78 Special Chromosomes and Sex Inheritance 



is noticeably thinner than the corresponding chromosome of the 

 female and is thinner than the autosomes of both sexes. The 



Y chromosome of the male forms part of the chromocenter. 

 The chromocenter is not a characteristic of dipteran salivary 



gland nuclei, although it is a prominent and characteristic fea- 

 ture of Drosophila. In forms like Chironomus and Sciara there 

 is no such union of heterochromatic material, and the chromo- 

 somes are independent units in the same number as in typical 

 somatic cells. 



"Lampbrush" Chromosomes 



Another unusual type of chromosome has recently received 

 considerable attention. In some animals, including amphibians 

 and birds, during the maturation of the egg, the chromosomes of 

 the first meiotic prophase may increase greatly in length. This 

 increase occurs in those forms whose developing eggs remain in 

 the prophase for a considerable time. The homologous chroma- 

 tids pair normally, but their chromonemata then proceed to 

 become extended by uncoiling As this continues, outgrowths 

 which are very fine threads appear from the sides of the chro- 

 monemata. They grow out approximately perpendicular to the 

 chromonemata and then bend around into loops. These loops 

 always appear in a certain order on the main thread, being of a 

 characteristic number and a characteristic distance apart from 

 one another. It has been suggested that they give forth sub- 

 stances into the cytoplasm which exert an action in directing 

 the development of the egg, but they need more study for a 

 completely satisfactory explanation of their behavior. 



QUESTIONS AND PROBLEMS 



1. Show by diagrams how the X chromosome and the Y chromosome 

 would pair at meiosis if the X chromosome were rod-shaped and the 



Y chromosome were J-shaped if: (a) the long arm of the Y chromosome 

 were completely homologous with the X chromosome; (6) the proximal 

 part of the long arm of the Y chromosome were homologous with an 

 interstitial segment of the X chromosome; (c) there were no homologous 

 parts in the X and Y chromosomes. 



2. Discuss the possibihties (a) that maleness is determined by the 

 presence of a Y chromosome and (6) that sex is determined in a diploid 

 animal by either one or two X chromosomes. 



