📄 genetic and genomic medicine

Prioritizing embryos with lower homozygosity may reduce disease risk in children of related individuals undergoing preimplantation genetic testing

This paper proposes that integrating embryo-level autozygosity quantification (FROH) into existing preimplantation genetic testing workflows allows couples from consanguineous unions to prioritize embryos with lower homozygosity, potentially reducing the risk of intellectual disability and recessive diseases by approximately 35–45%.

Original authors: Wolfram, T., Ahangari, M., Davidson, I., Wartschinski, L., Li, J. H., Eyre, M., Stern, D., Schleede, J., Haghighi, A., Carmi, S., Christensen, M.

Published 2026-06-04

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Wolfram, T., Ahangari, M., Davidson, I., Wartschinski, L., Li, J. H., Eyre, M., Stern, D., Schleede, J., Haghighi, A., Carmi, S., Christensen, M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: A Genetic "Lottery" with a Twist

Imagine two people who are cousins getting married. Because they share recent grandparents, they also share a lot of the same DNA. When they have children, there is a higher chance that the child will inherit two copies of the same "bad" genetic instruction (a recessive variant) from both parents. In genetics, this is called autozygosity.

Think of your DNA like a massive instruction manual for building a human. Usually, you have two copies of every page—one from Mom and one from Dad. If one page has a typo, the other copy usually fixes it. But when parents are related, they might both have the same typo on the same page. If the child gets that specific page from both parents, the typo becomes a problem, potentially leading to serious health issues or disabilities.

The Problem with Current Testing

Right now, doctors use genetic testing (PGT-M) to look for specific, known typos. It's like having a list of 100 known bad words and checking the manual to make sure those specific words aren't there.

The paper points out a flaw: This list is incomplete. There are thousands of other "bad words" (variants) that scientists haven't discovered yet. Current testing misses these hidden risks.

The New Idea: Measuring the "Length of the Bad Streaks"

The authors propose a new way to look at the embryo's DNA. Instead of just looking for specific known typos, they suggest measuring how long the "runs" of identical DNA are.

The Analogy: Imagine the DNA manual is a long strip of paper. In a child of unrelated parents, the strip is a colorful patchwork of Mom's and Dad's unique patterns. In a child of cousins, there are long, solid blocks where the pattern is exactly the same on both sides because they came from the same grandparent.
The Metric: The paper calls this measurement FROH (Fraction of the genome in Runs of Homozygosity). A higher FROH means longer, uninterrupted blocks of identical DNA, which means a higher risk of hidden genetic diseases.

The Solution: Picking the "Safest" Sibling

Here is the most important part of the paper: Even though the parents are related, their different embryos are not all the same.

The Analogy: Think of a deck of cards. If you deal five hands to five different people, even though they are all dealt from the same deck, the hands will be different. Some hands will have more "bad cards" than others.
The Discovery: The researchers used computer simulations to show that among a group of embryos from the same couple, the amount of "identical DNA blocks" (FROH) varies wildly. One embryo might have a lot of these blocks, while another has very few.

The Strategy (PGT-H):
If a couple is already doing IVF and has several embryos to choose from, the paper suggests measuring the FROH for each one and picking the embryo with the lowest score.

The Result: By picking the "lowest FROH" embryo from a group of five, the researchers estimate you can reduce the child's genetic risk by about 40% compared to just picking one at random.

What Does This Actually Fix?

The paper calculates that this strategy could significantly lower the risk of specific problems for children of first-cousin couples:

Intellectual Disability: Could be reduced by about 35–45%.
Congenital Anomalies (birth defects): Risk is lowered.
Common Diseases: It might even slightly lower the risk of things like Type 2 diabetes, which are influenced by many small genetic factors.

Crucially, the paper emphasizes:

It's a reduction, not a cure: This doesn't guarantee a healthy baby. It just lowers the odds of certain genetic problems.
It works alongside current tests: You would still use the old tests to find known bad genes (PGT-M), and then use this new method to pick the embryo with the least amount of unknown hidden risks.
It's optional and sensitive: The authors stress that this should be presented as a tool to reduce risk, not as a reason to tell people not to marry their cousins. It respects the family's choices while offering a way to make the outcome safer.

Summary

Think of this paper as proposing a new "safety filter" for IVF. If you are a couple who are related, you already know there is a higher risk of genetic issues. Current tests check for specific known monsters in the dark. This new method shines a light on the whole room to see how much "darkness" (identical DNA blocks) is in each embryo, allowing parents to choose the one with the least darkness, thereby giving their future child the best possible genetic start.

