TB color genes made easy (was Re: HYB: Seedling Color

Anner writes

:That was interesting, Tom, she said emphatically. Exceptionally clear and
:accessible. Might you please expand a bit on how different colors and
:patterns interact, maybe with a little bearded history thrown in? Feel free
:to oversimplify to make a point; I won't notice, I assure you!  And do the
:same tendencies obtain for the beardless stuff?

You may regret asking, but here goes.

First, the cast of characters--

1. GENES THAT DETERMINE IRIS COLOR

Beware: I've made a few simplifications for the sake of conciseness. And
only genes found in TBs are considered.

1a. Genes Affecting the Blue and Purple (Anthocyanidin) Pigments:

V (dominant), allows the flower to produce purple pigments
v (its recessive), no purple pigment produced

I (dominant), inhibits the expression of purple pigments everywhere in the
flower
i (its recessive), lets the purple be expressed

I-s (dominant), inhibits or reduces the expression of purple pigments in
the standards
i-s (its recessive), lets the purple be expressed everywhere

PL (dominant), lets the purple be expressed everywhere
pl (recessive to PL, dominant to pl-a), lets the purple be expressed only
along veins and petal edges (plicata)
pl-lu (recessive to PL, dominant to pl-a), lets purple be expressed only
between veins, leaving clear veins, styles, and hafts (luminata)
pl-a (recessive to PL, pl, and pl-lu) prevents the expression of purple

1b. Genes Affecting the Yellow and Pink (Carotenoid) Pigments:

Y (dominant), allows the flower to produce yellow pigments
y (its recessive), no yellow pigment produced

[Note--the situation with yellow is more complicated than this. There are
probably several genes involved; simple dominant/recessive model may not
always work.]

H (I just made this symbol up), causes the yellow pigment in the falls to
be distributed in a "halo". I don't know if this is dominant or not. Some
breeders may want to chime in.

A (I made this one up too), allows yellow pigment to be distributed
throughout the flower
a (its recessive), causes yellow pigment to be eliminated or reduced in the
standards.

T (dominant), keeps pigment in its yellow form
t (its recessive), changes yellow pigment to pink

2. GUESSING YOUR IRISES' GENES

Here are some rules to help you guess which of these genes a given iris may
have. You'll need to view this using a fixed-spaced font. An "x" means at
least one copy of the gene is probably present, a "o" means none of these
genes can be present. (Since TB irises are tetraploids, there is a total of
four of each type of gene.)

                                   V   I   I-s Y   A   H   T   PL  pl  pl-lu
2a. Selfs:
    blue or violet                 x   o   o   o               x
    red, brown, or black           x   o   o   x   x   o   x   x
    yellow                             x       x   x   o   x
    pink or orange                     x       x   x   o   o
    white                              x       o

2b. Bicolors:
    purple or blue amoena          x   o   x   o               x
    yellow amoena                      x       x   o       x
    pink or orange amoena              x       x   o       o
    variegata                      x   o   x   x   x       x   x

2c. Plicatas
    blue or violet on white ground x   o   o   o               o       o
    red or brown on yellow ground  x   o   o   x   x       x   o       o
    violet on pink/orange ground   x   o   o   x   x       o   o       o
    no plicata markings on standards (various ground colors)--same as above,
        but at least one I-s
    luminata (various ground)      x   o   o                   o   o

2d. Halos
    yellow stadards, falls white w/yellow halo (e.g., Debby Rairdon)
                                       x       x       x   x
    pink standards, falls white w/pink halo (e.g., Queen of Hearts)
                                       x       x       x   o
    brown standards, falls violet w/brown halo (e.g., Brown Lasso)
                                   x   o   o   x       x   x

If you study the chart for awhile, you'll find you get the "feel" of it,
and can guess the genes of color combinations not on the chart. For
example, 'Broadway' has clear yellow standards, and white falls with
red-brown plicata markings. It must have V (because the plicata markings
are made with the purple pigments, not the yellow ones), it cannot have I,
it must have I-s (because there are no purple markings in the standards),
it must have Y, it must have H (because the center of the falls are white,
but there must be some yellow around the halo to account for the redness of
the plicata marks), it must have T (because the yellow is yellow, not
pink), and cannot have PL or pl-lu.

3. WHEN YOU CROSS 'EM

As you can see from the chart above, in most cases we only know whether a
particular dominant gene is present (at least one) or absent (none). We
usually don't know exactly how many copies of a dominant gene may be there.
But if we're just making an "educated guess" of what might come out in the
seedling patch, that's good enough. Here are the rules:

3a. If neither parent has a particular dominant gene, NONE of the offspring
will have it. Thus crossing two plicatas can never produce a violet self,
because neither parent has the PL gene needed to make this happen.

3b. If only one parent has a particular dominant gene, AT LEAST HALF of the
seedlings can be expected to have it too, on average. (If more than one
copy of the gene is present, it may appear in most or all of the seedlings.)

3c. If both parents have a particular dominant gene, AT LEAST 3/4 of the
seedlings can be expected to have it too, on average.

Using these rules, you can make a good guess which of the dominant genes
the seedlings may end up with, and predict their color patterns.

4. GETTING FANCY

If this is not enough fun, and you want to get a better idea of how many
genes of each type the parents may carry, you need to do pedigree research.
If an iris comes from crossing a violet self and a plicata, for example, I
know it can have at most two PL genes out of its total of four; the other
two (which came from the plicata parent) are almost certainly pl. In this
way, you can sometimes narrow down the possibilities of what recessive
genes may be present, and in what dosage.

Ultimately, of course, if one knows the full genotype of the parents, one
can calculate in detail the percentages of seedlings having each possible
genotype. Genetics books have instructions and exercises for this sort of
thing.  In practice, though, we hardly ever know the complete genotype of
the parents, and we rarely care about the exact percentages that might
appear in the seedling bed (the exception being with controlled crosses
designed to study how a particular trait is inherited).

These "rules of thumb" are a _lot_ easier to use than the computations that
are explained in the textbooks, and in most cases give a very good idea
what to expect from a cross.

Happy irising, Tom


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tom Tadfor Little                   telp@Rt66.com
Santa Fe, New Mexico (USA)
Telperion Productions  http://www.rt66.com/~telp/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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