70 Special Chromosomes and Sex Inheritance 



half the sperm have neither an X nor a Y chromosome and are 

 male-determining. 



Autosomes 



In all organisms the eggs and sperm carry one member of 

 each of the other pairs of chromosomes in addition to the X or 

 Y chromosome. These other chromosomes are known as the 

 autosomes. In Drosophila melanogaster there are 6 autosomes 

 in the somatic cells of both the male and female and 3 in each 

 gamete; in man, there are 46 autosomes in each body cell and 

 23 in each gamete. 



Heterogametic Females 



In moths, butterflies, birds, and some fishes, the situation as 

 regards the sex chromosomes is the opposite from that in Dro- 

 sophila and man. In these organisms, the female is the hetero- 

 gametic sex. This type is generally referred to as the Abraxas 

 or bird type, and the sex chromosomes in the female are usually 

 designated Z and W whereas the male is ZZ. Inheritance of sex 

 in these organisms is shown in Table 1. 



The Y Chromosome 



Apart from its frequent difference in shape, the Y chromosome 

 differs in one marked respect from the X chromosome and from 

 the autosomes. It generally contains at most just a few genes. 

 Often no genes at all have been discovered in the Y chromosome, 

 and even where genes have been found they are frequently not 

 alleles of genes in the X chromosome. In the Y chromosome of 

 Drosophila melanogaster genes that have been discovered are 

 a gene for long bristles, which is an allele of the gene "bobbed" in 

 the X chromosome, and two genes for male fertility which appear 

 to have no corresponding allele in the X chromosome. The pres- 

 ence of the allele of bobbed indicates that there is one small seg- 

 ment of the X chromosome which is represented by a homologous 

 segment in the Y chromosome. The remaining parts of the 

 X chromosome have no counterpart in the Y chromosome, and 

 almost all the Y chromosome is completely nonhomologous with 

 the X chromosome. Apart from the few genes, the Y chromosome 

 of Drosophila appears to be made up of inert material usually 



