(). WlN(iK 1 .*];■) 



chn>inam>inos, tho ono (liplnbimit having i\\r fnnimla A A,, the nthcr 

 CI (I,. Oil loriuation of tho ^aiiu»tt*s, we can thou i»])tjuii four ditVrn'Mt 

 typi»s, A — ii, — tt — <i, ; two fnun each. In /',, we have 2x2 = 4- ^cno- 

 typically tliHt'ivnt <iiploi<i combinations, viz. Aa Aa^ /I, rt -/!//,, and 

 those will, on nHiuction (livisi<»n, aj^ain throw af^ thr four gametes 

 A — Ax — (I and (/,. In F,^ an<l the foliowini; oftspring gencrationH, we 

 Clin by five combination of these find 10 i^enotypically ditic'rent coMd»i- 

 niitions, viz. A A — -1 A , — Aa — Aa^ — A ^A , — A ,a — A ,rr, — (ut — rm, — fr,r/, , 

 of which four are homozygotic ; one for each gamete type. 



On considering in the same way an organism with two chromosomes 

 in the haplophjuse (.c = 2), and presuming the segregation ex|K'riment 

 to commence with the crossing of two individuals having different 

 genotypic value for all the chromosomes of the diplopha.se ((^ight in all 

 in the two individuals) then each individual will be able to form four 

 gametes, or eight diti'erent gametes from both. On crossing these forms, 

 we obuin U) ditferent biotypes in I\: and in F.,, where the gametes (16 

 different) can combine altogether freely, there can arise 100 genotypically 

 distinct diploid types, of which 16 will be homozygotic in all charactei-s, 

 i.e. one for each gamete type formed by F^ . 



Where .r = 3, we can by crossing two different individuals easily 

 obtain, theoretically, eight gametes from each, i.e. 16 diffc'rent in all. 

 In Fi, 64 types will have arisen, able to form in all an ecpial number of 

 gametes, and in Fo, 1000, of which 64 are homozygotic. 



Brieffy then, if a segregation experiment he commenced by crossing 

 two individuals of the species to be investigated, then tve can in F^ obtain 

 4* and in F^ and the folloiuing generations 10^ genotypically distinct 

 fornus, of which 4^ will be homozygotic in all characters, whei^e x indicates 

 the haploid cJiromosome number of tJie species. 



If, for instiince, we commence by crossing two entirely diff*erent 

 individuals, heterozygotic throughout, of a species with eight chromo.somes 

 in the haploid phase, then in F^, there can arise 65586, and in F., a 

 milliard different types. 



On extending the analysis so as to include a further number of 

 individuals, as to whose genotypic constitution nothing is previously 

 known, then the number of possible combinations will of course be far 

 higher even than this, as also when "crossing over" takes place. In this 

 last case we cannot reckon the possibilities beforehand. 



The advantage of working with self-fertilising organisms is thus 

 entirely evident. It would be even better if we could make our genetic 

 experiments with organisms where the haplophase was a richly developed 



