NOTES AND LITERATURE 



GENERAL BIOLOGY 



MendeUsm.* — In a well printed booklet of eighty-five 4 X 5^ inch 

 pages, R. C. Punnett of Cambridge, England, has presented an ad- 

 mirably clear and concise account of Mendelism. After reviewing 

 the simple and fundamental experiments of the Abbot of Brunn, the 

 writer describes the more recent discoveries to which they have led, 

 and in conclusion shows them to be of the highest practical and 

 scientific importance. Although the mendelian principles of heredity 

 are well known in America through the publications of Castle, Daven- 

 port, and others, a brief review of them as presented by Punnett may 

 still be of interest. 



It is found by experiment that when a certain pure bred tall variety 

 of plant is crossed vrith a dwarf, the resulting hybrid contains both 

 the factors for tallness and shortness. If A represents the tall factor 

 of one parent and a the short factor of the other the hybrid which 

 contains both is Aa. It is not of medium height, but is like its tall 

 parent. A character such as tallness in ])eas which is retained by 

 the hybrid is called dominant: one like dwart'iu ss which is latent in 

 the hybrid is named recessive. 



When hybrids Aa are bred together, they pnxhicf in the next gen- 

 eration 25% of pure tall forms, AA; 50* , of tall hyl)ri(ls. Aa; and 

 25% of dwarfs, aa. The famiHar fornuila may he written thus: 



1st generation Aa 

 2nd generation AA 2 Aa aa 



The tall hybrids Aa, and the pure tall plants A A, are indistinguish- 

 able except by further breeding. Then it appears that one in every 

 three contains only the factors for tallness. Such plants, like the 

 dwarfs, breed as true as if derived from an unbroken ancestry of pure 



It is not al\vay> the caM" that the hybrid resembles one of its parents. 



•Punnett. H, ('. M>n.i.li.^m. Second Edition. Cambridge, MacMillan 

 and Bowes. 1907. IGnio. vii + 85 pp. 



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