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Biallelic findings: when both copies of a gene are affected | CION Cancer Clinics
Biallelic means both copies of a gene carry a fault, one from each parent. For several cancer genes, that causes a different and often more serious condition than a single faulty copy. This page explains why two copies change the picture, how the laboratory confirms it, and why your parents, brothers, sisters or partner may be offered testing. At CION Cancer Clinics in Hyderabad, our oncologists review your family history with you and guide you to the right genetic counselling and testing.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- What does biallelic mean on a genetic report?
- Which genes behave differently with two faulty copies?
- How does the laboratory know both copies are affected?
- The words around a biallelic finding, in plain language
- One faulty copy compared with two
- What this page cannot tell you
- Four things families assume about two faulty copies
- Common questions about biallelic findings
The short answer
What does biallelic mean on a genetic report?
Biallelic means both copies of a gene carry a fault, one inherited from each parent. For several cancer genes, having two faulty copies causes a different and often more serious condition than having one. It can also change who else in the family should be tested, including your partner.
Why two copies are not simply double the risk
You carry two copies of almost every gene. For most inherited cancer faults, one faulty copy raises risk while the other copy keeps working. When both copies are faulty from birth, there is no working copy at all. With some genes that produces a distinct illness, sometimes starting in childhood, with its own screening and care. It is a different situation, not a stronger version of the same one.
Why it matters in many Telugu families
A biallelic finding usually needs both parents to carry a fault in the same gene. That is more likely when the parents are related, as in marriages between cousins or between an uncle and a niece, which remain common in parts of Telangana and Andhra Pradesh. Most children of such marriages are perfectly healthy. The chance of a biallelic condition is simply higher, and a counsellor will ask about it directly and without judgement.
Biallelic is a statement about how many copies are affected. What it means for you depends on the gene.Where it changes things
Which genes behave differently with two faulty copies?
These four are the ones families in cancer clinics meet most often. Your report may name another.
MUTYH
One faulty copy is common and usually adds only a small change in bowel cancer risk. Two faulty copies cause MUTYH-associated polyposis, meaning many growths in the bowel lining and a much higher bowel cancer risk, managed with regular colonoscopy.
The mismatch repair genes
MLH1, MSH2, MSH6 and PMS2 repair copying errors in DNA. One faulty copy causes Lynch syndrome in adults. Two faulty copies cause constitutional mismatch repair deficiency, a rare condition with cancers in childhood.
Can include
- Brain tumours in children
- Blood cancers
- Bowel cancer at a very young age
BRCA2
One faulty copy raises breast, ovarian, prostate and pancreatic cancer risk. Two faulty copies cause a rare childhood condition in the Fanconi group, in which the bone marrow fails and childhood cancers are more likely.
ATM
One faulty copy moderately raises breast cancer risk. Two faulty copies cause ataxia-telangiectasia, a childhood condition affecting balance and movement, the immune system and cancer risk.
Not sure whether this applies to you?
Ask an oncologistChecking the finding
How does the laboratory know both copies are affected?
Two variants are found in one gene
Or one variant is found on both copies. The report will usually say which of these it is.
The question of which copy
Two different variants might sit on separate copies, or both on the same copy. Standard testing often cannot tell these apart, and they mean very different things.
Testing the parents
If one variant came from each parent, they must be on separate copies. A blood sample from each parent usually settles it.
A confirmed or unconfirmed result
Once both copies are shown to be affected, the report is described as confirmed biallelic. If parents cannot be tested, it stays unconfirmed, and the counsellor will explain what that means for your care.
Planning care and family testing
Screening is planned for the condition itself, and brothers, sisters and partners are offered testing where it helps.
On your report
The words around a biallelic finding, in plain language
- Allele
- One of your two copies of a gene. One came from your mother and one from your father.
- Heterozygous
- A fault in one copy only. This is the usual finding in inherited cancer testing.
- Homozygous
- The same fault in both copies. This is more common when the parents are related.
- Compound heterozygous
- Two different faults in the same gene, one on each copy. It is also biallelic.
- In trans, in cis
- In trans means the two variants are on separate copies. In cis means both are on the same copy, leaving the other copy working.
- Recessive
- A condition that appears only when both copies are faulty. Each parent is usually a healthy carrier.
