PRINCIPLES Of HEREDITY 



663 



Pa-rents 



Female 

 white -eyed 



GaineteS 



Offspring 



Female 

 red- eyed 



Male, 

 red- eyed. 



Male 

 \A?hite-eyed 



Female 

 red- eyed. 



Male 

 red- eyed 



Female Female Male Male 



red-eyed red-eyed red- eyed v/hiteeyed 



Figure 32.5. Diagram illustrating sex-linked inheritance, the inheritance of red 

 vs. white eye color in fruit flies. See text for discussion. 



Not all the characters which differ in the two sexes are sex-linked. 

 Some, the sex-influenced traits, are inherited by genes located in auto- 

 somes rather than X chromosomes, but the expression of the trait, the 

 action of the gene which produces the phenotype, is altered by the sex 

 of the animal, presumably by the action of one of the sex hormones. The 

 presence or absence of horns in sheep, mahogany-and-white spotted 

 coat vs. red-and-white spotted coat in Ayrshire cattle, and pattern bald- 

 ness in man are examples of such sex-infiuenced traits. 



278. Linkage and Crossing Over 



In the discussion of Mendel's Law of Independent Assortment, we 

 stressed the fact that this law is valid only for two pairs of genes located 

 in different, nonhomologous chromosomes. The ratio of 9:3:3:1 is ob- 

 tained in the F.. generation of a dihybrid cross only if the pairs of genes 

 are located in different chromosomes. Since there are many hundreds of 

 inherited traits and a very limited number of pairs of chromosomes 

 (23 in man, 4 in the fruit fly), it is obvious that each chromosome must 

 contain many genes. All of the genes located in the same chromosome 

 tend to be inherited as a group and are said to be linked. In meiosis 



