The X-linked red- and green-pigment genes are arranged in a head-to-tail
tandem array1,
2,
3. The colour-vision defect of deuteranomaly
(in 5% of males of European descent) is associated with a 5´-green-red-3´
visual-pigment hybrid gene4,
5, which may also exist in males
with normal colour vision5,
6,
7. To explain why males with
a normal red, a normal green and a green-red hybrid gene may have either normal
or deutan colour vision, we hypothesized that only the first two genes are
expressed8,
9 and deuteranomaly results only if the green-red
hybrid gene occupies the second position and is expressed preferentially over
normal green-pigment genes occupying more distal positions. We used long-range
PCR amplification and studied 10 deutan males (8 deuteranomalous and 2 deuteranopic)
with 3 visual pigment genes (red, green and green-red hybrid) to investigate
whether position of the hybrid gene in the array determined gene expression.
The green-red hybrid gene was always at the second position (and the first
position was always occupied by the red gene). Conversely, in two men with
red, green and green-red hybrid genes and normal colour vision, the hybrid
gene occupied the third position. When pigment gene mRNA expression was assessed
in post-mortem retinae of three men with the red, green and green-red genotype,
the green-red hybrid gene was expressed only when located in the second position.
We conclude that the green-red hybrid gene will only cause deutan defects
when it occupies the second position of the pigment gene array.
