30 AN INTRODUCTION TO MODERN GENETICS 



cannot easily pretend that organisms are simply intermediate between 

 their two parents. It seems that one must conclude that an organism 

 does not inherit the whole of its parents' genotype, but only part of it; 

 in any given case some of the characters which might have been 

 inherited from the parent actually fail to appear. But theory went no 

 farther than this till Mendel was bold enough to leave out of considera- 

 tion the greater part of the characteristics of the organisms with which 

 he was working and to concentrate entirely on one or two sharply 

 marked features. In this way he proceeded to consider the phenotype 

 (the appearance of the adult animal) as a set of elementary characters; 

 and although it is obvious that an organism is not a mere assemblage of 

 isolated anatomical structures, this analysis revealed the fundamental 

 fact that the genotype consists of hereditary units which are very 

 largely independent. The relation between the genotype and the 

 phenotype, that is to say, between the hereditary constitution and the 

 appearance of an organism, can therefore only be understood after we 

 have discussed the facts which have been revealed by MendeUan analysis. 



2. Mendel's First Law: Factors and their Segregation 



Mendel's fundamental discoveries were made on the garden pea, 

 Pisum sativum. The typical, oft-quoted experiment was as follows: 

 tall-growing pea plants were crossed with short plants (Pi, the first 

 parental generation); their offspring (Fi, first filial generation) were aU 

 tall, and when self-fertilized, gave a second generation of hybrids (F2) 

 consisting of tall and short plants in the ratio of three to one. The short 

 plants from the F2 bred true for shortness when selfed, and a third of 

 the tails bred true for tallness, while the other two-thirds of tails again 

 gave tails and shorts in the three to one ratio which had been found in 

 the F2. Mendefs hypothesis was this: tallness and shortness are depen- 

 dent on a pair of alternative factors, which we may call T tall and t 

 short. Each fertilized zygote, and each cell of the organism into which 

 it develops, contains two of these factors, and may thus be TT, or Tt 

 or tt; but the gametes each contain only one factor selected out of the 

 two which are contained in the germ-mother cell out of which the 

 gamete is formed. The first cross was between TT and tt, and gave a 

 Fi of Tt; the fact that this Fi shows as tall plants must mean that 

 during development the T factor "dominates" over the t, which is said 

 to be "recessive." When the Fi is selfed, each Tt plant forms equal 

 nimibers of gametes with T and with t, and if these unite at random 

 they will give TT, Tt and tt plants in the ratio i : 2 : i. Thus there will 



