﻿358 
  

  

  Croonian 
  Lecture 
  : 
  — 
  Genetic 
  Segregation. 
  

  

  By 
  W. 
  Bateson, 
  M.A., 
  F.K.S. 
  

  

  (Received 
  September 
  20, 
  — 
  Lecture 
  delivered 
  June 
  17, 
  1920.) 
  

  

  The 
  later 
  developments 
  of 
  Mendelian 
  analysis 
  have 
  been 
  in 
  the 
  main 
  an 
  

   attempt 
  to 
  elucidate 
  the 
  scope 
  and 
  nature 
  of 
  Segregation. 
  Mendel 
  proved 
  

   the 
  existence 
  of 
  characters 
  determined 
  by 
  integral 
  or 
  unit 
  factors. 
  Their 
  

   integrity 
  is 
  maintained 
  by 
  segregation, 
  the 
  capacity, 
  namely, 
  to 
  separate 
  

   unimpaired 
  after 
  combination 
  with 
  their 
  opposites. 
  Our 
  first 
  aim 
  has 
  been 
  

   to 
  discover 
  specifically 
  what 
  characters 
  behave 
  in 
  this 
  way, 
  whether 
  there 
  is 
  

   any 
  limit 
  to 
  the 
  scope 
  of 
  segregation, 
  or 
  any 
  characters 
  or 
  classes 
  of 
  

   characters 
  which 
  are 
  determined 
  by 
  elements 
  unable 
  to 
  segregate 
  simply. 
  

   The 
  second 
  object 
  has 
  been 
  to 
  decide 
  the 
  time 
  and 
  place 
  in 
  the 
  various 
  life- 
  

   cycles 
  at 
  which 
  segregation 
  occurs. 
  It 
  is 
  with 
  the 
  latter 
  problem 
  that 
  I 
  

   propose 
  to 
  deal 
  more 
  particularly 
  in 
  this 
  Lecture, 
  but 
  a 
  brief 
  consideration 
  of 
  

   the 
  range 
  of 
  characters, 
  subject 
  to 
  segregation, 
  is 
  appropriate. 
  

  

  1. 
  The 
  Scope 
  of 
  Segregation. 
  

  

  Of 
  the 
  classes 
  of 
  features 
  by 
  which 
  animals 
  and 
  plants 
  are 
  distinguished, 
  

   most 
  have 
  now 
  been 
  shown 
  to 
  be 
  dependent 
  on 
  segregable 
  elements. 
  It 
  is 
  

   perhaps 
  noticeable 
  that 
  we 
  have 
  no 
  quite 
  clear 
  proof 
  that 
  the 
  factors 
  

   governing 
  differences 
  in 
  number, 
  meristic 
  characters 
  in 
  the 
  strict 
  sense, 
  

   commonly 
  behave 
  so 
  simply 
  as 
  those 
  determining 
  other 
  characteristics. 
  

   There 
  are 
  examples 
  of 
  parts 
  repeated 
  in 
  series, 
  such 
  , 
  as 
  the 
  extra 
  toe 
  of 
  the 
  

   fowl 
  (a 
  dominant) 
  and 
  the 
  leaf 
  of 
  the 
  monophyllous 
  Strawberry 
  with 
  a 
  single 
  

   leaflet 
  ( 
  a 
  recessive) 
  which 
  have 
  a 
  factorial 
  inheritance, 
  but 
  the 
  resulting 
  

   terms, 
  especially 
  the 
  heterozygotes, 
  are 
  indefinite. 
  In 
  the 
  polydactylous 
  

   fowl, 
  for 
  example, 
  the 
  heterozygote 
  has 
  a 
  great 
  variety 
  of 
  shapes. 
  The 
  hind 
  

   toe 
  is 
  most 
  often 
  represented 
  by 
  two 
  sub-equal 
  digits, 
  but 
  the 
  duplicity 
  may 
  

   be 
  so 
  slight 
  as 
  to 
  appear 
  externally 
  only 
  as 
  a 
  division 
  of 
  the 
  nail. 
  It 
  may 
  

   also 
  assume 
  another 
  very 
  different 
  form, 
  in 
  which 
  there 
  is 
  only 
  a 
  single 
  many- 
  

   jointed 
  digit 
  representing 
  the 
  usual 
  pair. 
  In 
  the 
  monophyllous 
  Strawberry 
  

   the 
  homozygous 
  recessive, 
  whether 
  before 
  or 
  after 
  immediate 
  extraction 
  from 
  

   the 
  heterozygote, 
  shows 
  fluctuations 
  to 
  a 
  2- 
  or 
  3-leaved 
  condition.* 
  Such 
  

   fluctuations 
  are 
  common 
  among 
  forms 
  distinguished 
  by 
  meristic 
  peculiarities. 
  

   There 
  is 
  not 
  the 
  uniformity 
  and 
  simplicity 
  which 
  is 
  such 
  a 
  striking 
  feature 
  of 
  

  

  * 
  Richardson, 
  C. 
  W., 
  'Jour. 
  Gen.,' 
  vol. 
  3, 
  p. 
  171 
  (1914). 
  

  

  