gene series likewise holds true for all genes 

 in each of the other three pairs of chromo- 

 somes in Drosophila. In fact there is one, and 

 only one, linear order that shows this rela- 

 tion. Accordingly, the only possible interpre- 

 tation of these facts is that the genes are ar- 

 ranged in the chromosomes in a precisely 

 determined linear order, like a string of 

 beads. 



As a result of such experiments, accurate 

 chromosome maps (Fig. 26-19) have been 

 prepared for Drosophila and, to a lesser ex- 

 tent, for a number of other organisms. Al- 

 though Drosophila has been studied much 

 more intensively than other organisms, not 

 all the genes of Drosophila are known. In 

 Drosophila there are probably about three 

 thousand genes, of which about half of those 

 known are shown on the maps. Map I is for 

 the long straight pair of the fruit fly's chro- 

 mosomes; II and III are for the smaller and 

 larger of the V chromosomes, respectively; 

 while IV is for the very small pair (Fig. 26- 

 14). All the genes in each pair of chromo- 

 somes assort independently of genes in the 

 other three pairs; and the genes localized in 

 each pair of chromosomes display a linked 

 inheritance — which is complete in male ani- 

 mals and incomplete in females. A cytological 

 study of the germ cells during the prophase 

 of the first meiotic division indicates that 

 synapsis, as it occurs in the male Drosophila, 

 is more transient and less intimate than is 

 the general rule. But precisely why crossing 

 over is entirely aborted in the male fruit fly, 

 whereas it occurs successfully in both sexes 

 of almost every other species of animal, can- 

 not be said with certainty. 



Morgan's Laws. The phenomena of link- 

 age apply to virtually all organisms, although 

 they were worked out by Morgan and his 

 students mainly in Drosophila. Thus it is 

 customary to designate the last four of the 

 six well-recognized laws of inheritance as 

 Morgan's laws: 



1. All the genes of a species are arranged 

 into linkage groups that correspond 

 exactly to the chromosomes of the spe- 



Heredity - 495 



cies; and the genes in any one pair of 

 chromosomes are assorted during hered- 

 itary transmission independently of the 

 genes in the other pairs. 



2. All genes in the same chromosome are 

 linked in heredity, maintaining their 

 original combinations throughout suc- 

 cessive generations, either always {com- 

 plete linkage), or more often than not 

 (incomplete linkage). 



3. The genes in each chromosome are 

 arranged in a specific linear order. 

 Thus the cross-over frequencies be- 

 txveen them have the same mathemati- 

 cal relations as distances between points 

 in a straight line. 



4. Crossing over at any one point inter- 

 feres with cross overs at neighboring 

 points; for short internals this inter- 

 ference is complete, so that for genes 

 relatively close together in a series, tlie 

 observed percentage of recombinations 

 is an accurate measure of the cross-over 

 frequency. 



INHERITANCE OF SEX 



As a general rule among sexual organisms, 

 males and females are produced in equal or 

 almost equal numbers in every generation, 

 and each of the sexes can mate successfully 

 only with the other. 



The inheritance of sex in a majority of 

 organisms shows that a single pair of chro- 

 mosomes is the differential that determines 

 whether an individual is to be male or fe- 

 male. Actually the situation is somewhat 

 more complex than this, as will become ap- 

 parent presently. Nevertheless it is certain 

 that the sex differences between individuals 

 of the same species are almost always trans- 

 mitted via genes, although in the case of 

 some animals, the developmental effects of 

 the "sex genes" are mediated largely through 

 the endocrine glands. 



In many species the chromosomes of the 

 female are visibly dissimilar from those of 

 the male. In the female Drosophila, for ex- 



