, Drosophila, he has discovered more than twenty-five such characters, applying to the color of the eyes and of the body, to the length of the wings, etc. A typical case is shown in Figs. 61 and 62. The eye color of this fly is normally red, but mutations have arisen in which the eye is white. Such a mutation always appears in males, though it may later be transferred to females, as we shall see. If now a white-eyed male and a red-eyed female are crossed all the F1s are red eyed, but if these F1s are interbred all the females of F2 have