The Genome’s Symphony: Primary and Secondary Variants

Our DNA isn’t a simple code where one gene equals one trait. It’s a vast landscape of millions of genetic variants, inherited from our parents or arising spontaneously. Some are common, others incredibly rare, but together they shape everything from our physical characteristics to our susceptibility to disease.

<p>While certain rare variants, like those causing sickle cell anemia and cystic fibrosis, can directly trigger illness, the story is often far more nuanced.  Most conditions aren’t caused by a single ‘primary’ variant, but are influenced by a chorus of ‘secondary’ variants scattered throughout the genome. Think of it like an orchestra: the melody (primary variant) provides the core theme, but the harmony and rhythm (secondary variants) dramatically alter the overall sound.</p>

<p>This explains why individuals with the same genetic mutation can experience wildly different symptoms – ranging from severe disability to being completely asymptomatic. This phenomenon, known as <a href="https://bio.libretexts.org/Courses/Lumen_Learning/Biology_for_Majors_I_(Lumen)/14%3A_Module_12-_Trait_Inheritance/14.12%3A_Penetrance_and_Expressivity">variable expressivity</a>, highlights the power of these secondary variants to amplify or modify the effects of a primary mutation. Generally, a greater number of secondary variants correlates with a more severe presentation of the disease.</p>

<figure>
  <img src="https://images.theconversation.com/files/694248/original/file-20251003-56-dcfggh.jpg?ixlib=rb-4.1.0&amp;q=45&amp;auto=format&amp;w=1000&amp;fit=clip" alt="Chromosomal deletion and duplication">
  <figcaption>Two types of chromosomal mutations include deletion and duplication. <span class="attribution"><a class="source" href="https://doi.org/10.3389/fncel.2020.611379">Gandawijaya et al./Biorender via Frontiers in Cellular Neuroscience</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption>
</figure>

<p>Sometimes, the interplay between primary and secondary variants can even lead to the co-occurrence of multiple disorders, such as <a href="https://doi.org/10.1002/ajmg.a.38315">Prader-Willi syndrome and Pitt-Hopkins syndrome</a>.  In other cases, secondary variants can subtly shift the balance, influencing whether a disease develops at all, as seen in some forms of <a href="https://doi.org/10.1016/j.ajhg.2015.10.013">heart disease in children</a>.</p>

<h2>Unraveling the Complexity: The 16p12.1 Deletion</h2>
<p>Recent research, including our work published in the journal <em>Cell</em>, has shed light on these complex interactions. We focused on a genetic change known as a <a href="https://www.ncbi.nlm.nih.gov/books/NBK274565/">16p12.1 deletion</a> – a missing piece of chromosome 16 – linked to developmental delays, intellectual disability, and psychiatric conditions like schizophrenia.  Interestingly, many parents carrying this deletion exhibit milder symptoms, different symptoms, or even no symptoms at all.</p>

<p>Analyzing data from 442 individuals across 124 families, we discovered that children with the 16p12.1 deletion possessed <a href="https://doi.org/10.1016/j.cell.2025.09.012">more secondary variants</a> elsewhere in their genomes compared to their parents. These secondary variants manifested as small changes, as well as larger deletions, duplications, and expansions of DNA.</p>

<p>Each type of secondary variant was associated with distinct health outcomes. Some correlated with smaller head size and reduced cognitive function, while others increased the risk of psychiatric or developmental symptoms. This suggests that the 16p12.1 deletion doesn’t directly *cause* a specific set of symptoms, but rather increases the genome’s sensitivity to neurodevelopmental disorders, with the specific manifestation determined by the accompanying secondary variants.</p>

<p>The story becomes even more intricate when considering that children inherit not only the 16p12.1 deletion from one parent, but also a unique combination of secondary variants from both parents.  Our research revealed a striking pattern: the symptoms exhibited by the parent with the deletion often mirrored those of their spouse. This phenomenon, known as <a href="https://theconversation.com/people-dont-mate-randomly-but-the-flawed-assumption-that-they-do-is-an-essential-part-of-many-studies-linking-genes-to-diseases-and-traits-194793">assortative mating</a>, suggests that individuals with overlapping genetic predispositions are more likely to pair up, leading to a concentration of risk in their offspring.</p>