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Side by side
One faulty copy compared with two
One faulty copy
Two faulty copies
Usually raises the risk of certain adult cancers
Can cause a separate condition, sometimes in childhood
Came from one parent
Came from both parents, one copy each
Each child has an even chance of inheriting it
Every child inherits one faulty copy
Partner testing is rarely needed
Partner testing may matter for the next generation
Brothers and sisters may carry one copy
Brothers and sisters may carry one copy, two, or none
Being straight with you
What this page cannot tell you
It cannot tell you whether your own finding is truly biallelic, or what it means for your health. That depends on the exact gene, the exact variants, how each is classified and whether your parents have been tested. What your specific variant means is a question for the counsellor who ordered the test.
It cannot plan screening for you
Care after a biallelic finding is specialised and differs from gene to gene. For some conditions it involves several specialists, including a paediatrician when a child is affected. A counsellor and your oncologist will build that plan together, and it should be written down.
Who this does not apply to
Most people who have genetic testing for cancer find one faulty copy or none at all, and this page does not describe them. If your report lists a single variant, or a variant of uncertain significance, the biallelic question is unlikely to arise for you.
If your report mentions two variants in one gene and nobody has explained it, call the helpline and ask for a counselling appointment.Commonly believed
Four things families assume about two faulty copies
"Two variants in one gene always means both copies are broken."
Only if they sit on separate copies. If both sit on the same copy, the other copy still works. Testing the parents is usually how this is settled.
"Two faulty copies simply means double the risk."
For several genes it means a different condition with different screening, not the same condition twice over. That is why the gene name matters so much.
"Our cousin marriage means our children will be ill."
Most children of related parents are healthy. The chance of a recessive condition is higher, and a counsellor can tell you whether any testing would help your family.
"Carrying one copy of a recessive gene means nothing at all."
For some genes one copy still adds a modest risk in adult life. It can also matter for your children if your partner carries a fault in the same gene.
Questions we are asked
Common questions about biallelic findings
Is biallelic the same as homozygous?
Homozygous is one kind of biallelic, where the same fault sits on both copies. Compound heterozygous is the other kind, where two different faults sit on one copy each. Both leave no working copy of the gene, which is what matters for your care.
My report lists two variants in one gene. Am I biallelic?
Not necessarily. If both variants are on the same copy, the other copy still works. The laboratory or counsellor may ask to test your parents to find out. Until that is settled, do not assume either answer.
Why do they want to test my parents?
To find out which parent passed on which variant. If each parent carries one, the variants must be on separate copies. Parents are usually healthy carriers, and the test is normally a simple blood sample from each of them.
Should my brothers and sisters be tested?
Often yes. When both parents are carriers, each brother or sister may have two faulty copies, one, or none. Which of those they have changes their care, so the counsellor will usually offer testing to them.
Does my husband or wife need to be tested?
If you carry a fault in a gene where two copies cause a serious condition, testing your partner can show whether your children could inherit two. It matters most before planning a pregnancy. Ask your counsellor whether it applies to your gene.
Can a biallelic condition be found before a baby is born?
When the family's variants are known, testing during pregnancy or before it may be possible. These are personal decisions with ethical and practical sides. Discuss them with a genetic counsellor before a pregnancy, not after.
Is a biallelic finding more common in India?
Marriages between relatives raise the chance that both parents carry a fault in the same gene, and such marriages remain common in parts of South India. Reliable figures for Indian families are limited, and studies so far are small.
Where do I start if my report says biallelic?
Book a genetic counselling appointment and bring the full report, not just the summary page. Bring what you know about both sides of the family. Call the CION helpline if you are not sure who to see, and someone will direct you.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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Sources
- MedlinePlus Genetics — What are the different ways a genetic condition can be inherited?
- GeneReviews (NCBI) — MUTYH Polyposis
- MedlinePlus Genetics — Constitutional mismatch repair deficiency syndrome
- MedlinePlus Genetics — Ataxia-telangiectasia
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Heterozygous, homozygous and compound heterozygousThe three words that describe how many copies are affectedReading variant nomenclature: c. and p. notationHow to read the variant names printed on your reportHow to read a germline genetic test reportA walk through every section of the reportReading your report with the familyWhen parents and siblings need to hear it too
Talk to us
Does your report mention two variants in one gene?
Bring the full report and we will arrange a counsellor to explain whether it is truly biallelic and what that means for your family. One helpline serves every CION centre.