GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 165 



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Figure 26. Section of the right end of second chromosome (salivary gland of 

 Drosophila melanogaster) with corresponding portion of the chromosome map (after 

 Calvin B. Bridges). (From paper in "Colloid Chemistry," Vol. 5, by J. Alexander, 

 Reinhold Publishing Corp., N. Y.) 



finds visual confirmation in the "genetic spectrum" there revealed. 

 Many more complete concordances are shown in the elaborate 

 salivary chromosome maps recently published in the Journal of 

 Heredity. Figure 27 outlines the 48 human chromosomes (24 

 pairs). 



Sex Determination 



In contradistinction to the main number of autosomes are the 

 usually smaller "sex chromosomes," so called because their segrega- 

 tion determines the sex of each offspring. The basic sex chromo- 

 some, known as the X-chromosome, has no discovered mate in 

 some species (XX-XO types). Only comparatively recently (1901) 

 was the F-chromosome found in human sperm, placing man in a 

 major XX-XY group in which the fertilized ovum that develops 



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Figure 27. The 48 human chromosomes in outline (after H. M. Evans arid O. 

 Swezy, Mem. Univ. Cal., Vol. 9, No. 1929). (From paper in "Colloid Chemistry," Vol 

 5, by J. Alexander, Reinhold Publishing Corp., N. Y.) 



