538 



EVOLUTION, HEREDITY, EUGENICS 



Morgan, T. H. 1915. Proc. Soc. Exp. Biol, and Med., vol. 13, p, 31. 

 RoxAs, T. A. 1926. Jour. Exp. Zool., vol. 46, p. S^. 



Genes. — Johannsen has given the name genes to those hereditary- 

 germinal factors that may sometimes need to combine to produce 

 a visible somatic character. Genotypes are individuals having the 

 same germinal composition, while phenotypes are a group of in- 

 dividuals with similar external 

 features. Genes may act in 

 single pairs or in several pairs. 

 Plural genes may be alike in 

 separate effects but cumula- 

 tively may produce another 

 effect. Again they may be 

 compleynentary or supplemen- 

 tary to other factors or they 

 may be lethal. 



1 2 A 2 B 2 C Chromosome Maps and 



Fig. 268. Crossing over during syn- Crossing Over. — Morgan and 



apsis of homologous paternal and maternal j^j^ coworkers have adequately 

 chromosomes. The segments indicate the 

 assumed linear arrangement of the genes 

 with allelomorphic genes opposite each 

 other. I, pair of chromosomes which have 

 entered and emerged from the synaptic 

 state without any crossing-over; Ila, chro- 

 mosomes winding about each other at r /o ^\ • 



TTi • f u u of genes (Sturtevant) is one 



synapsis; iib, separation or these chromo- ® ^ . 



somes, involving breaking at the points of that furnishes a satisfactory 



crossing; Ik, their emergence from synap- explanation of SUch phenomena 



sis with the members of the pairs of allelo- as linkage and recombination 



morphic genes interchanged. (After Wil- jnvolved in the " croSS-OVer." 



son, homWoodrxiS, Foundations of Biotozy. t > 1 ir r ^l 7* 



r \ f Tk i\/r 1! n \ Janssens theory ^^ of the chtas- 



Courtesy ot 1 he Macmillan Co.) •' ■' 



matype was elaborated by Mor- 

 gan and his students, who suggested that when the paired chromo- 

 somes are side-by-side at synapsis (Fig. 260) the /«c/orj' of the two 

 chromosomes may become exchanged. If two genes are close to- 

 gether, crossing-over of the two factors is unlikely to occur, but 

 as the distance between them increases, the chances of crossing-over 

 are much greater. By this means, Sturtevant, Bridges and others 



demonstrated that Roux's con- 

 ception of the chromosomes as 

 a linear series of smaller bodies 

 was the correct one. The 

 theory of linear aggregation 



1^ Janssens, F. 

 ibid., 34, 1928. 



1909. La theorie de la Chiasmatypie. La Cellule, Tom. 25; also 



