PART I – SOME BIOLOGY
by Sheila Sowter
Why bother with genetics?
For anyone who is interested in breeding an understanding of genetics can be a
guide to what you might get from particular doe and buck and what to mate
together to give the variety you want.
Firstly there are some technical terms to
recognise. The rat has 42 chromosomes, which are strings of DNA in its
cells. Along the strings are the genes, short lengths which give the
instructions about what the rat will be like. If it helps, you can think of
each chromosome as a book of instructions with each gene as a page which might
say “this rat should be pink-eyed”, while a page in another book might say “this
rat should have curly hair”. These pages are called loci (singular,
locus) and are referred to by letters. For instance, the gene that gives rise to
the Hooded pattern is on the H locus. Different genes on the same locus are
called alleles.
This may sound straightforward but the first
complication is that the 42 chromosomes occur in pairs, 20 where the two
chromosomes more or less match and one pair, the sex chromosomes, where a buck
has an X and a Y, whereas a doe has a pair of X’s. When sperm and eggs are made
each gets one of each pair of chromosomes, so when sperm meets egg to
start a new rat, this rat will have 21 chromosomes from its father and 21 from
its mother to give the 21 pairs.
If the alleles on a given locus are the same
from both parents the rat is homozygous at that locus, if they are
different, it is heterozygous. If a rat is homozygous at a given locus,
it’s simple – both alleles are giving the same instruction – but what happens
when the rat is heterozygous? Then there are two sets of instructions about the
same thing and either one allele ‘wins’ and is said to be dominant (the
other is recessive) or both have an effect (incomplete dominance) and you
can see that the rat is ‘in between’. By convention more dominant alleles are
referred to by capital letters, less dominant by small letters.
Let’s look at some of the more straightforward
examples. P is the pink-eyed locus. The dominant P leaves colour unaffected,
the recessive p turns black coat pigment to a light brown and black eyes to
pink. An Agouti or a Cinnamon homozygous for the recessive gene (pp) will be a
Silver Fawn, while a Black or Mink which is (pp) will be Champagne. A (Pp) rat
looks like a (PP) rat and you won’t know it is ‘carrying’ pink eyes unless you
know it had a pink-eyed parent or it gives pink-eyed kittens.
Re is the Rex locus. Here the effect is produced by a
dominant gene, so (re re) is normal hair, (Re re) is curly. (Re Re) should be
curly too. It is, but unfortunately having two Re genes is overdoing it a bit
as far as the coat is concerned and the fur tends to drop out, so (Re Re) rats
are semi-bald, though the hair does regrow, and what there is is curly. Show
rexes are almost always (Re re).
Some people have been using Du for the dumbo locus.
Dumbo ears are caused by a recessive allele so a (du du) rat has dumbo ears; a (Du
du) rat has normal ears but might have dumbo babies with a suitable mate, while
a (Du Du) rat will not have dumbo kittens with any mate at all.
I’ll end with some warnings firstly about Pink-eyed
Whites, which are albinos and have two recessive albino genes (cc) on the C
locus (more of that next time). This gene when homozygous effectively bleaches
the rat, the coat has no colour and the eyes look pink. Albinos bred together
can give nothing but albinos and albinos bred to non-albinos might give
anything. This is an example of the phenotype (what the rat looks like)
of pink eyes being caused by two different genotypes (what the genes
command) – (pp) and (cc). Other examples (mentioned earlier) are the Silver
Fawn and the Champagne, each of which may have at least two different genotypes
and you can’t tell which you have from the colour.
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