54 
MENDEL’S PRINCIPLES OF HEREDITY 
homogeneous materials, to consider each character separately, 
and to record separately the progeny from distinct individuals. 
He first worked on the edible pea (Pisum sativum). In 
crossing the tall with the short variety he found that the first 
cross-bred generation were tall, to the exclusion of the short- 
form. He therefore called the tallness a dominant character 
because it prevailed, the excluded character he called recessive. 
On self-fertilisation of the first cross family, the next generation 
proved to be mixed in the proportion of 75 per cent, dominant 
to 25 per cent, recessive. On the whole of these being again 
propagated, by self-fertilisation, it was found that the recessives 
bred true, while the dominants gave a proportion of tails 
breeding true to ‘ tallness ’ and a mixed generation in the pro- 
portion of two impure to one pure. 
This is well expressed in the diagram reproduced from 
Thomson (see p. 59). 
But note Bateson — ‘ Dominance is no inseparable attribute 
of Mendelian inheritance 9 (p. 50). 
A dominant character is due to the presence of a definite 
factor, while ‘ the corresponding recessive owes its condition to 
the absence of the same factor ’ (pp. 58, 54). 
The fact of segregation (splitting) was the essential dis- 
covery of Mendel. Take the case of the F x family in the 
diagram : — 
It is considered that here the germ cells divide into two 
kinds of egg cells and two kinds of pollen cells, one kind with 
the potential quality of tallness, one with the potential quality 
of shortness. The groups may be considered equal in size, and 
therefore on self-fertilisation the chances of any egg cell being 
fertilised by any pollen cell are equal. Take the case of four 
‘ tall ’ egg cells, and four ‘ short 5 egg cells, impregnated by 
an equal number of similarly arranged pollen cells : the formula 
would express the result : — 
2D + 4 D (R) + 2 R 
and since tallness is dominant there appears six tall and two 
short. 
This, shortly, is the theory of gametic segregation ; and 
explains the occurrence of pure and impure gametes. The Ds 
