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
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reserved © 1997-2000 Chris Rutt
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