Changes Involving Unbroken Chromosomes 



157 



We see, therefore, that meiosis produces 

 many aneusomic gametes when the number 

 of homologs is odd, as it is in triploids, 

 pentaploids, etc. In tetraploids, since each 

 chromosome can have a partner at meiosis, 

 the four homologs often segregate two and 

 two. Sometimes, however, the four homo- 

 logs form a trivalent and segregate three and 

 one, so that some aneusomic gametes are 

 produced by polyploids with even numbers 

 of homologs. 



Because the phenotypic effect of any gene 

 depends directly or indirectly upon the 

 phenotypic effects of most, if not all, of 

 the other genes present, it is expected that 

 a diploid individual contains, in its two sets 

 of chromosomes, a proper balance of genes 

 for the production of a successful phenotype. 

 It is not surprising, then, that a haploid in- 

 dividual mated to a diploid produces very 

 few progeny, since after fertilization most 

 zygotes are chromosomally unbalanced by 

 the absence of one or more chromosomes 

 needed to make two complete genomes. 

 Mated to a diploid the triploid individual 

 also produces zygotes that are imbalanced 

 but in the opposite direction, having one or 

 more chromosomes in excess of two ge- 

 nomes. 



In matings with diploids, however, the 

 triploid individual usually produces more 

 offspring than the haploid. This observa- 

 tion can be explained as the result of the 

 lesser imbalance brought about by the addi- 

 tion of chromosomes to the diploid condi- 

 tion than by the subtraction of chromosomes 

 from it. This effect can be seen by com- 

 paring how far from normality (diploidy) 

 each of the two abnormal conditions is. 

 When one chromosome is in excess, the ab- 

 normal chromosome number of three is one 

 and a half times larger than the normal num- 

 ber of two; when one chromosome is miss- 

 ing, the abnormal chromosome number of 

 one is two times smaller than the normal 

 number. Thus, the addition of a chromo- 



some makes for a less drastic change in bal- 

 ance than the subtraction. Accordingly, 

 knowing that the triple dose of a large auto- 

 some is lethal in Drosophila, we can cor- 

 rectly predict that the single dose is lethal 

 also. In these cases, death is attributable 

 to genetic imbalance due to an excess of the 

 genes present in a long autosome in trisomic 

 individuals and to a deficiency of these 

 genes in monosomic individuals. 



2. In Datura 



Chromosome addition and subtraction can 

 also be studied in Datura 3 whose haploid 

 chromosome number is twelve. It is pos- 

 sible to obtain twelve different kinds of in- 

 dividuals, each having a different one of the 

 twelve chromosomes in addition to the dip- 

 loid number. Each of these trisomies is 

 given a different name such as "Globe." It 

 is also possible to obtain viable plants that 

 are diploid but missing one chromosome of 

 a pair; these are monosomies or haplosomics. 

 Individuals with two extra chromosomes of 

 the same type (tetrasomics) or with two 

 extra chromosomes of different types (dou- 

 ble trisomies) are also found. 



Datura enables us to test the phenotypic 

 consequences of disturbing the normal bal- 

 ance among chromosomes. Compare, in 

 Figure 11-8, the seed capsules of the normal 

 diploid (2N) with those of diploids having 

 either one extra chromosome (2N + 1 ) of 

 the type producing Globe or two of these 

 (2N + 2). The latter two polysomics can 

 be called trisomic diploid and tetrasomic 

 diploid, respectively. Although the tetra- 

 somic is more stable chromosomally (each 

 chromosome can have a partner at meiosis) 

 than is the trisomic, the tetrasomic pheno- 

 type is too abnormal to establish a race, 

 since it has a still greater genetic imbalance 

 than the trisomic and produces a still greater 

 deviation from the normal diploid phenotype. 



3 Based upon work of A. F. Blakeslee and J 

 Belling. 



