﻿HERITABLE 
  VAEIATIONS 
  MULLER 
  353 
  

  

  trail 
  of 
  havoc 
  before 
  it 
  is 
  finally 
  stopped. 
  In 
  this 
  process, 
  many 
  of 
  the 
  

   atoms 
  through 
  which 
  the 
  electron 
  tears 
  have 
  one 
  or 
  more 
  of 
  their 
  

   own 
  electrons 
  torn 
  out 
  or 
  dislodged 
  from 
  their 
  proper 
  places; 
  this 
  

   change 
  in 
  the 
  structure 
  of 
  the 
  atoms 
  often 
  causes 
  them 
  to 
  undergo 
  

   new 
  chemical 
  unions 
  or 
  disunions 
  that 
  in 
  turn 
  alter 
  the 
  composition 
  of 
  

   the 
  molecules 
  in 
  which 
  the 
  atoms 
  lay. 
  If 
  a 
  gene 
  is 
  a 
  molecule, 
  then, 
  

   with 
  properties 
  depending 
  upon 
  its 
  chemical 
  composition, 
  it 
  can 
  be 
  

   shot 
  and 
  altered 
  by 
  the 
  electrons 
  resulting 
  from 
  the 
  absorption 
  of 
  

   X 
  rays 
  or 
  rays 
  of 
  shorter 
  wave 
  length. 
  The 
  only 
  question 
  would 
  be, 
  

   can 
  enough 
  mutations 
  be 
  caused 
  in 
  this 
  way 
  to 
  be 
  detectable 
  by 
  our 
  

   present 
  methods 
  with 
  doses 
  of 
  rays 
  small 
  enough 
  not 
  to 
  kill 
  or 
  steri- 
  

   hze 
  the 
  treated 
  organism? 
  

  

  With 
  these 
  points 
  in 
  mind, 
  the 
  author 
  undertook 
  in 
  the 
  fall 
  of 
  1926 
  

   a 
  series 
  of 
  experiments 
  designed 
  to 
  test 
  the 
  question 
  at 
  issue. 
  The 
  

   fruit 
  fly, 
  Drosophila, 
  was 
  used 
  since 
  it 
  is 
  so 
  easily 
  and 
  rapidly 
  bred 
  in 
  

   large 
  numbers 
  and 
  since 
  it 
  rendered 
  possible 
  the 
  employment 
  of 
  

   special 
  genetic 
  technique 
  for 
  the 
  finding 
  of 
  mutations, 
  that 
  had 
  been 
  

   elaborated 
  in 
  the 
  course 
  of 
  my 
  previous 
  work 
  on 
  linkage 
  and 
  mutation 
  

   in 
  this 
  organism. 
  It 
  would 
  take 
  us 
  too 
  far 
  afield 
  here 
  to 
  examine 
  

   this 
  technique 
  in 
  detail. 
  Stocks 
  of 
  flies 
  had 
  been 
  made 
  up 
  containing 
  

   in 
  given 
  combinations 
  certain 
  genes 
  with 
  conspicuous 
  effects 
  which 
  

   would 
  serve 
  to 
  notify 
  the 
  investigator 
  that 
  the 
  chromosome 
  under 
  

   consideration 
  was 
  present. 
  On 
  making 
  given 
  crosses 
  of 
  these 
  stocks 
  

   with 
  other 
  stocks 
  various 
  combinations 
  of 
  characteristics 
  would 
  be 
  

   expected 
  in 
  the 
  first 
  and 
  following 
  generations. 
  If 
  flics 
  A\dth 
  some 
  

   particular 
  expected 
  combination 
  were, 
  however, 
  absent 
  from 
  a 
  given 
  

   culture, 
  it 
  would 
  mean 
  that 
  a 
  mutation 
  had 
  occurred 
  that 
  had 
  given 
  

   rise 
  to 
  a 
  lethal 
  gene— 
  one 
  that 
  had 
  killed 
  the 
  flies 
  containing 
  it 
  before 
  

   they 
  had 
  a 
  chance 
  to 
  hatch. 
  By 
  noting 
  which 
  combinations 
  were 
  

   missing, 
  it 
  could 
  be 
  deduced 
  what 
  chromosome 
  of 
  the 
  fly 
  the 
  lethal 
  was 
  

   in, 
  and 
  at 
  what 
  place 
  in 
  the 
  chromosome 
  it 
  lay. 
  On 
  the 
  other 
  hand, 
  

   mutant 
  genes 
  having 
  visible 
  instead 
  of 
  lethal 
  effects 
  would 
  be 
  detect- 
  

   able 
  through 
  the 
  appearance 
  of 
  the 
  visible 
  variations, 
  and 
  these 
  too 
  

   could 
  be 
  traced 
  to 
  their 
  chromosome 
  position 
  through 
  studies 
  of 
  the 
  

   nature 
  and 
  frequency 
  of 
  the 
  combinations 
  in 
  which 
  they 
  appeared. 
  

   Mutant 
  genes 
  that 
  were 
  recessive 
  to 
  the 
  normal 
  type, 
  however— 
  and 
  

   most 
  mutations 
  are 
  recessive 
  — 
  would 
  not 
  have 
  a 
  chance 
  to 
  be 
  seen 
  or 
  

   found 
  until 
  the 
  second 
  or 
  third 
  generation 
  of 
  offspring 
  subsequent 
  to 
  

   their 
  origination. 
  The 
  reason 
  why 
  recessive 
  mutations 
  are 
  not 
  

   evident 
  at 
  once 
  has 
  been 
  explained 
  previously. 
  

  

  In 
  these 
  experiments 
  the 
  adult 
  flies— 
  in 
  some 
  cases 
  the 
  males, 
  in 
  

   other 
  cases 
  the 
  females— 
  were 
  placed 
  in 
  gelatin 
  capsules 
  and 
  subjected 
  

   to 
  doses 
  of 
  X 
  rays 
  so 
  strong 
  as 
  to 
  produce 
  partial 
  sterihty, 
  though 
  the 
  

   other 
  functions 
  of 
  the 
  flies 
  are 
  not 
  noticeably 
  disturbed 
  by 
  a 
  dose 
  

  

  