266 Practical Plant Biology. 



determinants will occur in the spore-mother-cells preliminary to 

 the formation of the gametes (sperms and ova). Four different 

 kinds of sperms will be found, viz. RY, R^, aY, ag in equal 

 numbers ; and four different kinds of ova, viz. RY, R^-, aY, ag, also 

 in equal numbers. The four different kinds of sperms will unite 

 indiscriminately with the four different kinds of ova, so that sixteen 

 distinct combinations or zygotes (oosperms) will result : RYRY, 

 RYR ? , RYaY, RY* R^-RY, R^, R^Y, Rgag, aYRY, aYRg, 

 aYaY, aYag, agRY, ag&g, agaY, agag. 



These combinations will be more easily followed by reference to 

 the accompanying diagram. Here along the upper side are shown 

 the four different types of sperms which can unite with the four 

 possible types of ova. The latter are indicated along the left side 

 of the square. The zygotes formed by these unions are shown in 

 the subdivisions of the square, the determinants contributed by the 

 sperms being in each case uppermost. The shape of the embryo 

 is indicated by a zone round the determinants in the subdivision, 

 and its colour by the marking of the enclosing zone. In the 

 figure dotting indicates yellow and green is represented by short 

 lines. 



Among these zygotes RYRY, RYR^, RYaY, RY^, R^RY, 

 R#Y, #YRY, tfYRo-, agRY, nine in all, will resemble one another 

 and will show the characters of roundness and yellowness ; the 

 zygotes R^R^, Rgag, ag&g, three in number, will appear round 

 and green ; the zygotes aYaY, aYag, agaY, three in number, will 

 appear angular and yellow. There is only one agag which will 

 appear angular and green, possessing the two recessives only. As 

 each of these combinations is equally likely to be formed, in a large 

 number of descendants we will get the ratios of these four different 

 categories to one another expressed as follows : 9:3:3:1. 



It will be evident that certain of these zygotes produced by 

 the union of two gametes having an identical constitution so far 

 as the characters considered are concerned when selfed must 

 breed true, viz. RYRY, R^R^, aYaY, agag. Of these "homo- 

 zygotes " the first and the last, called respectively the double 

 dominant and double recessive are new combinations and differ 

 from both of the original parents. Thus we see how new constant 

 varieties may arise from hybridisation. 



Recent research has shown that in all probability while the 

 dominant character is due to the presence of a determinant in the 

 individual, the absence of this determinant causes the correspond- 

 ing recessive character. Thus the recessive angularity of a quadrate 

 pea is the result of the absence of the determinant of roundness 



