310 



EVOLUTION AND GENETICS 



a 



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Br 



M 



Linkage in Other Organisms. Scientists have recorded Hnk- 

 age in other organisms. Among animals in which it is known to 

 occur may be mentioned poultry, pigeons, rats, mice, rabbits, silk- 

 worms and other insects; among plants, sweet peas, snapdragons, 

 primroses, corn, tomatoes, etc. The chromosome explanation would 

 lead to the belief that it is exceedingly common, but the accumula- 

 tion of evidence is a slow process. Moreover, Drosophila with only 



four kinds of chromosomes is an 

 especially favorable species for the 

 illustration of this phenomenon, 

 since one quarter of its characters 

 may be associated with a single 

 chromosome. Man, with twenty- 

 four kinds of chromosomes, may 

 have only one twenty-fourth of his 

 characters in one linked group. 



Localization of Genes. An im- 

 portant result of the theory of 

 crossing over, based on studies in 

 Drosophila, is the idea of localization 

 of the genes. Morgan and his co- 



_,,__„. . •„ 0^ . authors have published chromosome 



Fig. 177. — Diagram to illustrate r r, i i ■ i- 



double crossing over. In a the maps of Drosophila mdicatmg the 



positions of three genes, W, M theoretical location of genes for 



and Br, are indicated When ^^^^^^ ^f characters on all four of 



the chromosomes twist about . 



each other between these genes its chromosomes (Fig. 178). These 



as in b and break to form a new maps are based on a very logical 



assoc-iation of parts, as in c W analysis of percentages of crossing 

 and nr remain in the same chro- 

 mosome while M is shifted to the over between the various characters. 



other. (From Morgan et al., It is obvious that two filaments 



cannot twist about each other in a 

 space less than their diameter, 

 although they can be twisted several 

 times within a relatively short length. Consequently if genes 

 are located close together on a chromosome, there is less reason 

 to expect crossing over between them than if they are remote 

 from each other. It is not illogical to suppose that the per- 

 centage of cases in which crossing over actually occurs is in 

 proportion to the distance between the genes. 



Crossing over may occur between any characters of a linkage 



Mechanism of Mendelian He- 

 redity, with the permission of 

 Henry Holt and Company.) 



