242 



THE AMERICAN MUSEUM JOURNAL 



bearing the X chromosomes: they will, 

 when uniting with an egg, produce 

 female individuals and, since half of the 

 eggs have the factor for red while the 

 other half have the factor for white, half 

 of the resulting females will be pure red, 

 while the other half will be hybrid, but 

 will have red eyes because red is domi- 

 nant over white. In other words, all of 

 the females of this generation show red 

 eyes. When the sperm lacking X chro- 

 mosomes unites with the eggs, half of 

 which have the factor for red in their X 

 chromosomes and the other half white, 

 the result will be males, half of which 

 will be pure red and the other half of 

 which will be pure white. This gives us 

 the result stated above; namely, all the 

 females and half of the males red-eyed 

 while the other half of the males are 

 white-eyed. This case may perhaps be 

 more readily understood by reference to 

 page 240, and page 241 shows the details 

 of the seconfl case mentioned above, 

 which involves what is known as " criss- 

 cross" inheritance. 



The relatively complicated " sex- 



\ 



C^- 



linked" inheritance just explained be- 

 came simple when the explanation was 

 found, and comes near to demonstrating 

 that there is a relation between heritable 

 characters and chromosomes. It would 

 probal)ly be carrying scientific scepticism 

 too far to continue doubting that it is 

 a causal relation. Ordinary Mendelian 

 characters, that is, those which come out 

 in F2 in the 3:1 ratio, are related to or 

 borne by the ordinary chromosomes, that 

 is, those chromosomes which are alike 

 and paired in each sex. The interested 

 reader may make diagrams, similar to 

 the ones given here, which will show the 

 mechanism graphically. Now that we 

 think we know where the something 

 which transmits a given character lies 

 in the germ cell, we begin to wonder 

 harder than ever what that something 

 is and how it does it. A number of big 

 steps have been taken in the explanation 

 of heredity and, although the goal is 

 still far ahead, by looking back over the 

 ground already covered we are encour- 

 aged to belie^■e that it will finally be 

 reached. 



1^ 



PARENTS 1 



Siinplf^ Mendelism illus- 

 (rat<id l)y cross between white 

 and red races of Mirahilis 

 Jalapa, giving pink hybrids 

 in Fi, which when inbred give 

 one while to two pink to one 

 red 



Ct.iirlesv of (U>liifi.l.iu Vnivprxi'v l'r<ss 



