﻿348 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  to 
  another, 
  have 
  been 
  determined, 
  and 
  they 
  are 
  found 
  to 
  have 
  a 
  practi- 
  

   cally 
  universal 
  validity. 
  There 
  is 
  no 
  use 
  attempting 
  here 
  to 
  formulate 
  

   in 
  detu 
  . 
  these 
  rules 
  and 
  their 
  working 
  out. 
  Most 
  of 
  modern 
  genetics 
  

   has 
  been 
  occupied 
  with 
  tracing 
  down 
  these 
  "facts." 
  They 
  relate 
  

   essentially 
  to 
  the 
  method 
  of 
  transmission, 
  to 
  later 
  generations, 
  of 
  gene 
  

   differences 
  that 
  are 
  already 
  found 
  to 
  exist 
  between 
  individuals. 
  They 
  

   show 
  the 
  universality 
  of 
  these 
  differences, 
  their 
  comparative 
  perman- 
  

   ence, 
  and 
  their 
  recombining 
  capabilities 
  (as 
  when 
  a 
  man 
  derived, 
  sev- 
  

   eral 
  generations 
  back, 
  from 
  a 
  mixture 
  of 
  European 
  and 
  Indian 
  blood 
  

   exhibits 
  the 
  coloration 
  of 
  the 
  European 
  and 
  the 
  broad 
  face 
  of 
  the 
  

   Indian). 
  But 
  they 
  leave 
  untouched 
  what 
  now 
  becomes 
  the 
  major 
  

   question 
  — 
  how 
  do 
  such 
  differences 
  originate 
  in 
  the 
  first 
  place? 
  What 
  

   is 
  the 
  origin 
  of 
  variations? 
  

  

  THE 
  FINDING 
  THAT 
  THE 
  GENES 
  MUTATE 
  

  

  A 
  hitherto 
  rather 
  incidental, 
  yet 
  very 
  important 
  part 
  of 
  modern 
  

   genetics 
  has 
  had 
  to 
  do 
  with 
  this 
  latter 
  problem. 
  It 
  has 
  been 
  discovered 
  

   definitely 
  that 
  such 
  differences 
  do 
  arise, 
  de 
  novo, 
  as 
  it 
  were. 
  That 
  is, 
  

   not 
  all 
  the 
  gene 
  differences 
  now 
  existing 
  in 
  a 
  population 
  have 
  existed 
  

   in 
  it 
  from 
  the 
  beginning. 
  New 
  differences 
  are 
  continually 
  arising 
  

   somehow, 
  and 
  the 
  differences 
  now 
  existing 
  have 
  undoubtedly 
  arisen 
  

   in 
  the 
  past 
  in 
  a 
  manner 
  similar 
  to 
  these. 
  

  

  Each 
  gene 
  difference 
  arises 
  suddenly 
  and 
  full 
  fledged, 
  though 
  we 
  

   may 
  not 
  be 
  aware 
  of 
  it 
  at 
  once. 
  Thus, 
  in 
  a 
  population 
  of 
  gray- 
  

   colored 
  mice, 
  suddenly 
  in 
  a 
  certain 
  cell 
  of 
  one 
  individual, 
  one 
  of 
  the 
  

   genes 
  whose 
  cooperation 
  is 
  necessary 
  for 
  the 
  production 
  of 
  the 
  gray 
  

   color 
  undergoes 
  a 
  change 
  into 
  a 
  gene 
  of 
  different 
  composition 
  that 
  

   tends, 
  in 
  its 
  interaction 
  with 
  the 
  other 
  genes 
  for 
  color, 
  to 
  produce 
  

   a 
  yellow 
  tinge 
  instead 
  of 
  gray. 
  In 
  this 
  single 
  cell, 
  however, 
  the 
  

   change 
  will 
  not 
  be 
  observed 
  by 
  us. 
  But 
  if 
  this 
  cell, 
  or 
  one 
  of 
  the 
  cells 
  

   derived 
  from 
  it, 
  happens 
  to 
  be 
  a 
  germ 
  cell, 
  an 
  offspring 
  individual 
  

   may 
  be 
  formed 
  in 
  the 
  next 
  generation, 
  all 
  of 
  whose 
  cells 
  may 
  carry 
  

   this 
  new 
  gene. 
  Then 
  if 
  the 
  new 
  gene 
  is 
  dominant 
  (as 
  it 
  happens 
  to 
  

   be 
  in 
  the 
  case 
  of 
  yellow 
  and 
  gray 
  in 
  mice) 
  to 
  the 
  old 
  gene 
  for 
  gray 
  

   which 
  the 
  offspring 
  has 
  received 
  from 
  its 
  other 
  parent, 
  the 
  coat 
  of 
  the 
  

   new 
  animal 
  will 
  be 
  yellow, 
  and 
  we 
  will 
  see 
  that 
  a 
  mutation 
  has 
  

   occurred. 
  But 
  if 
  the 
  new 
  gene 
  had 
  been 
  recessive 
  (i. 
  o 
  , 
  il' 
  the 
  gray 
  was 
  

   dominant) 
  the 
  offspring 
  would 
  have 
  appeared 
  gray 
  like 
  its 
  parents 
  

   and 
  we 
  should 
  not 
  have 
  been 
  aware 
  of 
  the 
  mutation. 
  The 
  new 
  gene 
  

   might 
  persist 
  none 
  the 
  less, 
  and 
  be 
  inherited 
  by 
  generation 
  after 
  

   generation 
  in 
  invisible 
  fashion, 
  being 
  meanwhile 
  "dominated 
  over" 
  

   by 
  the 
  gray 
  from 
  the 
  other 
  parent. 
  If 
  in 
  a 
  later 
  generation 
  two 
  

   descendants 
  both 
  of 
  which 
  cr.rried 
  the 
  mutated 
  gene 
  happened 
  to 
  

   mate 
  together, 
  an 
  egg 
  with 
  the 
  yellow 
  might 
  become 
  fertilized 
  by 
  a 
  

   spierm 
  also 
  carrying 
  yellow, 
  neither, 
  therefore, 
  carrying 
  the 
  dominant 
  

  

  