366 LINKAGE PHENOMENA 



one of the means by which it is possible to tell in which 

 chromosome pair the genes for the different linkage 

 groups of characters are located. The first pair are 

 straight rods, the second and third ones are bent rods, 

 and the fourth pair are small round bodies. 



The Sex Chromosomes of Drosophila. — In the fe- 

 male the two members of the first pair of chromosomes 

 are indistinguishable and are called X chromosomes. In 

 the male, one of these chromosomes is slightly different 

 from its mate. One of them is like the pair in the female 

 and is in fact an X chromosome. The other one is called 

 the Y chromosome, and is not known to carry any genes. 



The Determination of Sex. — When the vinegar fly 

 forms its eggs, the two X's segregate from one another 

 along with the members of the other three pairs so that 

 every egg receives one X. In the male the X segregates 

 from the Y so that one half the sperms receive an X and 

 the other half receive a Y. There are thus two kinds of 

 sperms and but one of eggs. If an X-containing sperm 

 fertilizes an egg a female results (i.e., XX), but if a 

 Y-containing sperm fertilizes it a male (XY) results. 

 This general method of sex-determination appears to be 

 common among insects, birds and mammals, including 

 man. 



Linkage in Vinegar Fly Crosses. — Many hundreds 

 of experiments have been carried out in studies of linkage 

 with this insect, so that there is now a very large body 

 of data, whereas our knowledge of this phenomenon is far 

 less complete in respect to any other animal or plant. 

 For this reason the following paragraphs will deal with 

 vinegar fly crosses with the understanding that the prin- 

 ciples developed are in general applicable to other animals 

 and plants. 



The Back Cross is a method of breeding in which the 

 first generation hybrids are mated to pure recessives in- 

 stead of to one another. It is much used in analyzing 

 the genetic composition of hybrids because it gives simpler 



