Sealpoint siamese X bluepoint siamese = 100% sealpoint siamese, unless the sealpoint parent is carrying blue. Siamese (ch/ch) is separate from blue (d/d) so it's neither incomplete dominance nor co-dominance because those terms refer only to alleles situated on the same locus. The C-locus (where siamese sits) is the most complicated in mice. The D-locus (where blue sits) is one of the simplest.
For a moment, pretend that you are dealing with siamese and beige, which are on the same locus, the C-locus. In simple terms, when a siamese (ch/ch) is bred to a beige (ce/ce), you will get a whole litter of mice who are ch/ce. In this case, siamese is incompletely dominant to beige because you can see both phenotypes. You could equally say that beige is incompletely dominant to siamese.
Now pretend that you are dealing with a wild agouti (A/A D/D) mouse and a blue mouse (a/a d/d). When bred together, you will get all agouti mice (A/a D/d) carrying blue but when speaking of the blue-locus this is technically neither incomplete dominance nor co-dominance. It is simple (aka Mendelian) dominance. The A is dominant to the a. The D is dominant to the d. The A cannot really be said to be dominant to the D or d because they're in different locations (loci).
Things like ear size and placement, tail set-on, deepness of color, etc., are termed polygenetic which means they are controlled by alleles of multiple genes, some of which are always inherited together and some of which are not. And each of these alleles my have modifier alleles (commonly called simply "modifiers") in other places on the same chromosome (linked) or a different chromosome, which help push the phenotype a little bit in one direction or another.
With that said, there is no "proper ear placement" gene that makes the ears sit correctly; there is no "rich deep reddish-brown chocolate color" gene that makes a chocolate the right shade; instead there are numerous factors involved all at once and it's as much a game of luck as one of skill when you do get a line of mice who have nice ears (or tail, or whatever). This is why it's so much easier to breed pet-type mice and concentrate primarily on the color--color is usually determined by only one or two genes. It's much harder (but more rewarding, imo) and takes much more time and work to concentrate on type, temperament, and color for show. When you're breeding professional for show you're literally dealing with dozens of different variables, only a few of which have been scientifically measured, mapped-out, and assigned a letter (such as the A locus, B locus, etc).
P.S. You're very welcome. Mouse genetics is my major area of interest in life (gee, that makes me sound so boring) and I love helping people. lol
If you cite me, please use this format:
García, J. (2010, March 12). Re: Gene interactions.
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