THE ORIGIN OF VARIATION 



Block, Short-haired 



Fg PHENOTYPES 



9 Block, Short-haired 



3 Block, Long-haired 



3 Brown, Short-hoired 



I Brown, Long-haired 



Brown, Long-hoired 



PARENTS 0'^"" 'W' 



( ^ ) GAMETES [ |b|s| j [ |b|s| 



F| GENERATION 



GAMETES, ~ ~ 



bS] I [ Iblsl ; V blS^ i I Blsl I I Ibi 





Block, Short 



Block, Short 



Block, Short 



Block, Short 



Block, Short 



Block, Long 



|bg|s| 

 Block, Short 



iblsjsj 

 Block, Long 



^ (^>t*-^ Vf 



Block, Short 



IbgsJ 

 Block, Short 



Brown, Short 



[blbgsj 

 Brown, Short 



[bgjsj 

 Block, Short 



jb|sjsj 

 Block, bong 



Brown, Short 



|b|s|s| 

 Brown, Long 



Figure 72. A Dihybrid Cross Between a Black, Short-haired Guinea Pig and a 

 Brown, Long-haired One, Showing Independent Assortment. ( From Villee, "Biol- 

 ogy, The Human Approach," 3rd Ed., W. B. Saunders Co., 1957. ) 



Sex Linkage. Sex was the first character to be successfully interpreted 

 in terms of the chromosomes. It was found that many species of grass- 

 hoppers have 23 chromosomes in males and 24 in females. Thus 11 pairs 

 of chromosomes (autosomes— A) are identical in the two sexes, but the 

 twelfth pair is complete in the female yet represented by only one chro- 

 mosome in the male. This is the sex differential pair, or the X chromo- 

 somes. Consequently, meiosis ( maturation divisions ) results in eggs all of 

 which are similar, having llA -j- X. Sperm, however, are of two classes: 



204 



