HEREDITY AM) SEX 



239 



cntrrs and not at all upon the inoTlicr; 

 and the nunilx-r of each sex will, in the 

 long run, he eciual. All this is, of course, 

 subject to amendment hy further inves- 

 tifiation, and too s\veepin<;' ii;eneraliza- 

 tions should not be made, but it, or a 

 similar relation, seems to hold for other 

 strictly bisexual animals and it is the 

 only explanation for the following', 

 among other, facts. 



A few i)()mice Hies were fomid ha\ing 

 white eyes instead of red. This white 

 condition is recessive to red but in in- 

 heritance the proportions are not those 

 of simple ]\Iendelism. In what has gone 

 before nothing was said about sex, 

 because characters which have been 

 previously mentioned occur without 

 regard to it. This particular eye color 

 however, is one of a number of characters 

 which are "sex linked." If a white- 

 eyed male be mated with a pure red- 

 eyed female (see page 240), all the oflF- 

 spring, both male and female, will have 

 red eyes. If these offspring be mated 

 with one another, all the females of the 

 next generation will have red eyes, but 

 half of the males will have white eyes 

 and only half will have red eyes. On the 

 other hand, if a red-eyed male be mated 

 with a white-eyed female (see page 241), 

 all the male offspring will have white 

 eyes and all the female offspring will 

 ha\e red eyes. This is what has l)een 

 called "criss-cross" inheritance — the 

 sons being like their mother and the 

 daughters like their father. If these 

 offspring be mated with one another, 

 half of the male and half of the female 

 offspring will haxe white eyes, the re- 

 mainder ha\ing red eyes. 



The explanation is as follows: This 

 pair of characters, red eye versus white 

 eye, is associated with the X, or sex, 

 chromosome. In the first case men- 

 tioned the female was pure with respect 

 to this eye-color character; that is, both 



of the X chromosomes carried the factor 

 for red eye color (see page 240). The 

 male, since it showed the recessive char- 

 acter, nuist have been pure with respect 

 to white eye color and, furthermore, all 

 males are necessarily pure with respect 

 to this particular pair of eye colors, and 

 also with respect to all other sex-linked 

 characters, since they ha\e but one X 

 chromosome, and since that chromosome, 

 like any other, can l)ear the factor for 

 only one of a pair of characters. All of 

 the eggs, in this mating, carried the fac- 

 tor for red eye color. Half of the sperm 

 carried the factor for white eye color 

 and the other half had no factor con- 

 cerned with this pair of characters. If 

 a sperm bearing the factor for white eye 

 color united with an egg, the offspring 

 woidd be a hybrid since it contained 

 factors for both eye colors, but, since 

 red is dominant o\-er white in this case, 

 this individual woidd show the red color. 

 It would also be a female since the union 

 which produced it was with a sperm 

 ha\ing an X chromosome. If a sperm 

 not bearing the X chromosome (that is, 

 one with the Y) united with one of the 

 eggs, all of which bore the factor for red 

 eye color, the result would be a male 

 pure with respect to red eye color, since 

 the only factor concerned with this pair 

 of characters came with the egg and was 

 red. In other words, all the females of 

 this generation had red eyes and were 

 hybrids with respect to eye color, while 

 all the males had red eyes and were pure 

 with respect to eye color. Half of the 

 eggs which go to produce the next 

 generation bear the factor for red eye 

 color, and the other half bear the factor 

 for white eye color. Half of the sperm 

 have X chromosomes bearing the factor 

 for red eye color, and the other half 

 have no X chromosomes, and thus have 

 no influence upon eye color. Taking 

 up the first class of sperm, namely, those 



