sis Cat igh iti pat Bi 
PES. 2c oe ape 8 ee 
650 PHARMACEUTICAL BOTANY 
nant characters and recessive characters appearing in the 
progeny of crosses between plants examined for one set of con- 
trasting characters such as tallness and dwarfness, he found that 
by the same method experiments could be carried on in which 
two sets of contrasting characters would be involved. His 
variety of Pea plants that differed in one set of contrasting char- 
acters, when crossed, produced monohybrids or factors repre- 
sented by a suitable symbol such as Dd (Tallness D, Dwarfness 
d). Two pairs of contrasting characters such as a variety of Pea 
plant which Mendel found to produce yellow, wrinkled seeds 
and another plant producing the characters for smooth, green 
seeds, when cross-pollinated, developed spores which gave rise to 
four kinds of sperm and four kinds of eggs and not the character- 
istic two kinds of sperm and eggs as of the monohybrids. This 
condition of a progeny in which there are two separate con- 
trasted characters is called a Dihybrid. In the cross of a variety 
of Pea plants having green color, smooth surface seeds with 
another variety having yellow color, wrinkled surface seeds the © 
following progeny appears in the first filial generation: Factors for 
smooth (indicated by S$) and yellow (indicated by Y) are domi- 
nant; whereas factors for wrinkled (indicated by s) and green 
(indicated by y) are recessive. The F; diagrammatically is: 
sex eT 
ar 
SsYy (all smooth and yellow) 
In the second filial generation or Ff, after self-fertilizing the 
progeny of the first filial generation or F\, it is then noted that 
there are four possible kinds of male gametes and four possible 
kinds of female gametes. During mitosis, the male gametes 
(indicated by o*) unite with the female gametes (indicated by @ ) 
and there is a union of four different male characters (indicated 
by $,s,Y%y) with four different female’ characters (indicated by 
S,5,Yy). The multiple of such a union of gametes is sixteen, and 
indicates that there wil] arise an F, generation of zygotes with 
sixteen dihybrid characters in a definite proportion. The 
actual composition of the Ff, zygotes of this dihybrid may be 
represented by the following checkerboard diagram: 
