MUTATION OF TRISOMICS 355 



The 8-gamete will give with gandens a partly homozygous and 

 partly heterozygous trisomic : — 



(iii) A,B C^D j^^ p^^ ^^^ K,,L M^N 



BgC ^^^ ^^^^ ^^s^^ LgM NgA 



With velans it will give a different but analogous result : — 



D^E FgG H,K 



\ / \ / 

 E^F G.JI 



(iv) (A,B\ (C,,D\ ^^\ ^\ /^^ (K,,L\ (M,,N\ 



With the first type of 8-gamete any one of the twelve chromosomes 

 in the ring can be present in addition to the six (and O P) of 

 the opposite complex. In this way twelve kinds of 8-chromosome 

 gamete should be formed and these might combine with either 

 velans or gaudens normal gametes and thus give twenty-four 

 potential types of trisomic. With the second type the number of 

 gametic combinations is twenty-four, and these will give forty- 

 eight potential trisomic types. Including the trisomic derived 

 from the bivalent {OP — OP), we thus have seventy-three types in 

 all. Of these a large proportion will be homozygous for bivalent 

 forming chromosomes which in the diploid parent species are 

 heterozygous and within the ring. 



A large number of these potential types of genetically different 

 trisomies have actually been identified by Catcheside and Emerson 

 in (E. Lamarckiana ; the proportion of the seventy-three which 

 succeed depends of course on the viability of the combinations of 

 the new types of gamete with normal velans and gaudens, i.e., on 

 the distribution of differential segments (or lethal genes) amongst 

 the chromosomes. 



Some of the trisomies, as we saw, have the remarkable property 

 of being effectively true-breeding (Nos. (ii), (iii) and (iv) ). Type (ii) 

 is important. It will produce only two kinds of functional gametes, 

 one with a normal complex which is non-viable in the homozygous 

 state and one which has an extra chromosome and does not function 

 in the pollen. This trisomic type is homozygous for a particular 

 complex which exists only in the heterozygous state in the diploid 

 species from which the trisomic is derived. On a specific lethal 

 gene hypothesis it would be assumed that the particular extra 



