Ask Mr. Smarty Plants

The short answer is that I suspect that the flower colors (purple, lavender or pinkish and white) of Anemone berlandieri [syn.  A. heterophylla] (Tenpetal thimbleweed) are due to incomplete dominance; however, as far as I have been able to determine, there hasn't been formal research into its genetics.

Now, for the longer explanation, I'm going to remind you and other readers of some of the basics of genetics in general and plant genetics in particular. Every individual plant has in its DNA, two copies of each of the genes that determine how it looks and how it functions.  This includes the genes that are responsible for flower color.  In general, the pigments that produce the colors of flowers are formed in a biochemical pathway that requires converting one substance to another substance using enzyme catalysts at every step until the pigment is formed.  If an enzyme that catalyzes any of the steps in the biochemical pathway doesn't function properly, then the pigment is NOT formed.  The pigments that produce blue/purple flowers are called anthocyanins.  Let's suppose that there are two different forms (alleles) of one of the genes that makes an enzyme in the biochemical pathway to pigment.  One form of the enzyme works perfectly, the pigment pathway is completed and you get purple pigment.  The other form of the gene is defective and the enzyme it makes won't complete the pathway and there is NO pigment made.  There are then several possibilities.  Let's call the form of the gene that will result in one of the anthocyanins and purple pigment being made, 'A', and the form that won't produce purple pigment, 'a'.  There are 3 combinations—its genotype—for the two forms for any particular plant.  The plant could be A/A (have two copies that make pigment) or A/a (have one copy that does make pigment and one that doesn't make pigment) or a/a (has two copies that do NOT make pigment).  It's pretty easy to see that flowers with the genotype A/A will have purple flowers and those with the genotype a/a will have white flowers (lacking any pigment).  The puzzle is: what is the color of the flowers are made by the genotype A/a.  If the A form of the gene can make enough enzyme to complete the pathway for all the anthocyanin that needs to be produced, then the flowers will be purple and we have "complete dominance"—that is, it takes only one copy of A for the flowers of genotype of A/a to look just like the flowers of genotype A/A.  [This is the explanation for the flower color of Mendel's pea plants, although the symbols usually used in the explanation for Mendel's pea colors are 'P' and 'p', instead of 'A' and 'a'.]  However, if having only one copy of A doesn't make enough enzyme to always complete the pathway, then flowers with the genotype of A/a will have a color (lavender or pink) intermediate between purple and white. This is called "incomplete dominance" with the appearance (or phenotype) something between the two extremes.  One flower whose flower color genetics have been studied, Mirabilis jalapa (Four-O-Clock), a native of South America, shows "incomplete dominance" with an A/A genotype showing dark magenta flowers, an a/a showing white flowers and A/a having pale pink flowers. 

Co-dominance, on the other hand, indicates that both forms of a gene produce some product that is expressed.  One of the best examples of co-dominance is the A/B blood system of humans.  A and B are antigens that appear on the surface of red blood cells.  In this case, there are at least 3 forms (alleles) of the gene involved—'i' makes no antigens, 'IA' makes antigen A on the blood cell surface and 'IB' makes antigen B on the red blood cell.  An individual with genotype i/i has no antigens on his/her blood cell (Type O); individuals that have IA/IA or IA/i show only A antigens (Type A); IB/IB or IB/i has only B antigens (Type B); but individuals that are IA/IB have both A antigens and B antigens on their red blood cells (Type AB).   The two forms (or alleles) IA and IB are co-dominant. 

I'm not sure where you got your information about codominance in carnations but the information in the paper by A. L. Mehlquist and T. A. Geissman ["Inheritance in the Carnation (Dianthus caryophyllus).  III. Inheritance of Flower Color."  Annals of the Missouri Botanical Garden.  Vol. 34, No. 1 (Feb., 1947) pp. 39-74.] indicates that there are actually six genes responsible for the different flower colors (e.g., purple, white, pink, yellow, red, orange) and the solid or variegated pattern in the carnation. The six genes interact in different ways to produce all the various colors and variegation patterns.  However, there is one gene responsible for producing the anthocyanins and it has two basic forms, 'A' and 'a'.   Carnations with the genotype A/A and A/a produce anthocyanin and have purple flowers and flowers that are a/a have white flowers unless some of the other genes cause them to be modified to produce another color.  In other words, 'A' shows complete dominance to 'a'. Another gene with the symbols 'S' and 's' modifies the amount of anthocyanin made so that if the plant is A/A S/S or A/a S/S or A/A S/s or A/a S/s it will have purple flowers; whereas, plants that are A/A s/s or A/a s/s have lavender flowers.  The other color genes can modify the color in other ways.  As you can imagine, it gets rather complicated.  If you want to read more about the genetics of the carnation's flower colors, you can purchase a download from JSTOR at the link above, or, since you live in Austin, you can visit the Life Science Library at the University of Texas to inquire about access to a copy there.

Anemone berlandieri

Anemone berlandieri

Anemone berlandieri