Chapter 5 



SPECIAL CHROMOSOMES AND SEX 

 INHERITANCE 



Sex and the Sex Chromosome 



For most animals and a few plants, there is one outstanding 

 and regular exception to the statement that all the chromosomes 

 of a normal, diploid organism are in pairs and that each chromo- 

 some has a mate which is an exact duplicate of it in morphology 

 and in the loci of which it is composed. In all individuals of 

 Drosophila melanog aster, the second, third, and fourth chromo- 

 somes are present in pairs, and the first chromosome, rod-shaped 

 and of medium length, is paired in the female. In the male, 

 however, only one of these medium-sized rod-shaped chromo- 

 somes is present, and another 

 chromosome, which is longer, 

 J-shaped, and absent in a nor- 



1-^^ ^^ ^^ mal female, is also present. It 



m\ is customary to refer to the rod- 



shaped chromosome as the X 

 Fig. 22. Chromosomes in a fe- chromosome and the J-shaped 

 male-determining {lejt) and a ^^^ ^g ^j^^ y chromosome. At 



male-determining sperm of Dro- • • • ,1 a ±^ -v 



,., , , -Pk- ^ meiosis m the male, the X 



sophila melanogaster. Diagram- 

 matic. (After Bridges in Genetics.) and Y chromosomes separate 



so that all sperm carry one X 

 or one Y in addition to one chromosome from each of the three 

 pairs (Fig. 22). Since females have two X chromosomes, all 

 eggs have one X chromosome in addition to one member of 

 each of the other pairs. The X and Y chromosomes are there- 

 fore differentials in the determination of sex. When an X-bearing 

 sperm unites with an egg, the resulting individual has two X 

 chromosomes and is a female. Similarly, a male is produced by 

 the union of a Y-bearing sperm and an egg, since it has one X and 

 one Y chromosome (Table 1). 



68 



jk Jk 



