Nondisjunction 425 



type, and such flies die when only a few cell divisions past the 

 zygote stage. 



If known genes in the X chromosome are traced in nondis- 

 junction, the genetic ratios will be different from those expected 

 if meiosis is normal. In fact, it was the unexpected appearance 

 of certain phenotypes that led to the discovery of nondisjunc- 

 tion in Drosophila by Bridges in 1916. As demonstrated fre- 

 quently, a white-eyed female when mated with a red-eyed male 

 normally produces only red-eyed females and white-eyed males. 

 If, however, two X chromosomes each bearing a iv gene remain 

 in the egg, and if this egg is fertilized by a Y-bearing sperm from 

 a red-eyed male, the offspring will be a female, but will be white- 

 eyed because both its X chromosomes were derived from its 

 female parent (Fig. 117). If a nondisjunctional egg which is 

 deficient for an X chromosome is fertilized by a sperm contain- 

 ing an X chromosome on which is located a it'+ gene, the off- 

 spring will be male and will have red eyes because its X chro- 

 mosome was introduced from its red-eyed father. These white- 

 eyed females and red-eyed males are abnormal tjqoes and sug- 

 gest that something unusual has occurred. Normally, in Dro- 

 sophila, a male receives his only X chromosome from his mother. 

 Males that result from nondisjunctional eggs, however, receive 

 their X chromosome from their father and hence are known as 

 patroclinous males. Similarly, females normally receive one X 

 chromosome from each parent. The white-eyed females that 

 result from a nondisjunctional egg are called matroclinous fe- 

 males because they receive both their X chromosomes from their 

 mother. 



Nondisjunction is also found in both the sex chromosomes 

 and the autosomes of other animals and in many plants. In 

 fact, probably all monosomies and trisomies have arisen from 

 nondisjunction. Failure of pairing at zygotene or failure of 

 chiasmata probably are responsible for nondisjunction in most 

 plants and animals; but in those in which the chromosomes are 

 heterozygous for reciprocal translocations, nondisjunction may 

 arise from other causes. 



A good example of nondisjunction not resulting from failure 

 of pairing or of chiasmata is found in Oenothera Lamarckiana. 

 In Oenothera certain species are permanently heterozygous for 

 one or more translocations so that at meiosis their chromosomes 



