MENDELISM 5g^ 



practice, as we shall see later, many exceptions, so that they are 

 not ' laws ' in the sense in which that word is used in physics ; 

 nor were they enunciated in this form by Mendel, so that they 

 are better known as the Mendelian principles. Mendel's own 

 statement, which we have already given, combines the two in 

 one rule which is shorter and perhaps more elegant than the 

 common form. Mendel's essential discovery was that the characters 

 of a zygote are determined by unit factors in the gametes from 

 which it was formed. Nearly always a gamete contains, for a pair 

 of alternative characters, only a single factor, and sometimes the 

 distribution of the factors for one pair of characters is independent 

 of that of factors for another pair. When these two conditions 

 hold, the Mendelian principles are true. 



Alternative characters of the type with which Mendel dealt are 

 now generally said to be contrasted or allelomorphic, and the 

 essential part of his theory is that a gamete carries a factor for 

 only one character of such a pair, the factor being not the 

 character itself but a material something which tends towards its 

 production. 1 



When two gametes fuse to form a zygote two factors for a 

 pair of contrasted characters come together ; if they are ahke the 

 result is a homozygote, dominant or recessive as the case may be, 

 and if they are unlike the result is a heterozygote, which resembles 

 the dominant or is more or less intermediate in appearance. It is 

 obvious that we must distinguish between the appearance of the 

 individual, which is now called the phenotype, and its inherited 

 nature or genotype. Mendel's F^ tall peas had the same phenotype 

 as their tall parents, but were of a different genotype, as became 

 clear when they were bred from. To forecast the results of any 

 given breeding experiment we must know the genotypes of 

 the individuals to be crossed ; alternatively, from the results of 

 the experiment we may be able to determine the genotypes of the 

 parents. In either case, the first thing to do is to set out the 

 problem in the form of a genealogical tree and determine the factors 

 which must be present in the gametes, which are the connecting 

 links between the two generations. If the genotypes of the parents 

 are known, and the possible gametes are of more than one type, the 



1 This is the sense in which Bateson, who invented the term, used ' allelo- 

 morphic ', but it is also applied to the factors, which are called allelomorphs or 

 alleles, and some modern geneticists would restrict it to this usage. The context 

 will usually show in which way the word should be taken. 



