IIKREDITY AND SEX 



231 



generation is called, Fi) will have the 

 spots large but not joined. If these 

 hybrids mate, tlie next generation (Fo) 

 will, in till' lo!ig run, consist of one indi- 

 \idual with spots small and separate 

 to two with spots large and separate 

 (hybrids) to one with spots large and 

 joined. This is shown in the insect hall 

 and in the figure on page 280. Half of 

 tlie 1"\> generation are hybrids, and if 

 mated with similar hybrids will give 

 offspring in these F2 proportions, 1:2:1. 

 The rest are ])ure. If spots-small- 

 and-separate be mated with spots-small- 

 and-separate all the offspring will have 

 the spots small and separate, no matter 

 what the previous ancestors were. Like- 

 wise spots-joined mated with spots-joined 

 can give only .spots-joined. 



Although this case has not been as 

 thoroughly studied as the others to be 

 mentioned here, it is cited first because 

 it shows clearly which are hybrids. In 

 the others the law of dominance is so 

 prominent that the simplicity of Mende- 

 lism is obscured. Let us analyze this 

 case by means of symbols. We will let 

 6' stand for spots small and separate and 

 J for spots joined. As every individual 

 is made up of two parts, maternal and 

 paternal, we will indicate individuals 

 by two letters. The beetles with which 

 we started are therefore ^\S' and ././. 

 The former produces germ cells each one 

 of which carries the factor »S, and each 

 of the germ cells of the latter carries ./. 

 United, these make a hybrid individual, 

 SJ. Now the essential point is that a 

 given germ cell can carry the factor for 

 only one condition of a given character. 

 Therefore hybrid asparagus beetles pro- 

 duce two kinds of germ cells, one bearing 

 S and the other ./. There are equal 

 numbers of each kind. An S sperm has 

 equal chances of fertilizing an S and a ./ 

 egg, giving equal numbers of SS and SJ 

 offspring. There are just as many ./ 

 sperm, and they ha\'e equal chances of 



fertilizing an S and a ./ egg and therefore 

 we should get a similar number of SJ and 

 ././ offspring. The total would be one 

 SS to two SJ to one J J. "Q.E.D." 



A further test consists in mating pure 

 indi\iduals with hybrids. SS produces 

 only S germ cells, and SJ equal numbers 

 of S and ./ germ cells. Therefore, there 

 will be an equal number of the combina- 

 tions, SS and SJ. See page 230. 



The ordinary "sour fly" or pnmice 

 fly {Drosophila ampelophila) has been 

 used more than any other species of ani- 

 mal or plant in the experimental study 

 of inheritance. The two examples used 

 in the insect hall and shown on page 232 

 are illustrations of simple Mendelism 

 plus the law of dominance. This is a 

 very slight complication and consists 

 merely in the fact that when two char- 

 acters are joined in the hybrid only one 

 (the "dominant" one) is evident. The 

 "recessive" character is there however, 

 and half of the germ cells produced by 

 such a hybrid bear only the recessive 

 character. If a pomice fly having 

 aborted wings of a certain kind be mated 

 with a piu-e normal-wanged fly, all the 

 offspring (hybrids, or Fi) will have 

 normal wings, for ntnnnal wing is domi- 

 nant and al>orted w'ing is recessive. If 

 these hyl)rids be mated together we 

 shall get in the F2 generation, one pin-e 

 normal-winged to /(t;ahyl)rid (but having 

 normal wings), to one pure aborted- 

 wnnged. More briefly, the ratio is three 

 normal-winged to one aborted-winged. 

 Although the eye can not distinguish 

 between the two kinds of normal-winged 

 Fo, breeding shows that they exist in 

 the proportions just mentioned. In the 

 second illustration, normal body color is 

 dominant and black is recessive. 



Mendel used peas in his own experi- 

 ments, and on page 228 is shown part of 

 the exhibit in the Darwin hall illustrating 

 these. The pair of characters concerned 

 is yellow seed color (dominant) and green 



