484 



JOHN W. GOWEN 



— much like major genes — with given degrees of dominance. Within each 

 group the genes may have any known type of gene action so long as the quali- 

 ty of additivity of their effect between chromosomes is maintained. 



EFFECT OF CHROMOSOME LENGTH ON HYBRID VIGOR 



The analysis of variance shows that the effects of the three chromosome 

 pairs differed widely. These differences could be due to differential numbers 

 of gene loci within the separate chromosomes, to varying magnitude of gene 

 effects, or to both. The data do not allow us to positively distinguish between 

 these hypotheses. Proportionality between the effects of the three chromo- 

 somes and their sizes would favor the first interpretation. 



There are several different measures of chromosome size. These measure- 

 ments of the different chromosomes may be compared with their heterotic 

 effects in a least square test. 



The proportionality between heterotic effects and chromosome lengths 

 was as follows: 



Metaphase length 



Salivary length 



Salivary bands 



Per cent visible loci 



Cross over length 



Observed heterotic ^7 i„„ 



pffprt J 7 day.. 



^^^^^^ \A11 data 



Closeness of Agreement 



7-Day Data 



X 



X 



XX 



XX 



XX 



All Data 



X 

 X 

 X 



XX 

 X 



X significantly closer 1 : 20 or 

 X X highly significant 1 : 100 



Excellent agreements are observed between per cent of visible loci or the 

 crossing-over units with the heterotic effects of the chromosomes. Less agree- 

 ment is noted between the number of bands in the salivary chromosomes and 

 the heterotic effect. The metaphase lengths of chromosomes or the physical 

 lengths of the salivary gland chromosomes are less closely related to heterotic 

 effects. All comparisons of chromosome sizes with heterotic effect give excel- 

 lent to fair correlations. In general, the heterotic effect is distributed accord- 

 ing to random distribution of several genes to the various chromosomes. This 

 favors the view that the heterotic effect is due to many gene pairs in each 

 chromosome, rather than to one having a specific additive phenotypic effect. 

 These genes would be randomly distributed to the different loci within the 

 chromosome. 



GENOME CONTRIBUTIONS TO HYBRID VIGOR 



Average combining ability of one inbred line when mated to several lines 

 is called general. The genomes of an inbred line can be regarded as uniform 



