338 
MendeVs  Laws  of  Heredity. 
had  attempted  to  generalise  from  the  miscellaneous  mass  of 
forms  arising  from  cross-breds.  It  was  solely  because  Mendel 
examined  the  individual  and  its  progeny  separately  and  statis- 
tically that  these  far-reaching  discoveries  were  made. 
He  chose  the  genus  Pis  am,  using  for  the  most  part  varieties 
of  Pisum  sativum  (the  edible  peas)  and  some  other  closely 
related  species,  after  some  two  years  of  preliminary  trials, 
because  (1)  the  varieties  differ  from  one  another  in  characters 
which  are  practically  constant ; (2)  because  there  is  only  a 
remote  chance,  owing  to  the  structure  of  their  flowers,  of  their 
being  accidentally  cross-pollinated  ; and  (3)  because  successive 
generations  of  the  cross-breds  suffer  no  marked  diminution  in 
fertility.  Thus  nothing  which  could  be  foreseen  was  left  to 
chance,  and  the  whole  scope  of  the  problem  was  clearly  defined 
beforehand. 
Starting  first  of  all  with  simple  cases  where  the  varieties 
crossed  together  differed  from  one  another  in  a single  character 
only,  such  for  instance  as  two  peas  similar  to  one  another 
except  in  the  form  of  the  seeds  which  was  wrinkled  or  round, 
or  in  the  colour  of  the  pods  which  was  yellow  or  green, 
Mendel  found  that  the  cross-breds'  resembled  one  of  the 
parents  so  closely  that  one  of  the  characters  in  which  the 
varieties  differed  from  one  another  either  totally  escaped 
observation  or  could  not  l)e  detected  with  certainty.  Thus 
the  cross-bred  seeds  were  round  with  no  signs  of  wrinkling,  the 
pods  of  the  cross-bred  plants  green  with  no  blending  of  yellow. 
Precisely  the  same  results  were  obtained  with  reciprocal  crosses, 
both  round  x wrinkled  and  wrinkled  x round  giving  round 
seeds.  Mendel  called  the  character  appearing  in  the  cross- 
bred dominant,  the  one  which  appeared  to  be  swamped  the 
recessive  character. 
The  following  generation  obtained  by  allowing  the  cross- 
breds to  fertilise  themselves  consisted  of  plants  showing  either 
the  dominant  or  the  recessive  pod  chai’acter,  again  with  no 
transitional  forms,  and  a statistical  examination  brought  out 
the  fact  that  they  occurred  in  a ratio  of  3 : 1 on  the  average. 
Among  580  plants,  428  had  green  pods  and  152  yellow  pods. 
The  seeds  of  the  cross-bred  gave  a similar  result,  5,474  being 
of  the  round  type,  1,850  of  the  wrinkled — both  forms  generally 
occurring  in  the  same  pod.  The  first  generation  from  the 
cross-breds  consisted  of  75  per  cent,  with  dominant,  and  25 
per  cent,  with  recessive  characters. 
The  seeds  of  the  self-fertilised  individuals  were  again  sown 
separately,  with  the  following  results  : — (1)  The  recessive  types 
produced  recessives  only,  th^^  were  “fixed”;  (2)  some  of  the 
‘ \.e.,  The  seeds  produced  as  the  results  of  cross-polliiiatioii,  or  the  pods 
produced  by  the  cross-bred  plant. 
