Dominant  and  Recesaive  Charactera. 
339 
dominant  types  were  also  “ fixed  ” ; (3)  some  produced 
dominants  and  recessives  in  the  proportion  of  three  to  one. 
In  other  words  the  seeds  or  the  plants  showing  dominant 
characters  in  the  preceding  generation,  though  externallj^ 
similar,  had  not  a similar  constitution,  and  by  breeding  from 
them  this  difference  was  made  manifest.  On  counting  out 
the  plots  Mendel  found  that  the  75  per  cent,  with  dominant 
characters  of  the  ijreceding  generation  consisted  of  25  per 
cent,  which  were  “ fixed,”  50  per  cent,  which  “ broke,”  or 
produced  recessives  as  well  as  dominants. 
This  generation  therefore  in  reality  consisted  of — 
25  pure  recessives  ; 
50  cross-breds  ; 
25  pure  dominants  ; 
the  cross-breds  resembling  the  pure  dominants  as  before, 
making  it  appear  to  be  75  per  cent,  of  dominants  to  25  per 
cent,  of  recessives.  We  may  express  it  as  D — 2DE — R. 
Mendel  then  went  on  to  consider  the  case  of  a cross  between 
two  varieties,  AB  and  ah  differing  in  two  pairs  of  characters, 
A and  a,  B and  5,  of  which  ..4  and  B were  dominant  over  a 
and  h. 
The  cross-breds  were  again  all  similar,  and  showed  only 
the  dominant  characters,  A and  B.  They  thus  resembled  one 
of  the  parents.  Their  progeny  after  self-fertilisation  could  be 
grouped  into  four  classes,  as  follows  : — 
A B . Showing  both  dominant  characters. 
Ah  . Showing  one  dominant  and  one  recessive  character. 
aB  . Showing  one  dominant  and  one  recessive  character. 
ah  . Showing  both  recessive  characters. 
These  forms  appear  in  the  ratio  of  9M5  : ZAh  : : ZaB  : lah. 
On  raising  the  next  generation  from  the  individuals  com- 
posing these  groups  they  gave  rise  to  the  following  forms  ; — 
r 
All  AB, 
AB 
1 
1 
or  AB  and  Ah, 
individuals. 
] 
i 
or  AB  and  aB, 
or  AB,  Ah,  aB,  and  ah. 
Ah 
1 
f 
All  Ah, 
individuals. 
1 
or  ah  and  Ah. 
aB 
{ 
All  aB, 
individuals. 
or  ah  and  aB. 
ah 
individuals. 
/ 
1 
All  ah. 
Where  two  forms  appear  they  occur  in  the  ratio  of  three 
dominant  to  one  recessive  ; where  four  in  the  ratio  of  9 : 3 : 3 : 1 
as  before. 
