328 



DISCOVERY 



other. To keep out wind- or insect-borne pollen from 

 other plants, the flower which has been thus treated 

 must be covered with fine muslin or a greased-paper 

 bag. The seed resulting from this act of artificial 

 hybridisation grows into a plant called the Fj (first 

 hybrid generation). This, by fertilising itself, produces 

 seeds, and these grow into plants collectively called the 

 F, generation. From the seed of every F, plant a 

 family of plants (Fg) may be raised. The original 

 parents are designated Fo. That the form of the F^ 

 and the proportions of different forms in F^ were 

 governed b}' precise laws was the essence of Mendel's 

 discovery. 



In 1866, just before he became Abbot of the Augustin- 

 ian house at Briinn, Mendel communicated his dis- 

 covery, actually made in 1859, to a local scientific 

 society and by letter to some botanical contemporaries. 

 All failed to appreciate his work : the Origin of Species 

 published in 1859 had still a monopoly of biological 

 attention. Fortunately in 1900, through a brief refer- 

 ence in a book, the discovery was unearthed. It was 

 immediately confirmed and enthusiastically received. 

 Mendel had died sixteen years previously. His laws 

 may be thus briefly stated.' 



All plant characters are in pairs, the members of a 

 pair corresponding to the presence and absence of a 

 Mendelian " factor." In garden peas tallness and 

 shortness are such a pair. The Fj plant from the 

 cross TaU x Short is found to be tall. Consequently 

 taUness is said to be the " dominant " character and 

 shortness the " recessive." The dominant character 

 results from the presence m the plant of the factor for 

 taUness, the recessive from its absence. From the 

 seeds formed (by self-fertilisation) by the F^ plant, 

 there develop a number of plants (called the F.^ plants), 

 some tall and some short, there being 3 tails to i short, 

 i.e. a proportion of 3 : i. Of the 3 tails, i (i.e. a third 

 of the total number of tails) breeds true, all its progeny 

 being tall. The remaining 2 tails, when self-fertilised, 

 behave exactly like the Fj, their progeny being in the 

 proportion 3 tails to i short. The shorts of the F^ 

 breed true for shortness (i.e. their progeny consists 

 solely of short plants). Thus the F, falls into three 

 types : 



I tall (breeding true) ; 2 tall (giving tall and short 

 m their progeny) ; i short (breeding true). 



The proportion 1:2:1 remains constant however 

 often the experiment is repeated. This constancy 

 is the first salient feature, and Mendel's laws afford an 

 explanation of it in the following terms : 



The tall parent (F,) introduced the factor T for 

 tallness, while the short parent (Fq) lacked it. Absence 



1 Heredity, by L. Doncaster, Cambridge Manuals of Science 

 and Literature, is a simple and attractive little book for those 

 who seek further information about Mendelism. 



of the factor may be denoted by t. When the gametes 

 of the Fj plants combined, they formed a seed of 

 constitution Tt. From this the Fj plant grew, having, 

 of course, the constitution Tt. Mendel argued that 

 of its gametes, male and female alike, exactly a half 

 possessed the factor for tallness (to be denoted therefore 

 by T) and the other half lacked it (denoted by t). 

 When it became self-fertilised by the union of its own 

 male and female gametes, the two tj^es of male 

 gamete must have combined with the two types of 

 female gamete, giving four types of seed. "Thus T 

 male with T female = TT seed ; T male with t female 

 = Tt seed ; t male with T female = Tt seed ; t male 

 with t female = tt seed. The seeds formed by Fj. 

 and thus the F^ plants into which they grew, must 

 therefore be expected in the proportion i TT : 2 Tt : 

 I tt. Of these Fg types TT will breed true on self- 

 fertilisation, since it can contribute only T to all its 

 gametes, male and female. Similarly the tt must 

 breed true. But the 2 Tt will, like the Fj (also Tt), give 

 a 1:2:1 ratio in their progeny. This prediction 

 accords preciselj^ with the experimental result described 

 above. A diagrammatic representation maybe given. 



Short 



Gametes of F 

 o 



Therefore F, . 

 Gametes of F. 



Male 



-© 



t^ 



Female 



>T 



r^t 



(The arrows indicate the different possible combinations of 

 male and female gametes.) 



® & 

 Therefore F, = TT -(- Tt -f- Tt + tt 

 i.e. I TT: 2 Tt: i tt. 



Plants of constitution Tt which, when self-fertilised, 

 do not breed true, but in their progeny " split " oft' 

 different types, are called " splitters." In botanical 

 language they are " heterozygous " for T ; forms like 

 TT and tt are " homozygous." 



Crosses in which the parents differ in more than 

 one pair of characters illustrate Mendel's second law — 

 pairs of characters are inherited independently of one 

 another. Suppose that in the cross discussed the tall 

 parent had " round " seed and the short had 

 " wrinkled." Round and wrinkled may be denoted 

 by R and r, for experiment shows that they are a pair 

 of characters and that round is the dominant. To 

 foretell the results of the cross, it is simply necessary 

 to apply the principles explained for T and t above 

 jointly to both pairs of characters. All possible types 

 of gamete will be formed in equal proportions and all 

 possible combinations of male and female gametes 



