HYB: genes affecting pigments (original by Tom Little)
- To: iris-talk
- Subject: HYB: genes affecting pigments (original by Tom Little)
- From: L* M*
- Date: Tue, 09 Jan 2001 09:59:50 -0500
I re-posted this awhile back, but maybe it's worth re-posting again.
The original post was by Tom Little several years ago & is a
'simplified' version of the information in TWOI (The World of Iris), I
think. I find the chart Tom put together very helpful in sorting out
all these color inheritances. Keep in mind that this is the original
post, & I don't DARE try to update it with the more recent discussions
by Chuck Chapman et al. :
======= re-post of original post by Tom Tadfor Little =============
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
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.)[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 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 standards, 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 [Tadfor Little]
================================================
Linda Mann east Tennessee USA zone 7/8
