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Essential
Genetics for the Terrified
by Chris Rutt
Part 2:
Dominance
So, from a pairing of a pure
Marine (a more modern, and shorter, term for Pastel Blue, which is also
known as "Dutch Blue") with a pure Light Green, why all Light Green
offspring and not a mixture? We have now come to the idea of
"DOMINANCE" which I will now try to explain.
To do this let us consider what happens when we mate these Light Green
offspring (remember we call these F1) to produce F2 and find we get a
significant proportion, which over a large enough number of chicks will
be around one quarter, of Marine chicks.
Let us, in our heads, do an experiment. Take two large sacks of
marbles. The original parents could be thought of as a sack of light
green and a sack of Marine marbles. Each parent passes one "marble" to
each of its offspring, all of which will therefore have one light green
and one marine marble.
Each of the offspring (F1) can therefore be represented by a sack
containing equal numbers of Pastel Blue (which I will represent for the
moment as "M") and Light Green ("LG") marbles. Now, while blindfolded,
take out one marble from each of these evenly mixed sacks to make a
pair, and sort them, in the pairs as you took them from the sacks.
Some pairs will be (LG)(LG), others (M)(M) and the majority, (LG)(M).
If we count them, and we have made enough pairs, and we really didn't
peek, they will be in the following proportions:
| (LG)(LG) |
one
quarter |
| (M)(M) |
one
quarter |
| (LG)(M) |
the
remaining half |
I think we would all now at
least suspect that our Marine chicks in F2 are represented by the
occasions when the pair consist of two Marine marbles. Those which
received only light green will obviously be green themselves, and to
make up the numbers those chicks which received BOTH colours (one from
each parent) have also appeared as green.
Thus if the factor for light green is present, it suppresses the factor
for Marine, though this is still present and therefore capable of being
passed on to the following generation. This explains why, in the first
generation, no Marines appeared although ALL the offspring received a
Marine gene from the Marine parent - it, after all, only had Marine
genes to give.
This phenomenon is known as "DOMINANCE". Light Green is said therefore
to be DOMINANT to Marine. Conversely, Marine is said to be "RECESSIVE"
to Light Green.
In concluding this first part of the lesson, let us introduce some more
terms that may be helpful when reading other texts on this subject.
The inheritance controlling particles which pass from parent to
offspring are known as "genes".
Birds which have two identical genes for a particular characteristic
are called "homozygotes"and birds which have different genes
"heterozygotes". "Hetero" and "homo" are both familiar from other
contexts. "Zygote" means a fertilised egg.
In bird keeping terms, "homozygotes" are said to be "pure" for the
characteristic, "heterozygotes" are commonly described by bird keepers
as "splits" or "carriers". "Homozygotes" for a recessive gene (i.e. the
Marine birds in the above example) are sometimes called "visuals",
because you can see the presence of a recessive gene.
Part
3: Incomplete Dominance
All rights reserved
© 1997-2000 Chris Rutt
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