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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  9. 
  A 
  regularity 
  of 
  behavior 
  has 
  been 
  found 
  in 
  the 
  interference 
  of 
  

   one 
  crossover 
  or 
  linkage 
  break, 
  with 
  a 
  second 
  crossover 
  in 
  the 
  same 
  

   carrier, 
  which 
  says 
  that 
  the 
  per 
  cent 
  of 
  crossovers 
  varies 
  independ- 
  

   ently 
  of 
  the 
  per 
  cent 
  of 
  interference. 
  The 
  converse 
  is 
  also 
  true. 
  

  

  There 
  is 
  a 
  facination 
  to 
  this 
  picture 
  of 
  germplasm 
  architecture, 
  

   and 
  the 
  type 
  of 
  investigation 
  which 
  has 
  led 
  to 
  these 
  statements 
  will 
  

   undoubtedly 
  lead 
  to 
  still 
  greater 
  things 
  in 
  the 
  future 
  ; 
  but 
  we 
  should 
  

   do 
  well 
  to 
  realize 
  that 
  each 
  of 
  these 
  generalizations, 
  except 
  prob- 
  

   ably 
  the 
  sixth, 
  may 
  be 
  a 
  special 
  case; 
  and 
  that 
  new 
  and 
  different 
  

   special 
  cases 
  may 
  be 
  found 
  without 
  necessitating 
  a 
  change 
  in 
  the 
  

   first 
  five 
  conclusions. 
  

  

  There 
  is 
  just 
  one 
  other 
  generalization 
  to 
  be 
  mentioned. 
  It 
  is 
  the 
  

   only 
  one 
  concerned 
  with 
  fertilization, 
  and 
  upon 
  its 
  truth 
  has 
  largely 
  

   depended 
  the 
  discovery 
  of 
  all 
  the 
  others. 
  

  

  10. 
  There 
  is 
  no 
  selective 
  fertilization 
  between 
  complementary, 
  com- 
  

   patible, 
  functional 
  gametes. 
  

  

  In 
  other 
  words, 
  if 
  a 
  series 
  of 
  male 
  gametes 
  meets 
  a 
  series 
  of 
  female 
  

   gametes 
  sufficiently 
  alike 
  to 
  be 
  compatible 
  with 
  them, 
  fertilization 
  

   takes 
  place 
  by 
  chance. 
  Clearly 
  chaos 
  would 
  result 
  if 
  this 
  were 
  not 
  

   so. 
  Gametes 
  produced 
  by 
  a 
  single 
  hermaphroditic 
  organism 
  may 
  

   present 
  hundreds 
  of 
  hereditary 
  differences. 
  The 
  slightest 
  tendency 
  

   to 
  selective 
  fertilization, 
  therefore, 
  would 
  prevent 
  genetic 
  analysis 
  of 
  

   the 
  results. 
  Happily, 
  this 
  is 
  not 
  the 
  case. 
  Even 
  in 
  the 
  flowering 
  

   plants 
  where 
  varying 
  lengths 
  of 
  pistil 
  tissue 
  must 
  be 
  traversed 
  by 
  the 
  

   male 
  gametophyte 
  before 
  fertilization 
  is 
  possible, 
  evidence 
  has 
  been 
  

   offered 
  that 
  rapidity 
  of 
  passage 
  probably 
  is 
  not 
  affected 
  by 
  differ- 
  

   ences 
  in 
  gametic 
  constitution. 
  Genes 
  evidently 
  do 
  not 
  begin 
  to 
  func- 
  

   tion 
  as 
  such 
  until 
  the 
  life 
  cycle 
  of 
  the 
  new 
  generation 
  commences. 
  1 
  

  

  OTHER 
  FACTS 
  OF 
  HEREDITY. 
  

  

  There 
  are 
  other 
  categories 
  of 
  facts, 
  several 
  of 
  them 
  probably 
  not 
  

   flowing 
  out 
  of 
  the 
  above 
  conclusions, 
  that 
  are 
  as 
  interesting 
  to 
  the 
  

   general 
  biologist 
  as 
  the 
  abstract 
  laws. 
  Without 
  them 
  one 
  can 
  not 
  

   get 
  a 
  just 
  idea 
  of 
  the 
  actual 
  concrete 
  Avorking 
  out 
  of 
  the 
  heredity 
  

   mechanism. 
  

  

  First 
  let 
  us 
  speak 
  of 
  dominance. 
  Dominance 
  was 
  originally 
  de- 
  

   fined 
  as 
  an 
  observational 
  phenomenon, 
  the 
  appearance 
  of 
  the 
  effect 
  

   of 
  one 
  of 
  a 
  pair 
  of 
  different 
  homologous 
  genes, 
  as 
  opposed 
  to 
  the 
  

   disappearance 
  or 
  exclusion 
  of 
  the 
  effect 
  of 
  the 
  other. 
  Later, 
  it 
  was 
  

   taken 
  to 
  be 
  the 
  presence 
  of 
  an 
  effect 
  as 
  the 
  antithesis 
  of 
  its 
  absence; 
  

   and 
  this 
  idea 
  was 
  carried 
  to 
  such 
  lengths 
  that 
  many 
  geneticists 
  came 
  

  

  1 
  It 
  should 
  be 
  mentioned, 
  however, 
  that 
  in 
  Nicotiana 
  (several 
  species) 
  and 
  in 
  Zea 
  the 
  

   pollen 
  tubes 
  from 
  different 
  plants 
  exhibit 
  different 
  rates 
  of 
  growth 
  under 
  similar 
  condi- 
  

   tions. 
  The 
  rates 
  appear 
  to 
  be 
  controlled 
  by 
  the 
  genetic 
  constitutions 
  of 
  the 
  mother 
  plants 
  

   from 
  which 
  they 
  came. 
  

  

  