GENETICS OF HETEROMORPHIC CHROMOSOMES 469 



12, has opened with the long mstead of the short arms free, 

 but this is a different type of phenomenon.) 



d. Mating number I4 (pis. 3 and 4, i^ow 15 cf parent, row 16 

 9 parent, rows 17-28 cf offspring). Thisis the most interesting 



mating obtained so far, on account of one homologue of the 

 seventh pair in the father having a nearly median fiber attach- 

 ment, so that one free arm is about twice as long as the other 

 when the tetrad opens in the more usual manner. This condi- 

 tion is constant in the male parent. The other homologue of 

 this pair has the usual short free arm, so that the combination 

 is heteromorphic and atelomitic. In the mother the correspond- 

 ing pair is also heteromorphic; one member being telomitic, the 

 other atelomitic, so that four combinations are possible in the 

 offspring (text fig. B). Theoretically, these should occur in 

 equal numbers; for the twelve male offspring we should expect 

 a 3:3 : 3:3 ratio, actually it was 2:3 : 5:2, but when we take 

 into account that these are the recombinations in the tetrads of 

 such a small number of individuals, the result is as close as can 

 reasonably be expected. A more direct comparision is between 

 the number of homologues of each of the three types expected 

 and the number actually obtained. Since the father contributes 

 one long-armed atelomitic and the mother one telomitic, while 

 both parents contribute a short-armed atelomitic, we should 

 expect a ratio of 6:6:12. The actual ratio is 5:4:15. 



Chromosome pair number 8 is also heteromorphic in both 

 parents. One would therefore expect twelve of each of the two 

 types of homologues. The numbers obtained are ten atelomitic 

 to fourteen telomitic. The combinations in the tetrads where 

 one would expect 3:6:3 are 2:6:4. 



Pair number 1 is telomitic in both parents and in all of the off- 

 spring studied. Had the two types of homologues of this pair 

 followed their respective frequency of occurrence in the species, 

 four homologues should have shifted to the atelomitic condition. 



e. Mating number 17 {pi. 5, row 29 cf parent, rows 30-35 cf 

 offspring) . The female used in this cross died, hence knowledge 

 of her complex was not obtained, but if we accept the foregoing 

 evidence that the architecture of the chromosomes is transmitted 



JOURNAL OF MORPHOLOGY, VOL. 35, NO. 2 



