286 JAMES F. CROW 



OVERDOMINANCE 



The concept of a stimulating effect of hybridization began independently 

 with Shull (1908, 1911b) and East (1908). It was assumed that there was a 

 physiological stimulus to development which increased with the diversity 

 of the uniting gametes — with increasing heterozygosis. East (1936) elabo- 

 rated the idea further by postulating a series of alleles each having positive 

 action functions, and with these functions to some extent cumulative. As the 

 alleles became more and more divergent in function, the action was postu- 

 lated to become more nearly additive in the heterozygote. 



At the time when East and Shull first formulated the hypothesis, there was 

 no direct evidence of any locus at which the heterozygote exceeded either 

 homozygote. For a number of years, overdominance as an explanation of 

 heterosis largely was given up because of the failure to find such loci. 



Stadler (1939) pointed out that in certain of the R alleles in corn a situa- 

 tion obtains in which certain heterozygotes have more areas pigmented than 

 either homozygote. He suggested that genes acting in this manner could re- 

 sult in overdominance for such characters as size and yield. Other such loci 

 are known in corn. 



There are now several cases in the literature of single genes with heterotic 

 effects. In most of these it is not possible to rule out the possibility of close 

 linkages giving pseudo-overdominant effects. In particular, many cases may 

 turn out to be pseudoallelism, but the consequences for the animal or plant 

 breeder would not be changed. 



Several workers (Teissier, 1942a; Robertson, unpublished) have found per- 

 sistent lethals in Drosophila population cage experiments. If these are not 

 due to individually heterotic loci, extremely close linkage must be postu- 

 lated. Also certain recessive genes, such as ebony, come to an equilibrium 

 with their normal alleles in population cages. One of the most convincing 

 cases is that of the eye color mutant described by Buzzati-Traverso in this 

 volume. This mutant persists in the population, and was found in three in- 

 dependent stocks. It is quite improbable that in each of these cases the gene 

 happened to be linked in the repulsion phase with another harmful recessive. 



The idea of superior heterozygotes has been upheld by Hull (1945) who 

 suggested the word overdominance. Hull's original argument for overdomi- 

 nance is a simple one. He noted that in most cases the hybrid between two 

 inbred maize lines has a greater yield than the sum of the two inbreds. This 

 would not be possible with dominant genes acting in a completely additive 

 manner — unless it were assumed that a plant with no favorable dominants 

 had a negative yield. 



The validity of this argument depends on the unimportance of epistasis 

 in corn yields. Evidence on this point is very incomplete and somewhat con- 

 tradictory. Neal (1935) reported that the Fo yields were almost exactly inter- 

 mediate between the Fi and the average of the parents. This would suggest tha t 



