Decoding Dahlia Genetics: A Breeder's Guide to Creating Unique Varieties

Dahlias captivate gardeners and breeders with their extraordinary diversity in bloom colors, shapes, and sizes. For hybridizers, understanding the genetic foundation of this diversity is crucial. This guide explores the fascinating world of dahlia genetics, explaining how their unique genetic structure contributes to their incredible variation and how breeders can harness these principles to create stunning new varieties.

To call myself a “non-scientist” would be an understatement. In school I tended more to liberal arts and took only enough hard science classes to log those credits. Now that I have become immersed in growing and breeding dahlias, I regret the science education opportunities I squandered as a youth.

To compensate, I have become an enthusiastic learner, actively seeking science-backed knowledge to advance my own successes as a gardener. When I make mistakes, or my self-styled propagating or growing experiments fall short, I see those as opportunities to learn what happened and make improvements.

When I began breeding dahlias, I absorbed information like a sponge. Much of that shared knowledge proved enormously helpful, but some proved contradictory–even wrong–the more I researched and experimented on my own dahlias.

My mission with this article and others in this blog post series is to find and read research-backed scientific books, articles, theses and papers about dahlias, then put those concepts into practice. My laboratory is my garden and my greenhouse. Instead of a lab coat, I proudly wear work clothes and a sun hat. What I learn in the library and in the dahlia bed, I proudly share with my fellow dahlia enthusiasts.

Steve Lloyd
Washington State USA (Zone 8b)

To understand dahlia genetics, it helps to start with simpler systems. Humans, like most animals, are diploid organisms , meaning they inherit two sets of chromosomes—one from each parent. Each gene exists at a specific location on a chromosome, called a locus, and can have different forms known as alleles. These alleles determine traits like eye color, height, or hair texture.

For any given locus, an organism can inherit:

• Two identical alleles (homozygous): Both alleles are the same, which can either be dominant or recessive.
  
  

• Two different alleles (heterozygous): One dominant and one recessive allele, where the dominant allele typically determines the expressed trait.

For example:

• Homozygous dominant (AA): Both alleles are dominant, so the dominant trait is expressed.
  
  

• Heterozygous (Aa): One dominant and one recessive allele, where the dominant allele “overrides” the recessive one.
  
  

• Homozygous recessive (aa): Both alleles are recessive, so the recessive trait is expressed.

Understanding these patterns is crucial because they explain how traits are inherited—a foundational concept for hybridizers exploring dahlia genetics.

In contrast to humans, dahlias are octoploid organisms , meaning they inherit eight sets of chromosomes instead of two. At each genetic locus, dahlias can carry up to eight different alleles. This complexity drastically increases the potential combinations of genetic traits.

For example, while a diploid organism (like you and I) can inherit three allele combinations at a locus (AA, Aa, or aa), an octoploid organism can inherit exponentially more combinations. This genetic diversity explains why dahlias exhibit such a wide variety of bloom shapes, colors, and sizes compared to other plants.

From a breeder’s perspective, this octoploid nature opens up nearly limitless possibilities for creating new varieties. The challenge lies in managing this complexity to predict desirable traits effectively. By understanding this genetic richness, hybridizers can better select parent plants to achieve their goals.

In diploid organisms like humans, a single genetic locus with two alleles (A and a) can result in three combinations: AA, Aa, and aa. Across multiple loci, the combinations grow exponentially.

For example, with 10 loci, each with two alleles, there are:

3¹⁰ = 59,049 possible combinations.

In dahlias, the number of possible combinations at each locus is much higher due to their eight alleles. For 10 loci, the combinations expand to over a billion:

8¹⁰ = 1,073,741,824 possible combinations.

When considering that dahlias have tens of thousands of loci, the genetic possibilities are practically infinite. This means that even highly recessive traits, which might remain hidden across generations, have the potential to emerge when the right genetic combinations align.

For breeders, this implies a tantalizing possibility: characteristics never before seen in dahlias could someday manifest through continued hybridization. Traits influenced by rare or unexpressed alleles might combine to produce entirely new bloom shapes, colors, or growth patterns. This potential for discovery is part of what makes dahlia breeding so exciting and creatively fulfilling.

The  genotype is the genetic makeup of an organism—the combination of alleles it inherits. The  phenotype is the observable expression of these genes, influenced by both genetics and environmental factors.

• A dahlia may have a genotype for a perfectly round bloom shape, but environmental stress, such as extreme heat or drought, might result in irregular blooms.
  
  

• Similarly, a genotype coding for red petals may produce slightly different hues depending on soil pH or sunlight exposure.

This interaction explains why dahlias grown from different tubers cut from the same root clump can yield varying results under different conditions.

Imagine crossing a red dahlia (R) with a white dahlia (W):

• R (red) is dominant.

• W (white) is recessive.

• Parent 1 (Red): RR

• Parent 2 (White): WW

• All offspring inherit RW and appear red because R dominates.

• Parent 1 (Red): Rr

• Parent 2 (White): rr

• Offspring outcomes:

  • 50% Rr (red)

  • 50% rr (white)

This example illustrates how blending traits can yield predictable yet diverse results, with even recessive traits like white petals emerging in certain offspring.

Cross-pollinating two different dahlia varieties combines traits from both parents. For example:

• Crossing a red dahlia with a white dahlia might result in offspring with red, white, or pink blooms, depending on how their color genes interact.

