Trisomies 421 



genes that affect the gametophyte would show this unbalance, 

 for the specific effect of a gene that controls the shape of a sporo- 

 phytic structure such as a capsule would be the same in the 

 gametophyte whether it was present twice as frequently as other 

 genes or merely as frequently. Since very few gametophyte 

 genes have been identified, therefore, we must look merely for a 

 general effect rather than for a specific one in the gametophyte. 

 This general effect is often expressed, at least in part, in viabil- 

 ity, for n -\- 1 gametophytes on the whole do not survive as well 

 as normal n gametophytes. This lowered viability, furthermore, 

 is usually more apparent on the male than on the female side. 

 The reason for this is not too clear, but the loss oi n -\- 1 micro- 

 gametophytes is at least in part the failure of such pollen tubes 

 in competition with normal pollen tubes. This low viability on 

 the male side is extreme in Datura, where probably no n + 1 

 microgametophytes function. 



If specific, known genes are present on the chromosome that is 

 in the trisomic condition, the ratios that they will yield will 

 differ from the normal (Fig. 116). One of the best-known ex- 

 amples is the inheritance of the recessive gene eyeless in Dro- 

 sophila melanogaster. If a triplo-IV fly with three dominant 

 alleles of the gene eyeless is crossed with a normal fly homozy- 

 gous for eyeless ey, the Fi will be normal, wild-type Ey ey and 

 triplo-IV, wild-type Ey Ey ey. If a normal Fi fly is testcrossed 

 to a normal eyeless, the offspring will be half wild type and half 

 eyeless. When a triplo-IV Fi fly is crossed with a normal eye- 

 less, however, the offspring will segregate into flve wild type 

 to one eyeless. The eyeless will be normal as will two of the 

 five wild type, but the remaining three wild-type flies will be 

 trisomic. A 5 : 1 ratio is typical of a testcross of a trisomic 

 with two dominant genes and one recessive gene to a normal, 

 homozygous recessive. Of course, if the trisomic bears only one 

 dominant allele, the ratio in the offspring will be 1 : 1, and it will 

 not be possible to detect the trisomic condition from the breed- 

 ing ratios. 



The actual ratio which will be obtained in plants, however, 

 may differ from this because of the elimination of some of the 

 trisomic types in the gametophyte. It will also be disturbed if 

 crossing over occurs between the locus tested and the centromere, 

 a possibility which does not occur in the study of the eyeless 