<p>Over generations, this accumulation of secondary variants can result in children experiencing more severe symptoms than their parents.  Could this explain why some families seem disproportionately affected by certain genetic conditions?</p>

<p>What role does environmental factors play in this complex interplay? And how can we leverage this knowledge to develop more targeted and effective therapies?</p>

Addressing Bias in Genetic Research

Historically, our understanding of secondary variants has been hampered by biases in how genetic research is conducted. Most studies focus on individuals already diagnosed with a disease, often recruiting patients from specialized genetic clinics. While valuable, this approach can overestimate the impact of primary variants and overlook the subtle contributions of secondary variants.

<p>To gain a more comprehensive picture, researchers need to broaden their scope. Studying individuals from the general population – for example, recruiting participants from diverse community settings – would reveal a wider spectrum of genetic variation and symptom presentation. This would allow for a more accurate dissection of how different parts of the genome interact and influence disease development.</p>

<p>Our study demonstrated this principle. Individuals with the 16p12.1 deletion recruited from the general population exhibited milder symptoms and different patterns of secondary variants compared to those recruited from a clinical setting. This underscores the importance of inclusive research practices.</p>
<img src="https://images.theconversation.com/files/694249/original/file-20251003-56-me8ctz.jpg?ixlib=rb-4.1.0&amp;q=45&amp;auto=format&amp;w=1000&amp;fit=clip" alt="Scientist working in a lab">
<figcaption>Who’s included in genetics research influences how scientists understand disease. <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/scientist-pipetting-sample-into-a-vial-for-dna-royalty-free-image/1334095553">Cavan Images/iStock via Getty Images Plus</a></span></figcaption>

A New Era of Personalized Medicine

The future of genetics lies in embracing this complexity. Instead of viewing genetic inheritance as a deterministic process – one mutation, one outcome – we must adopt a more holistic model that accounts for the intricate interplay between genetic variants and environmental factors. This shift has profound implications for clinical practice.

<p>Currently, a child diagnosed with a genetic variant may be immediately labeled with the associated disease.  However, in the future, doctors may analyze a child’s entire genetic profile to predict their developmental trajectory, assess their risk of psychiatric conditions, and tailor therapies accordingly. Families will receive more nuanced counseling, understanding the *probability* of disease development rather than assuming a predetermined fate.</p>

<p>This is an evolving field.  Larger, more diverse datasets and sophisticated computational models are needed to capture the subtle effects of genetic variants and environmental influences. But one thing is clear: secondary variants are not merely secondary in importance. They are integral to understanding the full spectrum of human health and disease.</p>

<div style="background-color:#fffbe6; border-left:5px solid #ffc107; padding:15px; margin:20px 0;"><strong>Pro Tip:</strong> Consider participating in genetic research studies. Your contribution can help accelerate discoveries and improve healthcare for future generations.</div>

Frequently Asked Questions About Genetic Variants

• What are genetic variants and how do they contribute to disease?

  Genetic variants are natural differences in our DNA sequence. While some are harmless, others can increase our susceptibility to certain diseases, particularly when combined with other variants.

• What is the difference between a primary and secondary genetic variant?

  A primary variant is a mutation that directly triggers a disease, while secondary variants modify how that disease develops and progresses, influencing its severity and presentation.

• How does assortative mating impact the inheritance of genetic conditions?

  Assortative mating occurs when individuals with similar genetic predispositions are more likely to pair up, increasing the concentration of risk factors in their offspring.

• Why is diversity important in genetic research?

  Studying diverse populations helps researchers identify a wider range of genetic variants and understand how they interact with different environmental factors, leading to more accurate and inclusive results.

• Will understanding secondary variants lead to new treatments for genetic diseases?

  Yes, by identifying the complex interplay of genetic variants, researchers can develop more targeted and personalized therapies that address the root causes of disease.