210 GENETICS IN RELATION TO AGRICULTURE 



quadruple allelomorphs, the factors for striped, moricaud, normal, and plain 

 larval pattern, and a pair of factors for yellow and white cocoon color. 

 Sturtevant has pointed out that the experimental results are explicable if 

 there is no crossing-over in the female, the sex-heterozygote in this case. 

 Typical results are given by crosses involving striped, P t , and plain, p, 

 larval patterns and yellow, F, and white, y, cocoon colors. Thus striped 

 yellow (P,F)(P.F) crossed with plain white (py}(py) gives in FI striped 

 yellow individuals of the constitution (P t Y)(py) in both sexes. When 

 FI males were crossed back to plain white females, there were obtained 

 2907 individuals of which 865 were striped white and plain yellow, which 

 are the cross-over classes. This gives a value of 29.8 per cent, for cross- 

 ing-over in the male. Similarly striped yellow males of the genetic con- 

 stitution (P,y)(pY) crossed back to plain 1 whites gave 488 individuals 

 of which 151 were striped yellow and plain white, which are the cross- 

 over classes in this case. The value for crossing-over in this latter case 

 is 30.9 per cent., substantially in agreement with the previous calculation. 

 These results are to be compared with those obtained by crossing back 

 striped yellow females of the genetic constitution (P s F)(p?/) to plain 

 white males. From such crosses 1183 offspring were reared all of which 

 were either striped yellow or plain white, consequently non-cross-overs. 

 In both types of sex-inheritance, therefore, no crossing-over occurs in the 

 sex-heterozygote. However, in plants which have the male and female 

 organs in the same individual, crossing-over takes place both in the 

 formation of pollen grains and ovules. 



The relations exhibited in sex-determination in some insects are 

 extremely complex and present many differences from the simple types 

 which have been described above. Much painstaking cytological in- 

 vestigation has been done in determining these intricate relations with 

 results which for the most part confirm our general observation as to the 

 essential role played by the chromosomes. One of the simplest cases is 

 that of the honey bee. As is well known there are three forms of the 

 honey bee; the queens, the drones, and the workers. Worker bees are 

 females with their sex organs undeveloped as a result of the kind of food 

 furnished them during the larval state. Queen bees lay fertilized or 

 unfertilized eggs. From the former, worker bees and queen bees develop 

 according as to whether they are provided with royal jelly in the larval 

 stage. Unfertilized eggs on the other hand always give drones. Along 

 with these observations it should be noted that under exceptional con- 

 ditions worker bees lay eggs and these always develop into drones. 

 From a chromosome standpoint, therefore, queen bees and worker bees 



1 This cross as reported by Tanaka actually involved moricaud larval pattern 

 not plain larval pattern, but, as previously stated, it has been proved that the factor 

 for moricaud occupies the same locus as the factor for plain. 



