214 Biology in America 



The results of the crosses above described can be most 

 readily explained by assuming that one of the X chromosomes 

 cariies the determiner for red eye color and another X the 

 determiner for white. They are shown graphically in the 

 accompanying diagrams, in which the chromosomes carrying 

 the determiner for red eyes are shown as capitals, and those 

 carrying the white eye determiner as small letters. 



Many other cases of sex-linked inheritance occur in animals, 

 notably in moths, fish, birds and mammals, including man. 

 In the latter, color blindness and haemophilia (imperfect clot- 

 ting of the blood causing continuous flow from wounds) are 

 examples, although the manner of inheritance here is slightly 

 different than that of eye color in the fruit fly. 



While the phenomena of linkage are clearly shown in the 

 fruit fly, not only in the eye color but also in color of wings 

 and body, size of wings, etc., the linkage in many cases is 

 imperfect, some of the combinations of characters in the off- 

 spring being different from those shown by the parents. 



Some fruit flies have vestigial wings and black, bodies, 

 while the ordinary type has long gray wings and a light yellow 

 body, banded with black on the abdomen. Black body and 

 vestigial wings tend to stay together in inheritance, indicating 

 that their determiners are both lodged in the same chromo- 

 some. But there are some exceptions to this rule. If a 

 black fly with vestigial wings be crossed with one having long 

 gray wings, the offspring will all have long gray wings, this 

 type dominating the former. If now we "back cross" these 

 offspring with the black vestigial parent, we obtain a curious 

 result. If on the one hand we back cross a hybrid male with 

 a black vestigial female, we obtain only black vestigial and 

 gray long flies. The characters have "stuck together," com- 

 ing out of the cross in the same combination in which they 

 entered it. The linkage is perfect and the evidence is strong 

 that the determiners for the characters tested are lodged in 

 the same chromosome (black vestigial in one, gray long in 

 another), which retains its identity throughout the matura- 

 tion and fertilization processes. But if, on the other hand, 

 we back cross a female hybrid and a black vestigial male we 

 obtain a very different result; namely, 41.5% of black ves- 

 tigial and gray long offspring respectively, and 8.5% of black 

 long and gray vestigial. In other words, some of the charac- 

 ters have become mixed in the shuffle and the cards are not 

 dealt "according to Hoyle." 



Is such a result a fatal blow to the chromosome hypothesis ? 

 On the contrary it furnishes indirectly one of the strongest 

 evidences in its favor. 



We have seen above that the chromosomes pair with one 



