Let's face it: people make the rules. You'll
never find a group of star-nosed moles sitting around and discussing
the differences between one population of earthworms and another. Yet,
scientists discuss differences in populations of various animals and
plants all the time. But how can one rationally discuss differences
between populations unless one has a standard, i.e., a base line group
to compare these differences to? For our purposes, this base line group
will be known as the "wild-type".
The ancestor to our domestic pigeons is
the wild species variously known in English as the rock-dove,
rock-pigeon, or common pigeon. It's also got many other names in
various languages as well, e.g., pigeon biset (French), paloma
doméstica (Spanish), Klippedue (Norwegian), Αγριοπερίστερο (Greek),
etc. Its scientific
name is Columba
livia.
This species is a wide ranging one found
from Scotland to Iran and from northern Europe to northern Africa. When
we look at it, what we find overall is a fairly standard dove-gray bird
weighing about 8 oz. (227 g.) with two black bars on its wings and a
black sub-terminal tail bar. Yet, in its vast range there are some
obvious differences in various population. Some show red orbital
skin around the eyes rather than the more common dark skin. Some have a
dark rump rather than a light one. The shade of blue also varies; with
some populations being darker than others. Some groups have youngsters
with dark feet in the nest, some with light. There are also some minor
differences in voice and size. So how do we decide which group is
the one we'll use as our baseline? Simple, we pick one. In
biology, the baseline group is the one which is first formally
identified and described. In the case of the common pigeon, this is
that population of pigeons found in western Europe. Here the common
pigeon tends to have orange-red eyes, light rump and a fairly light
blue color. It is this
population of birds which is considered to be the wild-type. For our
purposes in pigeon genetics, this bird is the median against which
everything is judged.
Any
mutation, or change, from this wild-type is compared to it and
described. For example, the wild type pigeon has no crest. At
some point in time, there was a mutation (change) which caused the
feathers of the rear of the neck or head to grow reversed. This
mutation was described and given a genetic
symbol (cr). When I say it was described, I don't simply mean that
someone said a crest is a group of feathers which stand up in the back
of the head. Rather, I mean that someone performed breeding tests
to check how crest is inherited. They found that if a crested
bird was paired to a wild type bird that all the resulting young were
non-crested. These young, called the F1 generation, paired together
brother and sister produced both crested and non-crested young.
Further testing indicated that the ratio of crested to non-crested
young showed that this particular mutation is inherited as a simple
(autosomal) recessive condition.
In
like manner, Ash-red, a mutation from wild-type was described. In
this case, the mutation was found to be dominant to the wild type and
also found to be sex-linked
in its inheritance. But notice, everything is referred back to
the wild-type. To all intents and purposes when we test domestic
pigeons for color or structural changes from the wild-type, the bird we
consider to be the standard tester is the blue bar homer. For
most, not all, testings, this is considered to be the closest we can
get to actually using the wild pigeon. Wild type also is used to simply
mean the wild type condition for the factor we are testing. For
example, suppose I want to test a black pigeon to see whether it is
homozygous (carries two genes) or heterozygous (carries only one gene)
for Spread. I simply need as my mate for this bird any non-spread
pigeon. I don't care whether it's check, bar, barless. I don't
care whether it's brown, blue or Ash-red. I simply want a bird which is
wild-type (non-spread) at that particular spot on the chromosome where
the mutation for spread would normally be found.
The
important thing to remember about wild type is that it's simply the
standard against which we judge everything. The genetic symbol for
wild-type is (+). That's because we have no idea what goes into making
the wild pigeon what it is. ONLY when there is a mutation from the
normal wild type can we test. Some spots on the pigeon's chromosome
seem to be more prone to mutation than others -- or the effect caused
by a mutation there is more easily observed by us. The spot (locus) on
the sex-chromosome where there has been a mutation to Almond
is one such place. Here there are multiple alleles known. There might
well be other spots on other chromosomes where there are also mutiple
alleles, maybe even many more than in the Almond group, but such
alleles may simply code for differences in blood type or the production
of different enzymes in the bird's body. Things like this would seldom,
if ever, be observed by the average fancier - and might easily escape
notice in a laboratory setting as well.
So, recapping quickly. Wild type is the
original type and is the standard against which we compare any
mutation. If a mutation needs only one copy of itself in the bird's
body to show its effects, we call it a dominant mutation. If it needs
two copies of itself to show its effect, we call it a recessive
mutation. So, we can say crest is a recessive mutation (to wild-type is
always understood.) In like manner, Indigo is a dominant mutation (to wild type,
again understood.) Wild type is ALWAYS our base line.
(Information I
received in personal conversation on Sept. 26, 1997, with both Drew
Lobenstein and Leon Stephens indicates there is a dominant crest
mutation found as part of the geneotype of Jacobins. I've asked both
men to please write up their findings. If the information is
indeed accurate, then whoever first formally describes the mutation and
its effect will have the honor of choosing the symbol which will
represent it.)
