o^AB Ab aB 



ab 



AB 



Ab 



aB 



ab 



in the diagonal scries from AB to ab. All the others will be heterozygote, and will 

 continue to Mendelize out on being interbred to the F, generation, and so on. 



In this case since the original parents 

 were homozygote of the type Ab, aB, two 

 new homozygote combinations appear as new 

 types; e.g. the double dominant and the 

 double recessive : the latter is recognized at 

 once, but half the crop shows double domi- 

 nant appearance as well as the pure ^ : to 

 sort the latter out it will be necessary to grow 

 self-fertilized seed of a number of such 

 plants another season, and select the pure 

 strain which does not break. Assuming 

 tallness as dominant to dwarf habit, and 

 purple pigment dominant to white colour in 

 absence of pigment, the above scheme would 

 illustrate the crossing of a tall white pea with 

 a dwarf red. The same scheme holds for a 

 tall red crossed with a dwarf white ; in the 

 latter case the new types would be of the form Ab and aB. 



Trihybridiflm, involving working in terms of 3 factors simultaneously, would 

 extend similarly to 64 squares (8 gametes, o* and 9), giving a27*9*9*9*3*3*3-i 

 ratio, as 64 types of individual, but in 8 different aspects (phenotypes), and would 

 be correspondingly difficult to handle. But the idea suffices to show the possibilities 

 of increasing complexity, as organisms may be built up of wholly indefinite numbers 

 of factors, and two parents may differ in many characters. In practice it is sufficient 

 to consider one pair (allelomorph, presence or absence of one factor) at a time. 



Cy tological Interpretation : since Mendelism postulates that every gamete 

 carries one factor only of each pair derived from the parents, and these parental 

 factors are obviously mingled in syngamy, the conclusion appears unavoidable that 

 segregation of factors takes place at the complementary phase of meiosis. The 

 sorting out of P and M chromosomes, in giving the haploid number, is the visible 

 expression of such segregation. In the first meiotic spindle each daughter-nucleus 

 takes either a P or an M chromosome, but not both, for each set of characters the 

 individual chromosomes convey. 



This implies that all the factors of an organism are inherited through a 

 mechanism, in which they are divided into as many groups as there are in the 

 haploid number as a specific constant ; the possibility of such combinations of P and 

 M chromosomes is considerable, giving a great range of variation in the gametes ; 

 this being again increased by the possibilities of fertilization-fusion of any 2 such 

 combinations. The haploid number for Angiosperms ranges from 3 to 45. 



Neo-Mendelism introduces indefinite complications ; in connexion with : 



(1) Cytology, the chromosomes being regarded as factors (often causal) in the 

 mechanism ; cf. * sex '-chromosome, and number of associated units. 



(2) Mutations, since hybridization by affecting the zygote at a crucial period may 

 induce ' sports ' or ' mutants '. 



(3) The mechanism of segregation being plasmic and biological, may go wrong 

 in any respect, at any time. 



Hence the 3 component conceptions of Mendelism may fail ; e. g. : 

 (a) Dominance may be imperfect (case of ' blends ' and ' mosaic hybrids '). 

 (ft) Segregation may be affected, by influence of factors on one another ; as, com- 

 plementary factors, inhibiting factors, supplementary and cumulative factors, 

 reversion to type, ' crossing over '. 

 (y) Unit characters may go wrong, fail, or be lost ; non-Mendelian inheritance. 



From a comparatively simple new idea the subject rapidly grows in complexity ; 

 though undoubtedly the most exciting branch of modern Biology, emphasizing the 

 significance of minute variations in the progeny of the same parents ; all such progeny 

 in nature being heterozygote in some respect, however minute. This appears to be 

 the aim of all mechanisms of cross-fertilization, an absolutely pure strain is a fiction 

 of cultivation and autogamy. 



22 



