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Essential
Genetics for the Terrified
by Chris Rutt
Part 6: Alleles
...but before getting into
this, I left you with the question: Can we tie up the ratios to
specific types of mating and inheritance? Yes we can. 3:1 is the
proportion of colours we get when we mate two (genetically identical)
birds which are HETEROZYGOTES carrying a simple dominant:recessive pair
of genes. 1:2:1 is the expected proportion if we mate HETEROZYGOTES
carrying an incomplete dominant pair of genes. OK?
If you have, as I hope, managed to get to grips and become confident
with the previous parts, you should already be making use of your new
found expertise. So let us move on to the idea of Alleles. It is not
terribly complicated, and will be one of the shorter sections.
In fact any pair of partner genes, like "Dark" / "Not Dark", are known
as "alleles" of each other. So far we have considered that each Gene in
a pair has only one possible partner. However, in some cases a specific
gene has more than one potential partner. Let us look at the
inheritance of the "Whitefaced Blue", or "Lavender". I will use the
name "Lavender" because it saves my typing fingers, and while they are
not truly lavender coloured, neither are they completely white faced or
completely blue. I will use the code " m' " for this mutation. In our
letter code I will describe the Lavender as " m'm' ". It is small "m"
because it is recessive to the "normal" Light Green gene. When paired
up with the Marine gene "m" (with no " ' ") it appears to act like an
INCOMPLETE DOMINANT, so that we have these possibilities:
| MM |
Light Green |
| mm |
Marine (Pastel or Dutch Blue) |
| 'm' |
Lavender |
| Mm |
Light green "split for" Marine
(you remember the term HETEROZYGOTE) |
| Mm' |
Light green "split for" Lavender |
| mm' |
APPLE GREEN |
In these types of
inheritance, we could perhaps envisage that while there are three genes
waiting to occupy that particular piece of the master plan, there is
only space for two. With the knowledge you now have, you will be able
to work out the potential outcome of a pairing of any of these
varieties. Use the grid layout, and if you need to, refer back to the
Sections on DOMINANCE and INCOMPLETE DOMINANCE. It is worth at this point discussing the variations in
colour among birds which are described above as simple single gene
mutations.
Experienced breeders will be well aware that among Jades, the degree of
darkness of the green feathers varies considerably. Marines, Lavenders
and Apple Greens vary considerably in how blue they are. Some Lavenders
turn out very green indeed, particularly when bred from two birds of
the configuration Mm' as compared with those from mm' x m'm or m'm' x
m'm'. Another common example of this type of inheritance occurs in the
relationship between the Lutino and the Australian Cinnamon in the
Peachfaced Lovebird. The fact is that very few mutations are solely
controlled by a single gene. Other genes, and there may be many,
contribute to the phenotype and lead to the more subtle differences
which we observe. Very often these MODIFIER genes pass with the main
one, but for reasons we will see in a later section, are more likely to
be rearranged during transmission.
Part
7: Chromosomes
All rights reserved
© 1997-2000 Chris Rutt
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