T H E 
HEW PflYTOIiOGIST. 
Vol. VIII., Nos. 9 & io. December, 1909. 
[Published January 13th, 1910.] 
RECENT ADVANCES IN THE STUDY OF HEREDITY 
(A Course of Lectures, for the University of London, delivered in the 
Summer Term, 1909). 
By A. D. Darbishire. 
LECTURE V. 
The 1:2:1 and 3:9:4 Ratios. 
10 R the theme of this lecture I propose to select the phe- 
J] nomenon of reversion, and to discuss the relation between 
this and dominance, and a possible alternative interpretation of 
the 1:2:1 proportion in P 2 . 
Hitherto we have been dealing with allelomorphic pairs of 
characters, the constituent unit-characters of which were inde¬ 
pendent of, and unaffected by, those belonging to other pairs of 
characters. In the case which I am about to describe, there is a 
specific and very definite effect exerted by a unit-character 
belonging to one allelomorphic pair upon a unit-character belonging 
to another allelomorphic pair. 
Let us proceed to the description of the two parent types 
crossed. It will be remembered that in the description of the 
last cross we described a Pea, the markings on whose seed-coat 
consisted of a fine purple spotting on a grey background, 
and it will be remembered that in the P 2 generation, produced 
by crossing this with a maple Pea, there occurred a Pea in which 
this grey seed-coat existed without the purple spots. This 
grey seed-coat is a characteristic of many commercial varieties of 
Pisum, as for instance, many of the Pield Peas. This grey is one 
of the characters used in the cross we are about to describe. The 
other character is that of the type of Pisum which has no pigment 
in its seed-coat at all. This has been the case in all the races 
which we have employed in crosses which concern cotyledon 
