Heredity — Genetics 697 



are said to be linked. This linkage force keeps the genes in proper as- 

 sociation. It is known that during meiosis (reduction divisions during 

 gametogenesis) the homologous pairs of chromosomes separate as units, 

 one of each pair passing to each gamete. Linkage between genes is 

 usually not complete. During the process of synapsis (temporary fusion 

 of homologous chromosomes), associated with gametogenesis (discussed 

 later in this chapter), the homologous chromosomes often mutually ex- 

 change segments and their contained genes. This mutual exchange is 



PARENTS 



FEMALE 



MALE 



GAMETES 



Fig. 344. — Inheritance of sex in the fruit fly (Drosophila sp.), shown somewhat 

 diagrammatically. Observe that in both male and female there are two pairs each 

 of chromosomes known as II, III, IV. In the female there is an additional pair 

 known as the X chromosomes. In the male there is an additional X chromosome 

 and a Y chromosome. When gametes are produced, there is a separation of the 

 members of each pair. When offspring are formed, the males have the three pairs 

 of chromosomes in addition to an X and a Y chromosome, while the females 

 have an additional pair of X chromosomes instead. Note the two kinds of male 

 gametes. (From Parker and Clarke: Introduction to Animal Biology, The C. 

 V. Mosby Co.) 



