XX 



THE GREAT RELAY RACE 



Preview. Seed and soil • Independence of the germplasm • Lines of 

 approach • The experimental method : The usefulness of hybrids ; Mendelism ; 

 what Mendel did ; monohybrids, dihybrids, trihybrids, and other crosses : 

 Unit characters and factors, modified ratios, different kinds of factors • 

 Practical breeding : Selection, mass selection, pedigree breeding, progeny 

 selection; inbreeding and cousin marriage; outbreeding and hybrid vigor; 

 asexual propagation ■ The germplasmal method : Chromosomes ; genes ; 

 linkage and crossing-over; chromosome maps • The role of cytoplasm • 

 Sex in heredity • Suggested readings. 



PREVIEW 



"Now these are the generations of Pharez : Pharez begat Hezron, and 

 Hezron begat Ram, and Ram begat Amminadab, and Amminadab begat 

 Nahshon, and Nahshon begat Salmon, and Salmon begat Boaz, and Boaz 

 begat Obed, and Obed begat Jesse, and Jesse begat David." 



As will be remembered, along came Ruth at the Boaz stage and 

 injected a welcome bit of romance into these dry statistics. It is not, 

 however, the vivid story of this Moabite woman, who was in her day 

 so young and charming, that is the reason for introducing this quo- 

 tation from the Book of Ruth, but rather the bare record of names 

 in itself, together with the indispensable "begats," that claims our 

 immediate attention now. The generations of mankind have always 

 been hooked up in this chainlike fashion. The spark of life has 

 always been borne forward for certain intervals of time by indi- 

 viduals, and then transmitted to individuals of another generation to 

 carry on. This is the Great Relay Race, participated in alike by all 

 human beings, lower animals, and plants. It depends upon the 

 co-operation of long lines of separate mortal individuals who play 

 their temporary part and then inevitably die, while the immortal 

 enduring line of life itself persists. The science of genetics attempts 

 to explain how such a relay race is run. 



A single microscopic streptococcus, a solitary wandering housefly, 

 or a chance weed pulled up from the wayside, each can boast of a 

 longer pedigree than can the King of England. This universal 



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