408 



Intrachromosomal Aberrations 



gous chromosomes may be identified as 1-2, 3-4, 5-6, 7-8, 9-10, 

 11-12, 13-14, 15-16, 17-18, 19-20, 21-22, and 23-24. As this 

 is a pure-breeding type, each chromosome is represented twice. 

 When a second prime type (PT2; formerly, line B) is crossed 



Fig. 112. Pollen grains in semi-sterile maize. The large, round, full, 

 deeply staining grains are viable, but the small, shriveled, nonstaining 

 ones are inviable, being nothing but empty pollen grain walls. (Courtesy 

 of Dr. R. A. Brink in the Journal of Heredity.) 



with a plant of PTl, ten pairs and a circle of four chromosomes 

 are found in the first meiotic prophase of the hybrid. The ten 

 pairs signify that PTl and PT2 have ten chromosomes which 

 are identical, whereas the circle of four indicates that one inter- 

 change between segments of two chromosomes of PTl has dif- 

 ferentiated PT2. The interchange had occurred between chro- 

 mosomes 1-2 and 17-18 so that PT2 is homozygous for the new 

 chromosomes 1 • 18 and 2 • 17. In other words, the ends numbered 



