402 RADIATION BIOLOGY 



at this near-saturation level is another matter, for it would have been 

 subject to regulation by mutations in accessory genes as well as in the 

 "primary" gene. In this way the level could l)e prevented from being 

 set at an unduly high mark, despite the fact that the primary gene was 

 at near-saturation activity. The details of the phenomenon of "dosage 

 compensation," alluded to above, have made it clear that this adjustment 

 of the near-saturation level of the developmental reaction was as impor- 

 tant as its attainment of stability. 



14. DOMINANCE 



An important consequence of the high activity attained by the normal 

 gene, and of the strong similarity resulting therefrom between individuals 

 with different doses of the normal gene, is the fact that individuals having 

 a normal gene from one of their parents and a hypomorphic or amorphic 

 mutant allele from their other parent closely resemble, or are sensibly 

 identical with, those having a normal gene from both parents. That is, 

 the normal gene tends to be dominant, the mutant gene recessive. This is 

 to be expected if the effects of the two different alleles in an individual 

 having one normal and one hypomorphic or amorphic mutant gene tend 

 to be cumulative, hke those of added doses. For in such an individual 

 the character in question would be developed at least as strongly as in an 

 individual with but one dose of the normal gene and with a deficiency in 

 the homologous chromosome, and, as we have seen above, this level of 

 the character is practically as high as in the individual with two doses of 

 the normal gene. Some mutant genes, to be sure, exercise a somewhat 

 competitive action (see p. 404) and thus cause the effect, in an individual 

 having one such gene and one normal gene, to be less than that to be 

 expected from a simple addition of the two respective doses, but this 

 reduction is seldom enough to cause a perceptible deviation from the 

 normal phenotype. 



A few more definitions are in order before proceeding further. An 

 individual that has received from its parents two genes of identical type, 

 lying in a given position or locus in a given pair of homologous chromo- 

 somes, is said to be homozygous, or a homozygote, with respect to the genes 

 at that locus, while an individual which has received two different alleles 

 of that gene is termed heterozygous, or a heterozygote, for that locus. 

 Dominance, in the sense in which the word is sometimes used, implies 

 only that the phenotype of the heterozygote does not, in the case of a 

 given pair of alleles, stand midway between those of the two contrasted 

 homozygotes. In other words, some degree of dominance may be said to 

 exist whenever the heterozygote resembles one of the two homozygous 

 types distinctly more than it does the other one, and the gene for the 

 first type is then termed the dominant one. 



