Essential
Genetics for the Terrified
by Chris Rutt
Part
4: Multiple Genes (1)
In
the example we have just dealt with, where we
bred Light Green to Olive producing all Jades,
and Jade to Jade, producing one quarter Olives,
one quarter Light Greens and half Jades, we have
in fact been dealing with the transmission of TWO
genes from grandparents to grandchildren, but one
of them has been the same in every individual,
the gene for "Green-ness". How can we
calculate the expectation if both genes are
different between the parents, for example if we
mate a pure Olive Green to a Marine which has no
genes for "Darkness"? We need to have
some more marbles, if we are to work it out in
the way we used before. However, we need first to
be clear as to the genetic make up of each parent
in relation to the characteristics with which we
are dealing.
In this case, the Olive parent has two genes for
Darkness (often referred to as "having two
dark factors"), and two genes for
"Green-ness". The Marine must be
considered as having two genes for Marine, and
two genes for "NOT dark". This idea of
genes for "Not ..." is important. Genes
NORMALLY work in pairs, for reasons I will deal
with in the next article. Birds must NORMALLY
have two genes for each characteristic. (The
exceptions will be dealt with in a later
section). Visualise therefore each parent being
represented by TWO sacks. As its contribution to
the colour of the offspring EACH parent takes, at
random, one marble from each sack. In this case,
as we are dealing with pure (=
"homozygous") parents the pairs of
marbles/genes produced by the Marine Parent will
all be "Marine & Not dark" and from
the Olive parent "Green & Dark" .
So, we find that the four genes (in two pairs) in
all the offspring are "Marine:Green"
"Dark:Not dark".
Before I go on, take a moment to consider what
the phenotype of the F1 is. (Translation: What do
these first generation chicks look like?)
Part 5: Multiple Genes (2)
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reserved © 1997-2000 Chris Rutt
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