New World Rattery: Genetics

Home

About Rats

Our Rats

Kittens

Shows

Links

About Us

Genetics 3

Punnet diagrams

The diagram below shows what happens when two husky carriers are bred together. Don't worry if it looks a little complicated. This particular example is the most difficult single-gene case, so if you get this one, you're well underway. If you don't then I would recommend picking up a GCSE science book (all fifteen-year-olds will get taught this in one shape or another) and see how they tackle this point before continuing as their approach may work for you than the summary below.

Illustration of a carrier crossed with a carrier

This is usually shown as:

x	Q	q
Q	QQ	Qq
q	Qq	qq

The powerful thing about this diagram is that it can be interpreted in two ways:

Genetically	Appearance
QQ (On average, 1 in every 4)	Self (On average, 3 in every 4)
Qq (On average, 2 in every 4)	Self (On average, 3 in every 4)
qq (On average, 1 in every 4)	Husky (On average, 1 in every 4)

Now we have identified three genotypes, we can now go through the six different breeding possibilities.

First, the two obvious ones:

QQ x QQ: Breeding a true-bred self with a true-bred self gives a litter of true-breeding selves.

x	Q	Q
Q	QQ	QQ
Q	QQ	QQ

qq x qq: Breeding a husky with a husky gives a litter of huskies every time.

x	q	q
q	qq	qq
q	qq	qq

Next is the example from the last page that we used previously to show dominance:

QQ x qq: Breeding a true-bred self with a husky gives a litter where all the babies will be husky carriers, Qq.

x	Q	Q
q	Qq	Qq
q	Qq	Qq

The next two are the result of crossing with a carrier:

Qq x qq: Breeding a husky carrier with a husky gives a mixed litter. Half the babies will be inherit Q and q and become husky carriers. The other half will inherit q from both parents and will be huskies.

x	Q	q
q	Qq	qq
q	Qq	qq

QQ x Qq: Breeding a true-bred self with a husky carrier gives a litter that are all selves (these inherit H from both parents), but half are true-breeding while the other half are carriers (these inherit Q from one parent and q from the other).

x	Q	q
Q	QQ	Qq
Q	QQ	Qq

The last one is the one we covered at the top of the page:

This is the crossing of a carrier with a carrier.

x	Q	q
Q	QQ	Qq
q	Qq	qq

For simplicity, the term true-breeding has been used in preference to homozygous, and carrier in preference to heterozygous. These and other scientific terms such as phenotypes and alleles are important and their use is encouraged, although these pages are only intended as an introduction.

Here, I have used the letter Q to denote "husky". When I published this in the NERS magazine Rattitude series I used the letter H. That turned out to be an oversimplification too far for some. Formally, the H locus is used to denote the Hooded gene, and by extension the other associated alleles. I did think of using the letter R, but that is used elsewhere too. So, I'll try Q for a change, but I do not imply (because I don't know either for or against) whether there's any connection with the other marked genes or any other gene that might use the letter Q. I simply know that a gene turns the roaning on and off, and a second gene is responsible for the causing the stripe on banded roans.

<< Page 2 | Page 4 >>