222 THE SECOND-CHROMOSOME GROUP 



STREAK (5,). 



(Plate 5, figure 5, and Plate 10, figure 2.) 

 ORIGIN OF STREAK. 



In a stock culture from a pair of flies with the mutant called 'Top- 

 wing" (culture C 149, November 27, 1912), Bridges found a single 

 female which had a prominent broad, dark streak down the middle of 

 the thorax. 



STOCK OF STREAK. 



This female (non-virgin) was mated to several of her brothers and 

 produced many streaks among the offspring. It was assumed that 

 the character was recessive and that some of the brothers had been 

 heterozygous. No Fi counts were made and not much attention was 

 paid to the character. 



Several of the streak individuals were mated together to provide 

 stock. In this F2 culture somewhat more than half (no counts) of the 

 offspring were streak where all had been expected to be streak. This 

 was thought to indicate a ''poor" character, which, like truncate, club 

 (wings), and others, shows in a variable proportion of the flies of the 

 same genetic constitution. The stock was carried on in this way for 

 two more generations, when it was decided to throw it away as being 

 too poor to repay further labor. This would have been done had not 

 Morgan seen in this character a bearing on a selection problem which 

 he had been carrying out for over two years on the thorax pattern 

 of "with" flies. In the course of selections for a still darker pattern 

 three notable successes had been obtained, all of which turned out to 

 be simply new mutations (speck, oUve, and band) which had occurred 

 in the selected stocks, but which gave no further variability or progress 

 when once the stocks were pure for them. In streak there was an 

 example of a dark thorax character which closely resembled in pattern 

 the darkest of the long selected "bands," though not as dense in pig- 

 mentation, but which had arisen entirely independent of any selection 

 whatsoever. 



Later the trident mutant "trefoil" (plate 5, fig. 6) likewise arose 

 independent of selection. These independent mutations and the fact 

 that during this same period over a hundred other mutations affecting 

 every part of the body had appeared in Drosophila, left no basis what- 

 ever for the supposition that the selection had had any effect what- 

 ever on either the frequency or the nature of the mutations, or that any 

 other process, aside from the production of three definite mutations, 

 had contributed to the success of the selection. 



Morgan selected the streak stock for about six months, though not 

 very vigorously, without increasing the intensity of the pattern or the 

 frequency of the streak individuals. 



