424 Aneuploids and Nondisjunction 



be n + 1 and two will be n — 1. This abnormal behavior, as the 

 result of which the members of a homologous pair fail to sepa- 

 rate normally so that both members become incorporated into 

 one of the daughter nuclei while the other daughter nucleus has 

 neither, is known as nondisjunction. It actually results, of 

 course, from nonconjugation. 



Since there are four pairs of chromosomes in Drosophila 

 melanog aster, each gamete should have four chromosomes if 

 meiosis is normal and regular. Since one of the pairs in the 

 female consists of two X chromosomes, each egg should have 

 one and only one X chromosome. Any other result indicates 

 some irregularity in the meiotic system. Such irregularities do 

 occur, although not with any great frequency, and, when they do, 

 nondisjunction results. 



When nondisjunction of the X chromosome occurs, either the 

 two X chromosomes remain in the egg or they both pass into the 

 polar body, leaving the egg with no sex chromosome. If both 

 remain in the egg, such eggs will have three autosomes and two 

 X chromosomes. They can be fertilized by either an X chromo- 

 some-bearing sperm or by a sperm that has a Y chromosome. 

 If a sperm with an X chromosome unites with such an egg, the 

 resulting individual will have three pairs of autosomes, as in 

 the normal diploid fly, but will have three X chromosomes in- 

 stead of two. A fly with such a chromosomal complement is a 

 triplo-X fly and is trisomic for the X chromosome. The chro- 

 mosomal unbalance in this case is so great, however, that 

 triplo-X flies normally die before reaching the adult stage. If 

 an egg with two X chromosomes is fertilized by a sperm that 

 bears a Y chromosome, the resulting individual wdll have four 

 pairs of chromosomes like any normal female, but will have a 

 Y chromosome in addition. It will behave as a normal female. 



If the two X chromosomes go into the polar body leaving none 

 in the egg, the egg will carry only the three autosomes. If this 

 egg is fertilized by a sperm with an X chromosome, the indi- 

 vidual which results will have three pairs of autosomes and one 

 X chromosome. Such a fly will be a male, but it will be sterile. 

 If the egg deficient in an X chromosome is fertilized by a Y 

 chromosome-bearing sperm, the resulting zygote will have three 

 pairs of autosomes and a Y chromosome, but it will have no X 

 chromosome. The lack of an X chromosome proves fatal to this 



