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THE POPULAR SCIENCE MONTHLY 



Gametes 



Fi 



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-lim- 

 ited. The development of the sec- 

 ondary sex characters is usually 



determined by the ovaries or 

 Fig. 60. Diagram Showing Sex as a j 



Mendelian character, the Female being testes, which are the primary sex 

 homozygous, the Male heterozygous for characters, though in some in- 



Sex. The female forms gametes all of ' ° 



which contain the x-chromosome ; the male stances they may develop in ani- 



fprms two sorts of gametes one half of malg which haye logt their ovar j es 

 which contain the X-chromosome and the 



other half lack it. All possible combina- or testes, but in the last analysis 



tions of these gametes give a 2 : 2 or 1 : 1 both pr i mary am } seCOndarv sex 

 ratio of females to males. 



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. 



Sex-linked Inheritance 



In this connection we may consider another class of characters, 

 which are linked with sex but are in no wise connected with sexual repro- 

 duction. 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" inheri- 

 tance) in exactly the way in which the sex chromosomes (X) are trans- 

 mitted. Morgan has therefore concluded that the factors for these 

 characters are carried by the sex chromosomes and has named them sex- 

 Unked 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 F,s 

 are red eyed, but if these F x s are interbred all the females of F 2 have 



