TY  - CHAP
M1  - Book, Section
TI  - Color Vision and Its Genetic Defects
A1  - Motulsky, Arno G.
A1  - Deeb, Samir S.
A2  - Valle, David L.
A2  - Antonarakis, Stylianos
A2  - Ballabio, Andrea
A2  - Beaudet, Arthur L.
A2  - Mitchell, Grant A.
Y1  - 2019
N1  - 10.1036/ommbid.278
T2  - The Online Metabolic and Molecular Bases of Inherited Disease
AB  - Color  vision  has  intrigued  scientists  for  several  hundred  years.  Red-green  color  vision  defects  are  common  and  among  the  first  recognized  X-linked  traits.  The  molecular  genetics  of  the  visual  pigments  mediating  normal  and  defective  color  vision  have  been  elucidated  and  provide  the  basis  for  an  understanding  of  the  genetic  basis  of  normal  and  abnormal  color  vision.The  retina  is  a  displaced  part  of  the  brain  and  includes  four  different  photoreceptors:  rods  containing  rhodopsin  and  cones  containing  photopigments  sensitive  to  either  blue  (short-wave),  green  (middle-wave),  or  red  (long-wave).  Normal  color  vision  is  trichromatic  and  is  subserved  by  these  three  cone  pigments.  Rhodopsin  is  used  for  dim-light  vision,  whereas  the  various  cone  photopigments  mediate  vision  in  bright  light  and  color  vision.  The  photopigments  have  characteristic  absorption  maxima  with  wide  regions  of  overlap.  The  four  human  visual  pigments  are  similar  in  their  amino  acid  sequences  and  are  members  of  the  heptahelical  transmembrane  receptor  family  that  includes  olfactory  receptors.  The  red  and  green  pigments  differ  by  at  most  15  amino  acids.  Differences  at  three  residues  (180,  277,  and  285)  largely  account  for  spectral  differences  between  these  pigments.The  autosomal  genes  for  rhodopsin  and  the  blue  pigment  are  located  at  chromosome  3q21–24  and  7q31.3–q32,  respectively.  The  red-green  pigment  gene  complex  maps  to  a  subterminal  site  on  the  long  arm  of  the  X  chromosome  (Xq28)  linked  to  the  loci  for  adrenoleukodystrophy,  glucose-6-phosphate  deficiency,  and  hemophilia.  The  red-green  gene  arrays  are  composed  of  a  single  red  pigment  gene  (six  exons)  and  one  or  more  green  pigment  genes  (six  exons)  located  downstream  (3′)  of  the  red  gene.  About  25  percent  of  male  Caucasians  have  a  single  green  pigment  gene,  whereas  the  rest  have  two,  three,  or  more  green  pigment  genes.  Almost  half  of  Japanese  and  African-American  males  only  have  a  single  green  pigment  gene.  The  high  homology  of  the  red  and  green  opsin  genes  (including  introns)  predisposes  to  unequal  crossover  and  accounts  for  the  numerical  polymorphism.  Gene  expression  studies  suggest  that  only  the  two  most  proximal  among  several  pigment  genes  are  expressed  in  the  retina.  The  ratio  of  red  to  green  pigment  mRNAs  in  human  retinas  varies  widely  (1–10,  with  a  mode  of  4).Illegitimate  recombination  between  the  red  and  green  pigment  genes  causes  deletions  or  the  formation  of  hybrid  genes  and  explains  the  genetic  basis  of  the  majority  of  color  vision  defects.  The  deletion  of  green  pigment  genes  leaves  a  single  red  pigment  gene  that  is  characteristically  associated  with  deuteranopia  (G−).  Affected  individuals  are  dichromatic  because  they  completely  lack  functional  green  cones.  The  finding  of  severe  trichromatic  deuteranomaly  (G′)  in  a  few  individuals  with  this  genetic  makeup  remains  unexplained.Exon  5  (which  includes  two  residues  that  account  for  two-thirds  of  the  spectral  difference  between  the  red  and  green  pigments)  plays  a  major  role  in  spectral  tuning.  The  recombinational  exchange  of  exon  5  produces  hybrid  pigments  with  large  spectral  shifts  and  corresponding  effects  on  color  vision.5′  Green–red  3′  hybrid  genes  with  or  without  additional  green  genes  usually  are  associated  with  deuteranomaly—a  milder  type  of  color  vision  defect  with  a  red-shifted  absorption  maximum  for  the  green  ...
SN  - 
PB  - McGraw-Hill Education
CY  - New York, NY
M3  - doi: 10.1036/ommbid.278
Y2  - 2024/04/18
UR  - ommbid.mhmedical.com/content.aspx?aid=1184070884
ER  -