CHROMOSOMES AND SEX 391 



nificance of this with respect to sex and other features has not been 

 determined. 



The first known case of XF-chromosomes in digametic females in 

 plants was that of Fragaria elatior. The genetic evidence that the car- 

 pellate plants in Fragaria produce two sorts of megaspores, and therefore 

 female gametes, is supported by cytological studies on F. elatior. In the 

 megasporocyte there are 21 pairs, including an unequal XY pair which 

 disjoins in the first meiotic mitosis, each member then dividing in the 

 second. Fertilization of X-eggs and F-eggs by X-sperms thus gives 

 staminate plants (X'X) and carpellate plants {XY), respectively. The 

 breeding behavior of Thalictrum Fendleri shows that this species also is 

 heterozygous for sex in the female. ^"^ 



Sex-linkage. — As explained in Chapter XVII, characters w^hich are 

 dependent upon differential genes located in the same chromosome are 

 "linked" in inheritance. Since the sex-chromosomes carry genes affect- 

 ing non-sexual characters also, such characters exhibit a peculiar rela- 

 tion to sex in successive generations: they are sex-linked characters. ^^ 

 The following example will make this clear. 



When a red-eyed male Drosophila is mated to a white-eyed female, 

 the Fi individuals are white-eyed males and red-eyed females; each 

 eye-color is, as it were, transferred to the opposite sex (Fig. 225). When 

 the Fi flies are interbred, the resulting F^ generation comprises four 

 types: red-eyed males and females, and white-eyed males and females. 

 It is observed that red eye-color appears in every fly, male or female, 

 which possesses a derivative of the X^-chromosome of the original red-eyed 

 male ; whereas, white eyes characterize every male deriving the X-chromo- 

 some from the original white-eyed female, and in every female having two 

 such chromosomes. This is because the original male's X-chromosome 

 carries a dominant factor for red eyes, while each of the original female's 

 X-chromosomes carries a recessive factor for white eyes. This explana- 

 tion also applies to the reciprocal cross (white-eyed male X red-eyed 

 female), in which, however, the relative proportions of red-eyed and 

 white-eyed flies in Fi and /^2 are different: in Fi all flies of both sexes have 

 red eyes, while in F2 all of the females and one half of the males have red 

 eyes, white eyes appearing only in one half of the males. 



'^ Genetic evidence in Fragaria: C. Richardson (1914), Valleau (1923), Correns 

 (1928a). Cytological: Kihara (1926, 19306). Thalictrum: Kuhn (1930d). 



^^ Sex-linked characters are not to be confused with sex-limited characters. The 

 latter are those found exclusively in individuals of one sex and are usually called 

 "secondary sexual characters." In several cases it has been shown that their genes 

 are located in the autosomes. It is suggested by Winge (1922a) that inheritance of 

 secondary sexual characters through autosomes be referred to as "sex-limited" 

 inheritance, that of characters through the sex-chromosome common to both sexes 

 (X) as "sex-linked" inheritance, and that of characters through the sex-chromosome 

 occurring in only one sex (F) as "one-sided" inheritance. 



