OPERATION OF THE GENETIC SYSTEM 63 



somes Present. — Bridges discovered further that sometimes 

 the number of autosomes becomes changed, and he deter- 

 mined the results of such changes. In the normal individuals 

 there are two sets of autosomes, one from each parent; 

 that is, the chromosomes are in pairs, AAXX (figure 7). 

 Certain individuals in the fruit-fly were discovered to have 

 their chromosomes in sets of three instead of in pairs. Such 

 individuals are known as triploids; their chromosomes may 



4}k 



B 



Figure 21. Chromosomes In diploid (A) and triploid (B) Droso- 

 phila, after Bridges (1922). In A is shown the normal condition 

 in the female: four pairs of chromosomes. In B are shown the 

 chromosomes of the triploid female: four groups of three chromo- 

 somes each. 



be represented as AAAXXX. How these individuals were 

 produced is not known. It seems probable that in some way 

 germ cells were produced in which the two chromosomes 

 of the pairs did not separate, so that these germs cells were 

 AAXX. These being fertilized by the usual sperm, AX, 

 gave the triploids. It is important to understand that the 

 existence of these triploids is demonstrated under the mi- 

 croscope. Their chromosomes in microscopic preparations 

 are seen to be in threes (figure 21). 



When such triploids form germ cells, some of the latter 

 are of the usual haploid type, AX, while others have their 

 chromosomes in pairs, AAXX; that is, they are diploid. 



