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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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© 1997-2000 Chris Rutt
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