﻿GENETIC 
  PROGRESS 
  — 
  EAST. 
  293 
  

  

  variable 
  degrees 
  of 
  thoroughness. 
  They 
  are 
  not 
  set 
  down 
  here 
  with 
  

   the 
  idea 
  they 
  are 
  necessarily 
  more 
  inviolable 
  than 
  the 
  laws 
  of 
  the 
  

   Medes 
  and 
  Persians, 
  but 
  merely 
  with 
  the 
  annotation 
  that 
  no 
  experi- 
  

   mental 
  data 
  have 
  thus 
  far 
  overstepped 
  them 
  except 
  those 
  on 
  the 
  plas- 
  

   tids. 
  They 
  may 
  be 
  called 
  provisional 
  laws 
  of 
  heredity. 
  

  

  The 
  first 
  five 
  of 
  these 
  generalizations 
  describe 
  the 
  mechanism 
  of 
  

   heredity 
  in 
  the 
  preparation 
  of 
  the 
  germ 
  cells 
  for 
  fertilization. 
  

  

  1. 
  There 
  is 
  segregation 
  of 
  paternal 
  genes 
  from 
  maternal 
  genes, 
  

   each 
  unchanged 
  by 
  the 
  association. 
  The 
  pedigree 
  culture 
  evidence 
  

   supplements 
  the 
  cytological 
  evidence 
  in 
  favor 
  of 
  the 
  idea 
  that 
  this 
  

   segregation 
  takes 
  place 
  at 
  the 
  reduction 
  division 
  of 
  the 
  chromosomes 
  

   in 
  the 
  maturation 
  of 
  the 
  germ 
  cells, 
  when 
  homologous 
  paternal 
  and 
  

   maternal 
  genes 
  pair 
  and 
  separate, 
  one 
  of 
  each 
  pair 
  passing 
  to 
  each 
  

   of 
  the 
  two 
  daughter 
  cells. 
  

  

  2. 
  There 
  may 
  be 
  any 
  combination 
  of 
  the 
  choice 
  of 
  one 
  out 
  of 
  each 
  

   pair 
  of 
  genes 
  in 
  making 
  up 
  the 
  genetic 
  constitution 
  of 
  the 
  gametes. 
  

  

  3. 
  In 
  transmission 
  to 
  the 
  two 
  daughter 
  cells, 
  certain 
  sets 
  of 
  genes 
  

   are 
  always 
  manipulated 
  independently 
  of 
  all 
  other 
  genes. 
  This 
  is 
  

   a 
  statement 
  of 
  independent 
  inheritance, 
  or 
  rather 
  of 
  independent 
  

   genetic 
  recombination, 
  without 
  reference 
  to 
  the 
  chromosomes. 
  It 
  is 
  

   an 
  unworthy 
  piece 
  of 
  quibbling, 
  however, 
  not 
  to 
  accept 
  the 
  simple 
  in- 
  

   dication 
  of 
  cytology 
  that 
  this 
  law 
  is 
  the 
  result 
  of 
  the 
  operation 
  of 
  

   the 
  chromosomes 
  acting 
  as 
  gene 
  carriers. 
  It 
  results 
  from 
  the 
  fact 
  

   that 
  No. 
  1 
  pair 
  of 
  chromosomes, 
  no 
  matter 
  how 
  it 
  may 
  be 
  packed 
  with 
  

   genes, 
  carts 
  its 
  cargo 
  independently 
  of 
  all 
  other 
  pairs. 
  

  

  4. 
  The 
  manipulation 
  of 
  genes 
  within 
  a 
  given 
  series 
  is 
  always 
  de- 
  

   pendent. 
  This 
  is 
  the 
  phenomenon 
  of 
  linkage, 
  or 
  association 
  in 
  in- 
  

   heritance. 
  Concretely, 
  we 
  assume 
  that 
  the 
  genes 
  packed 
  within 
  each 
  

   freight 
  carrier 
  pair, 
  an 
  homologous 
  pair 
  of 
  chromosomes, 
  are 
  mu- 
  

   tually 
  dependent 
  to 
  various 
  degrees 
  in 
  their 
  recombinations 
  with 
  

   each 
  other. 
  

  

  5. 
  The 
  number 
  of 
  dependent 
  series 
  or 
  linkage 
  groups 
  is 
  limited. 
  

   In 
  the 
  fruit 
  fly, 
  the 
  number 
  corresponds 
  to 
  the 
  number 
  of 
  chromo- 
  

   somes, 
  and 
  the 
  presumption 
  is 
  that 
  this 
  is 
  the 
  case 
  in 
  every 
  species. 
  

  

  Added 
  to 
  these 
  five 
  laws 
  are 
  four 
  more 
  generalizations 
  which 
  refer 
  

   particularly 
  to 
  the 
  architecture 
  of 
  the 
  germ 
  plasm 
  within 
  the 
  carrier. 
  

  

  6. 
  There 
  is 
  a 
  stable 
  orderly 
  arrangement 
  of 
  genes. 
  

  

  7. 
  This 
  arrangement 
  is 
  linear. 
  The 
  genes 
  appear 
  to 
  be 
  strung 
  

   together 
  much 
  as 
  a 
  string 
  of 
  magnetized 
  steel 
  balls. 
  

  

  8. 
  Rearrangement 
  of 
  genes 
  after 
  linkage 
  breaks 
  is 
  stable, 
  orderly, 
  

   and 
  linear. 
  By 
  this 
  it 
  is 
  meant 
  to 
  say 
  that 
  rearrangement 
  of 
  the 
  

   package 
  after 
  interchange 
  of 
  the 
  contents 
  of 
  a 
  pair 
  of 
  carriers 
  is 
  also 
  

   constant 
  and 
  of 
  the 
  same 
  linear 
  order 
  as 
  in 
  No. 
  6 
  and 
  No. 
  7. 
  

  

  55379—24 
  20 
  

  

  