172 AN INTRODUCTION TO MODERN GENETICS 



example of the effects of chromosome balance. The haploid chromo- 

 some number is twelve, and there are twelve primary trisomies, from 

 which one may determine the developmental effect of each chromo- 

 some; as might be expected, the effects are usually not confined to a 

 single part of the plant, but are rather general. The effects on the 

 shape and spikiness of the seed-capsules have been very fully described 

 and figured, and we shall confine our considerations to that organ. 



The effect of a given chromosome, as expressed in the trisomic 

 diploid, becomes exaggerated in the tetrasomic diploid, in which it is 

 represented four times, but is reduced in the pentasomic tetraploid 

 (4« + i); the 4^ + 2 form is very Hke the 2w + i, and the 4« + 3 is 

 sUghdy less extreme than the in + 2. Thus we have a clear case of the 

 dependence of phenotypic expression on the balance between opposing 

 tendencies ; one can compare this with the balance theory of sex deter- 

 mination in Drosophila and the cases of balance in individual "quantity 

 genes" discussed on p. 165. 



In the secondary trisomies, the redupUcated chromosome consists of 

 two similar ends; they can be compared, as regards dosage, with a 2w + 2 

 form, but here it is only half a chromosome which is reduphcated, not 

 a whole one. Typically the secondaries are less abnormal in appearance 

 than 2« -f- 2 plants, and, when compared with the primary from which 

 they arose, show an exaggeration of some but not all of its features. 

 Thus the primary Rolled has a redupUcation of the chromosome whose 

 ends are labelled as i : 2. From it two secondaries can be derived 

 according to which end is doubled; in one secondary, Polycarpic, the 

 trisome consists of two i-ends, while in Sugarloaf it consists of two 2- 

 ends. Each of these secondaries has an exaggeration of some of the 

 abnormal features of Rolled, which is more or less intermediate between 

 them. 



A rather similar state of affairs is found in the doubly trisomic 

 diploids, i.e. 2« + 2 forms in which the two extra chromosomes are 

 not aUke. The double trisomic usually combines the characters of the 

 two primaries. More complicated examples of balance occur in tertiary 

 trisomies, which are 2w + i forms in which the extra chromosome 

 consists of two parts from two other chromosomes, e.g. 2w + i : 6, 

 where i is a chromosome end from the trisome in the primary Rolled 

 and 6 is an end from the trisome in another primary Buckling. The 

 tertiary trisomic is, as might be expected, more or less intermediate 

 between the two secondaries to which it is related, e.g. 2« + i : 6 is 

 intermediate between 2« + i : i and 2w + 6 : 6. 



