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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