290 



ANIMAL BIOLOGY 



Aside from his masterly foresight in realizing that success de- 

 pended on simplifying the problem by dealing with definite al- 

 ternative characters, Mendel's 

 claim to fame lies chiefly in his 



P discovery of a simple principle by 

 which the results may be ex- 

 plained. Since the hybrids when 

 inbred always give rise to hybrids 

 and also to each of the parental 

 types in a pure form, it must be 



]? that the factors (genes) which de- 

 termine the characters in question 

 are sorted out, or segregated, in 

 the ripe germ cells, or gametes. 

 Assuming for illustrative purposes 

 that a single gene determines a 

 given character, it follows that 

 after segregation the genes are 



F 2 distributed so that some gametes 



bear the gene for one character 



and other gametes bear the gene 



for the other character, but no 



gamete ever bears the genes for both 



characters. If the gametes of the 

 Fig. 181B. — Diagram of a Men- . . . _. , 



delian monohybrid. Results of cross- original tall parent contained the 

 ing tall (S) and dwarf (s) Pea plants, gene for tallness (S), and those 



The circles represent the zygotes and of the dwarf Qt the for 



the characters ot the soma (pheno- x . 



type); the letters within the circles, dwarf ness (s) - - then the hybrids 



the germinal constitution (genotype), will arise from a zygote which 



The letters outside the recombination combines both genes (&), and 



square represent the gametes. JNote . . ° 



that each of the parents (P) repre- since tallness is dominant over 



sents a different phenotype and gen- dwarf ness all will be tall. Further, 



otype; all the F, (one shown) belong when the ceUg of thig h brfd 



to the same phenotype and genotype; 



while the F 2 represent two pheno- (Ss) mature, and these genes are 



types and three genotypes. The rel- segregated SO that half of the 

 ative number of individuals compos- gametes bear S and ha lf bear S, 

 mg the 1 2 phenotypes is 3 : 1. ° . . , 



then when such plants, each with 



this germinal constitution, are inbred there will be equal chances 



for gametes bearing the same and for gametes bearing different 



genes to meet in fertilization. 



