ORCHID CONFERENCE. 



415 



altogether absent from the zygote, and the plant v/ill be zerozygous, 

 or wanting in regard to that factor. 



Peactical Consequences. 



The practical consequences of these first principles of Genetics are 

 that, as regards any one heritable character represented by a factor, 

 there are three distinct kinds of individual plants : (1) homozygous, or 

 pure ; (2) heterozygous, or impure ; and (3) zerozygous, or wanting. Each 

 of these three kinds of individuals will give a different result when bred 

 from. With regard to outw^ard appearance, (3) will as a rule be easily 

 distinguishable from (1) and (2), representing as it usually does the 

 hypostatic or recessive form, from which the epistatic or dominant 

 character is quite absent. 



If, as sometimes happens, a single dose of the dominant factor 

 produces the same effect in the zygote as a double dose, we get the 

 phenomenon of complete dominance, and in such cases (1) and (2) are 

 indistinguishable in outward appearance, and their pure or impure 

 nature can only be determined by breeding from them. 



If, however, as often happens in Orchids, a single dose of the 

 dominant factor produces a different effect in the zygote from a double 

 dose, we get the phenomenon of incomplete dominance. In such cases 

 (1) and (2) are. distinguishable in outward appearance, and their pure 

 or impure nature is determined at sight without breeding from them. 



The apparent blending of characters observed in* many Orchid 

 hybrids is no doubt due to the effect of the single dose of one factor 

 reacting on the effect of a single dose of another factor, resulting in a 

 mosaic. 



When more than one heritable character is involved in the breeding 

 problem, as usually happens in Orchids, the different kinds of individual 

 plants become exceedingly .numerous. One plant, for instance, may 

 be pure for one character, impure for another, and wanting for a third 

 character, and so on. Each individual plant, however, has a definite 

 germinal constitution, which can be ascertained by cross-breeding 

 it with other plants. In this way a factorial analysis of the two plants 

 can be made, and, as regards each heritable character, each plant 

 will be found to be either pure (homozygous), impure (heterozygous), 

 or wanting (zerozygous), and as such it will behave when bred from. 

 Once the factorial analysis of a plant has been made, all future breeding 

 from that plant is reduced practically to a certainty. 



Colour and Albinism, 



Recent results show that in certain cases an apparently simple 



heritable character is due to the presence of more than one distinct 



factor. For instance, in Orchids we have already one case of that 



description, and no doubt there are many more yet undiscovered. 



From evidence collected by the writer during the past five years,''' 



* Bateson, W., MendeVs Principles of Heredity (1909), pp. 96-97; Hurst, C. ('., 

 ^ Inheritance of Albinism in Orchids," Gard. Chron. (1909), i. p. 81; Hurst, C. C, 

 Mendel's Law of Heredity and its Application to Horticulture," Journ. Boy. Hort . 

 ^oc. vol. xxxvi. (1910), pp. 44-48, figs. 20-34. > 



