The Source of Variability 



73 



of one or two chromosomes or by the addition of complete normal 

 sets of chromosomes. 



In the normally precise process of meiosis, each chromosome pair 

 divides, one of the pair going to each daughter cell. Extremely 

 rarely, an occasional pair of chromosomes fails to divide, so that 

 one daughter cell receives an extra chromosome and the other loses 

 one. Frequently the progeny of these mutants (called aneuploids) 

 are inviable, but in many known cases, especially in organisms 



Usually dies 

 (superfemale) 



Always 

 dies in the 

 egg stage 



Fig. 27. Aneuploidy in Drosophila melanogaster. The progeny of the unusual 

 eggs WW or lacking w are aneuploids, in that they have more than or less than 

 the normal number of chromosomes. (From Dobzhansky, Evolution, genetics, 

 and man. John Wiley & Sons, Inc.) 



