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HYB: color genes made easy (orig post 7/17/97)

Turns out Tom Little's post wasn't about Punnett's square after all -
sorry for the confusion.  Took some serious searching even when I knew
what I was looking for and I had to go to my printed copy to find it, so
figure it's worth posting again.  Here is the original post:


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
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
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
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
be distributed in a "halo". I don't know if this is dominant or not.
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

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


Here are some rules to help you guess which of these genes a given iris
have. You'll need to view this using a fixed-spaced font. An "x" means
least one copy of the gene is probably present, a "o" means none of
genes can be present. (Since TB irises are tetraploids, there is a total
four of each type of gene.)
[for these tables, to get them to line up right, try using courier or
other fonts - lm]
                                   V   I   I-s Y   A   H   T   PL  pl
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
        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
it must have I-s (because there are no purple markings in the
it must have Y, it must have H (because the center of the falls are
but there must be some yellow around the halo to account for the redness
the plicata marks), it must have T (because the yellow is yellow, not
pink), and cannot have PL or pl-lu.


As you can see from the chart above, in most cases we only know whether
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
But if we're just making an "educated guess" of what might come out in
seedling patch, that's good enough. Here are the rules:

3a. If neither parent has a particular dominant gene, NONE of the
will have it. Thus crossing two plicatas can never produce a violet
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
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

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.


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
If an iris comes from crossing a violet self and a plicata, for example,
know it can have at most two PL genes out of its total of four; the
two (which came from the plicata parent) are almost certainly pl. In
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,
can calculate in detail the percentages of seedlings having each
genotype. Genetics books have instructions and exercises for this sort
thing.  In practice, though, we hardly ever know the complete genotype
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
are explained in the textbooks, and in most cases give a very good idea
what to expect from a cross.

Happy irising, Tom [Tadfor Little]

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