10 THE ORIGIN OF GYNANDROMORPHS. 



The gynandromorph must have received the normal second chromo- 

 some from its mother (since normal autosomal characters only ap- 

 peared) and a second chromosome from its father with the three 

 recessive genes. Since neither male nor female parts show these 

 recessive genes, two second chromosomes must have been present in 

 all the nuclei, both in the male and in the female parts. 



FREQUENCY OF OCCURRENCE OF GYNANDROMORPHS. 



In general, we have no record of the frequency of the occurrence 

 of gynandromorphs. They are found from week to week, their number 

 being roughly in proportion to the number of flies passing under 

 observation, and also in proportion to the care with which the flies 

 are scrutinized in detail. On four occasions, however, the frequency 

 of their appearance was recorded. 



In the first case (in 1914) a cross, involving yellow flies, white-eyed 

 and eosin-eyed flies, and wild-type flies, seemed to give gynandromorphs 

 more often than usual. It is to be noticed that the striking color 

 differences of eye and body in this combination would, as a rule, make it 

 easy to detect hybrid gynandromorphs, and their frequency may have 

 been due to this fact. In all 32 gynandromorphs were found in a total 

 of 42,409 flies, or 1 in 1,325. 



Duncan, in 1915, made a careful examination of hybrid flies and 

 found 3 gynandromorphs in a total of 16,637 flies, or 1 in 5,500. All 

 flies were so thoroughly scrutinized that probably most of the gynan- 

 dromorphs that occurred were found. 



The third set of observations was made on material that was chosen 

 because, in addition to sex-linked factors, autosomal genes were present, 

 which should give an answer to the three contrasted hypotheses de- 

 scribed in the preceding pages. In all, 2 gynandromorphs were found 

 in a total of 4,979 flies. 



A fourth record made by Sturtevant also involved autosomal as 

 well as sex-linked characters. Forked females were mated to males 

 with normal bristles. The female was heterozygous for the second- 

 chromosome genes, black, purple, curved; the male homozygous for 

 the same genes; 3 gynandromorphs were found in about 24,000 

 offspring. 



Taking all these results together, the observed ratio is 1 gynandro- 

 morph in 2,200 flies. 



Whenever the chromosomal elimination occurs at an early stage in 

 development, or when the color or structural difference involved is 

 striking, the gynandromorph is more likely to be found than when the 

 contrary conditions are present. If elimination occurs late in develop- 

 ment the region affected may be so small as to escape detection. 

 It seems probable, therefore, that such irregularities may be more 

 frequent than the figures given above indicate. 



