The Genetics Behind Flower Color in Plants

Have you ever wondered why some flowers are purple while others are red yellow or white? The color of a flower is determined by pigments called anthocyanins that are produced in the petals. Some plants have the genetic ability to make purple anthocyanins, while others do not. In this article, we’ll explore the genetics behind flower color in plants, focusing on how a plant can produce either purple flowers or flowers of another color.

Genes Control Flower Color

Flower color is controlled by genes that code for enzymes involved in pigment production. Specifically, it is genes that regulate anthocyanin biosynthesis that determine whether a plant will have purple flowers or not. Anthocyanins are a class of pigments that appear red to blue to purple depending on pH. They are found in the vacuoles of petal cells.

The main genes involved in regulating anthocyanin production are

• P gene – Codes for an enzyme that starts the anthocyanin biosynthesis pathway
• A gene – Codes for an enzyme later in the pathway
• B gene – Codes for an enzyme that modifies anthocyanins to make them purple

For a plant to produce purple anthocyanins, it must have dominant alleles for all three of these genes (P^, A^, B^). If any one of these genes is recessive, it will block anthocyanin production and result in flowers of another color.

Purple vs Non-Purple Flower Genotypes

Many plant species have varieties that produce purple flowers and varieties that do not. This is because over generations, different alleles of the P, A, and B genes have become fixed in different populations.

Let’s look at some examples:

• Snapdragon – A plant homozygous for dominant alleles of P, A, and B genes (P^P^ A^A^ B^B^) will have purple flowers. A plant with a recessive p allele (P^p A^A^ B^B^) will be unable to make anthocyanins and will have white flowers.

• Petunia – A Petunia with P^P^ A^a B^B^ genetics will have purple flowers due to a dominant A allele. A petunia with p^p a^a B^B^ genetics will not produce anthocyanins and will have white flowers, as it lacks a functional A gene.

• Morning Glory – Morning glories with the genotype P^P^ aa B^B^ have pink flowers rather than purple. This is because they lack a functional dominant A allele needed to complete anthocyanin biosynthesis.

As you can see, the genetics rapidly gets complicated when all possible allele combinations are considered! But in simple terms, the presence versus absence of key anthocyanin genes P, A, and B determines if a flower will be purple or not.

Beyond Purple – Other Flower Colors

While this article focuses on purple flower color, it’s important to note that other pigments besides anthocyanins play a role in creating the diverse palette of flower colors we see in nature.

Some examples:

• Red and pink flowers get their color from anthocyanins, but the specific pH of the vacuole determines if they appear red versus purple.

• Orange and yellow flowers get their color from carotenoid pigments like lutein

• White flowers lack any pigment and reflect all wavelengths of light equally

• Blue flowers contain delphinidin-based anthocyanins modified by additional enzymes that make the pigments appear blue

So while the P, A, and B genes control purple anthocyanin production specifically, additional genes influence the production of other colored pigments in flowers. There is still much to uncover about the full complexity of genetic controls over flower color!

Mendel’s Experiments on Flower Color

The understanding of flower color genetics built off important foundational experiments done by Gregor Mendel in the 19th century. Mendel, known as the “Father of Genetics”, studied flower color inheritance in pea plants.

Some key findings from Mendel’s work include:

• He cross-bred purple-flowered and white-flowered pea plants. All the first generation offspring had purple flowers, indicating purple flower color was a dominant trait.

• When the hybrid purple-flowered plants were self-pollinated, the second generation had a 3:1 ratio of purple to white flowers. This demonstrated that discrete units of inheritance (“genes”) controlled this trait.

• The principles of dominance, segregation, and independent assortment that Mendel defined through his pea plant experiments formed the basis of genetics.

Today, we know Mendel’s purple flower color trait was governed by a dominant P allele for one of the anthocyanin biosynthesis genes. However, at the time, the molecular basis was still unknown. Nonetheless, Mendel’s pioneering work opened the door for the discoveries about flower color genetics that followed in the 20th and 21st centuries.

Applications of Flower Color Genetics

Understanding the genetic basis of flower color allows for targeted breeding of new flower varieties by both amateur and professional horticulturists. Some examples of how flower color genetics can be applied include:

• Breeding hybrids that mix novel or atypical flower colors, like a true blue rose

• Selectively breeding plants to be more resistant to stressors like drought, disease, or pests while retaining desired flower traits

• Creating color-changing flowers, like hydrangeas, by engineering pH-sensitivity into pigment production

• Modifying color patterns, like introducing variegated spots or stripes, by restricting expression of key pigment genes

• Bioengineering entirely new colors not found in nature by altering anthocyanin biosynthesis genes

The possibilities are endless! From hobby gardeners to large-scale growers, understanding flower color genetics enables endless creativity and innovation in floriculture.