This involves repeatedly crossing a hybrid with one of its parent plants. This technique helps breeders retain specific desirable traits while introducing new characteristics.

Emerging tools from the Dahlia Genome Project allow breeders to use genetic markers to identify plants with specific traits. This reduces the trial-and-error approach, making breeding more efficient.

Research shows that dahlias have remarkable differences in traits like bloom size, stem strength, and flowering time, all of which can be passed down through generations. Breeders can use this natural variety to select parent plants with the best traits, creating new hybrids while keeping the species’ incredible diversity intact.

Studies of dahlia varieties reveal that some groups of plants are more alike than others, while some are quite different. Breeders can combine plants from different groups to create stronger and more unique hybrids with exciting new traits.

Some dahlia traits, like the way petals are arranged or how strong the stems are, tend to occur together in certain groups of plants. Breeders can use this knowledge to focus on specific traits, making it easier to develop dahlias that are both beautiful and sturdy.

New tools like DNA markers are helping breeders see exactly how plants are related and which traits they are likely to pass on. These tools will make it easier to plan crosses and speed up the creation of exciting new dahlia varieties.

In dahlia circles, growers often refer to a phenomenon called "genetic breakdown" to describe the gradual decline in a cultivar's performance or appearance over time. This might include changes in floret arrangement, bloom size, coloration, or overall plant vigor.

While this term suggests that a plant's DNA has somehow degraded or become corrupted, scientific evidence does not support this interpretation. Instead, other biological processes are likely at play.

While true genetic mutations can occur, they are relatively rare and unlikely to account for widespread degradation in dahlias. Most vegetatively propagated plants, like dahlias, maintain their genetic integrity across generations unless exposed to unusual genetic instability or mutagens.

Environmental factors such as stress, temperature fluctuations, or nutrient deficiencies can cause epigenetic changes (chemical modifications to DNA), which alter how certain genes are expressed without modifying the underlying DNA sequence. These changes may lead to visible differences in blooms or plant health and can persist across growing seasons.

Localized genetic mutations in non-reproductive cells can lead to visible changes, such as altered bloom shapes or colors, but these mutations are typically confined to specific areas of the plant and do not affect its overall genome. However, if propagated from a mutated segment, the changes could become more apparent.

A more common explanation for perceived degradation is the accumulation of pathogens such as viruses, bacteria, or fungi over time. Viral infections, in particular, can alter bloom structure, reduce flower size, and diminish overall vigor, mimicking the effects of genetic instability.

Repeated propagation from weaker stock or improper storage practices can reduce tuber vitality, leading to decreased performance. This decline is physiological rather than genetic, but it can significantly affect the plant’s appearance and productivity.

By understanding these processes, growers can take proactive measures to mitigate the effects of "genetic breakdown," such as maintaining clean propagation practices, managing environmental stressors, and ensuring optimal tuber health.

Led by Dr. Virginia Walbot at Stanford University, the Dahlia Genome Project aims to map the complete dahlia genome. This research will provide insights into the genes responsible for traits like color, shape, and disease resistance.

1. Genome Sequencing: Identify all genes and their functions.

2. Functional Genomics: Study how specific genes influence traits.

3. Genetic Diversity Studies: Analyze variations across cultivars to guide breeding efforts.

4. Breeding Tools: Develop genetic markers to streamline trait selection.

Dahlia genetics offers a world of possibilities for hybridizers. By understanding their octoploid genome and applying strategies like hybridization and marker-assisted selection, breeders can unlock endless potential for creating stunning new varieties.

With each generation, the chance to uncover unseen traits and combine rare genetic combinations grows, ensuring that the future of dahlia breeding remains as colorful and diverse as ever.

As the Dahlia Genome Project continues to reveal genetic insights, the future of dahlia breeding looks brighter—and more colorful—than ever.

• Dahlia Genome Project Overview from The American Dahlia Society.

• “ Dr. Keith Hammett: Shaping Dahlia Genetics and Breeding for the Future ”. An article on the Dahlia Doctor “Knowledge Center” blog by Steve Lloyd.

• An interview with New Zealand dahlia breeder Dr. Keith Hammett on The Quince Flowers Podcast (November 19, 2021).

• “Dahlia Growing & Breeding with Kristine Albrecht” on The Joe Gardener Podcast  (February 17, 2022).

• An interview with Canadian dahlia breeder Trevor Hoff on The Quince Flowers Podcast (March 6, 2023).

• Devi, Maguluri Sree, et al. " Assessment of genetic variability among dahlia (Dahlia variabilis L.) genotypes for productivity and quality traits. " Journal of Pharmacognosy and Phytochemistry 9.4 (2020): 3134-3137.

• Singh, Simrat, Kiranjeet Kaur Dhatt, and Pankaj Kumar Bodla. " Exploring genetic diversity of Dahlia (Dahlia variabilis Desf.) germplasm using multivariate statistics. " Journal of Horticultural Sciences 18.1 (2023): 67-76.

• Behr, H., and T. Debener. " Novel breeding strategies for ornamental dahlias : Analysis of the Dahlia variabilis breeding system with molecular markers. " European Journal of Horticultural Science 69 (2004): 177-183.

• Hegde, Bhagyashree Narayana, et al. " Genetic Divergence Analysis of Dahlia (Dahlia variabilis L.) Genotypes. " International Journal of Plant & Soil Science 34.16 (2022): 48-54.