MENDELISM AND OTHER METHODS OF DESCENT 209 



That polarit}^ of inheritance also exists in nature apart from 

 sex-inheritance is shown in numerous instances of dimorphism 

 and dichromatism. Some animals have dichromatism super- 

 posed upon sex, and many plants are dimorphic in which the 

 sexes are not separated. 



Partial Dominance. — Partial dominance is a form of polar 

 inheritance in which the diverse characters of the parents are 

 brought to expression in unequal numbers of the conjugates 

 and perjugates. The relation between the parent gametes is 

 not uniform, one of the divergent characters gaining expres- 

 sion as a dominant in one conjugate individual, but remaining 

 abeyant in another. Nevertheless, the gametes formed by 

 these unlike conjugates may follow the law of polar inheri- 

 tance. Equal numbers of gametes would then represent the 

 expression of the divergent parental characters, though the 

 partial dominance of the conjugate relation would again affect 

 the proportion in which the characters would be expressed 

 among the perjugate organisms. 



A good example of partial dominance is to be found in crosses 

 between CEnothera lata^ one of De Vries's mutations, and the 

 parent type O. laniai'ckiana. About 15 percent show charac- 

 ters of O. lata, that is O. lata is dominant to the extent of 15 

 percent, O. lamarckiana to the extent of 85 percent. As this 

 mutation does not itself produce any pollen, its characters must 

 be considered as receiving expression only in this condition of 

 partial dominance. The instance is interesting because of the 

 further fact that the lata mutation has very recently been found 

 to have only 14 chromosomes in the somatic cells, while the 

 lamarckiana form of the cross has at least 20.^ This shows 

 that dominance, at least, does not depend upon the number of 

 chromosomes. 



There is no assurance, of course, that a character once reces- 

 sive is always recessive, or that the proportions of expression 

 may not change. Even in Mendelian experiments different 

 crossings of the same stocks often show different results, as in 

 the case of the tailless fowls and other deviations from the 

 Mendelian expectations described by Professor Davenport. 



'Gates, R. R., 1907. Preliminarv Note on Pollen Development in CEnothera 

 lata De Vries and its Hybrids. Science, N. S., 25 : 259, and Bot. Gaz. 44 : 1-2 1. 