Technical Summary: Prioritizing Embryos with Lower Homozygosity in Consanguineous Couples

Problem Statement
Consanguineous unions (reproductive partnerships between individuals sharing recent common ancestors) are prevalent globally and increase the burden of rare recessive disorders in offspring. This risk arises from elevated autozygosity, where genomic regions are inherited identically by descent (IBD) from both parents. While current reproductive genetic screening (PGT-M) focuses on a limited set of known pathogenic variants, it leaves the majority of recessive risk unaddressed, particularly variants at loci not yet identified or screened. Furthermore, existing approaches do not account for the genome-wide burden of recessive variants that contribute to both rare monogenic disorders and complex polygenic outcomes. The clinical challenge is how to mitigate these elevated genetic risks in consanguineous couples without discouraging the unions themselves, adhering to principles of non-directive genetic counseling.

Methodology
The authors propose a framework termed PGT-H (Preimplantation Genetic Testing for Homozygosity), which integrates embryo-level autozygosity quantification into existing IVF workflows. The study employs the following methodological approaches:

Forward Simulations: The authors simulated 1,000 independent families for two relationship types: first-cousin and double-first-cousin unions. Using realistic sex-specific recombination maps, they modeled the transmission of founder chromosomes through the pedigree to generate 1,000 embryos per couple.
Quantification of FROH: They defined FROH (Fraction of the genome in Runs of Homozygosity) as the total length of autosomal ROH segments divided by the total autosomal genome length. ROH segments were identified as tracts where homologous chromosomes were inherited from the same founder haplotype.
Selection Modeling: The study modeled the reduction in FROH achievable by selecting the embryo with the lowest FROH from cohorts of varying sizes ( $n = 2$ to $20$). They calculated the expected minimum FROH relative to a random selection.
Risk Translation: To estimate health benefits, the authors combined baseline risk data from outbred populations with relative risk estimates derived from consanguinity studies. They applied two modeling assumptions:
- Monogenic Outcomes: For conditions like intellectual disability and congenital anomalies, they assumed excess risk scales linearly with FROH. They incorporated diagnostic yields from exome sequencing to model the residual risk after PGT-M.
- Polygenic Outcomes: For conditions like type 2 diabetes and childlessness, they modeled the log-odds of disease as proportional to FROH, based on within-sibling analyses.

Key Contributions

Demonstration of Within-Family Variability: The study establishes that despite a fixed parental relatedness, realized FROH varies substantially among full siblings due to the stochastic nature of meiotic recombination and Mendelian segregation.
Feasibility of PGT-H Integration: The authors outline a workflow where FROH can be computed from ultra-low-pass sequencing data already generated during routine PGT-A (Preimplantation Genetic Testing for Aneuploidy) or PGT-M, requiring no additional biopsy or sequencing steps.
Quantification of Risk Reduction: The paper provides specific estimates of absolute risk reduction for various outcomes when prioritizing low-FROH embryos, demonstrating that this strategy complements, rather than replaces, variant-centric screening.

Results

FROH Reduction: In simulations of first-cousin couples, selecting the lowest-FROH embryo from a cohort of five reduced realized FROH by approximately 40% on average compared to random selection. For double-first-cousin couples, the proportional reduction was approximately 30% (an absolute reduction of ~3.8 FROH percentage points).
Disease Risk Estimates:
- Intellectual Disability: For first-cousin couples, the absolute risk of intellectual disability is estimated at ~5% without screening. PGT-M alone reduces this to ~3.5–4.1%. The addition of FROH-based screening (PGT-H) further reduces the risk to 2.8–3.2%.
- Congenital Anomalies: For first-cousin couples, risk decreases from ~6.6% to 4.0–4.5% with combined screening.
- Type 2 Diabetes: Risk decreases from ~28% to 24.5% for first-cousin couples.
- Infant Death: Risk decreases from ~1.3% to 1.0% for first-cousin couples.
Complementary Mechanism: The results indicate that PGT-M removes risk mediated by known pathogenic variants, while FROH-based prioritization reduces the remaining genome-wide recessive burden. The two strategies act in a complementary fashion to address both known and unknown recessive risks.

Significance and Claims
The paper argues that autozygosity-based embryo prioritization offers a "principled way" to address a component of recessive risk that current variant-centric approaches miss. The authors claim that this approach:

Addresses the "Unknown": It captures the burden of recessive variants at loci not yet associated with disease or included in carrier screening panels.
Provides Actionable Biomarkers: FROH serves as a genomic biomarker that can be integrated into standard clinical workflows without disrupting existing PGT-A or PGT-M procedures.
Offers Modest but Meaningful Reduction: The authors emphasize that while PGT-H does not eliminate disease risk, it can reduce the absolute risk of severe outcomes (such as intellectual disability and major congenital anomalies) by several percentage points and reduce excess risks (compared to unrelated couples) by roughly one-half to two-thirds.
Ethical Alignment: The framework is presented as a non-directive risk-reduction tool that respects cultural values regarding consanguinity while providing evidence-based options for informed decision-making.

The authors conclude that these risk reductions are meaningful but must be communicated realistically, acknowledging that residual risks remain due to non-recessive etiologies, unscreened genetic mechanisms, and the fact that not all embryos will result in a live birth.