Medical Oncologist
CION Cancer Clinics
Heterozygous, homozygous and compound heterozygous, explained | CION Cancer Clinics
You carry two copies of almost every gene, and zygosity is the word for whether one or both carry a fault. This page explains heterozygous, homozygous and compound heterozygous in plain terms, and why the pattern found changes who else in the family is worth 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.
NG
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
17+specialists on panel
15,000+patients treated
35+centres across Telangana & AP
4.8★ / 800+Google rating
Decoding the report
What does heterozygous or homozygous actually mean?
You carry two copies of almost every gene, one inherited from your mother and one from your father. Zygosity is simply the word for whether one of those copies has a fault, or both. Heterozygous means one copy is affected. Homozygous means both copies carry the identical fault. Compound heterozygous means both copies are affected, but by two different faults rather than the same one.
Why the report bothers to state it at all
For some genes, one working copy is enough to keep a cell behaving normally, so a single faulty copy still raises risk on its own. For a smaller group of genes, one working copy is enough to cover for the other entirely, and a problem only shows up once both copies are affected. Which pattern applies to your gene decides how much the word on your report actually changes.
Why this word affects the rest of the family
Whether a fault behaves as heterozygous or needs to be homozygous to matter changes who in the family is worth testing, and how urgently. A parent who is heterozygous for a fault that only matters in double dose may never be affected themselves, yet still have something worth knowing about for family planning. Two such parents, each carrying one copy of the same fault without symptoms, can each pass it to a child, who then inherits two affected copies and a very different picture from either parent's own history.
Zygosity describes how many copies are affected. It does not by itself say how serious the fault is or what it means for treatment.Three words, three patterns
Heterozygous, homozygous and compound heterozygous, side by side
Each describes a different combination of the two copies of a gene that everybody carries.
Heterozygous
One copy of the gene carries the fault, and the other copy is working normally. This is the pattern behind most of the inherited cancer genes families are tested for. Each child of a heterozygous parent has an even chance of inheriting the faulty copy.
Homozygous
Both copies carry the exact same fault, one inherited from each parent. It is uncommon, and it is more likely when parents are closely related to each other.
Compound heterozygous
Both copies carry a fault, but two different faults rather than an identical one, again one from each parent. The overall effect can be similar to being homozygous.
Why the distinction matters
Some conditions only appear when both copies are affected, whether by the same fault or two different ones. Knowing which pattern applies changes how relatives are counselled, not just the patient. A sibling of someone with a heterozygous finding faces a different conversation to a sibling of someone found to be homozygous or compound heterozygous, even when the gene involved is exactly the same.
Not sure whether this applies to you?
Ask an oncologistWorking it out
How does a laboratory decide which pattern applies to you?
Every gene is checked in pairs
The laboratory reads both copies of the gene, one inherited from each parent, rather than assuming they match.
Each copy is compared against the reference
Any difference from the standard reference sequence is recorded separately for each copy, not combined into one result.
The pattern across both copies is named
If only one copy differs, the result is heterozygous. If both differ identically, it is homozygous. If both differ but not identically, it is compound heterozygous.
For compound findings, parents may be tested too
Testing both parents can show whether the two faults sit on separate copies, which matters for predicting a child's risk of inheriting both.
Terms on the report
The words that go with zygosity
- Allele
- One of the two copies of a gene you carry. Zygosity describes the relationship between your two alleles.
- Biallelic
- Affecting both copies of a gene, whether by the same fault or two different ones. It covers both homozygous and compound heterozygous results.
- Monoallelic
- Affecting only one of the two copies. This is another way of saying heterozygous.
- Dominant inheritance
- A pattern where one faulty copy is enough to raise risk on its own. Most well-known inherited cancer genes work this way.
- Recessive inheritance
- A pattern where both copies must be affected before risk rises meaningfully. A single faulty copy usually causes no problem.
- In cis and in trans
- Whether two faults sit on the same copy of a gene, or one on each. It is only relevant once a compound heterozygous result is found.
- Carrier
- Someone with a single faulty copy of a gene that only causes a problem when both copies are affected. A carrier is usually well themselves and finds out only through testing.
Leave a number, we will call you
One field. No form to fill in, and no charge for the call.
Did you know
Finding two different faults in the same gene does not automatically mean they came one from each parent. Occasionally both sit on the copy inherited from a single parent, which behaves quite differently for a child's risk. This is exactly why parents are sometimes asked to test alongside a compound heterozygous result.
Being straight with you
What this page cannot tell you
It cannot tell you whether your specific gene behaves as dominant or recessive, or what your particular zygosity result means for your children. That depends on which gene is involved, which only a counsellor reviewing your full report can confirm. Nor can it tell you how closely you should be watched from here.
It cannot tell you whether parental testing is needed
Whether a compound heterozygous finding calls for testing both parents depends on the gene, the specific faults found, and what the family already knows. This is decided case by case, not from this page. Two families with the same two faults in the same gene can still be given different advice, because their wider family history is not the same.
Who this does not apply to
If your report does not mention zygosity at all, it may not be relevant to the gene you were tested for, and nothing here changes your result. This page is for people whose report already uses one of these three words.
If your report uses a term from this page that confuses you, bring the exact wording to your counsellor rather than guessing which pattern applies to your family.Questions we are asked
Common questions about zygosity results
Is homozygous always more serious than heterozygous?
Not automatically. It depends on the gene. For genes that only cause problems when both copies are affected, a homozygous result is the one that matters most. For others, a single heterozygous fault is already the significant finding.
Does a compound heterozygous result mean both my parents carry the fault?
Usually, since each fault typically comes from a different parent, but not always. Occasionally both sit on the copy from one parent, which is why parental testing is sometimes recommended to clarify.
Why would my doctor ask my parents to be tested too?
To work out whether your two faults came from separate parents or the same one, which changes the risk calculation for your own children. It is a targeted, specific request, not a general screening exercise.
Is homozygous more likely if my parents are related?
Yes. Related parents share more of their genetic background, which raises the chance of both passing down the identical fault in the same gene. This is one reason family relationships are asked about during counselling.
Can a heterozygous result still matter for cancer risk?
Yes, for many of the genes tested in cancer care, a single faulty copy is already enough to raise risk. Whether that applies to your gene is something your counsellor will confirm directly.
Should my siblings be tested if I am heterozygous?
Often yes, since each sibling has an independent chance of carrying the same single fault. Your counsellor can explain the specific chance for your family and gene.
What does "in trans" mean on my compound heterozygous result?
It means your two faults sit on different copies of the gene, one from each parent, rather than both on one copy. It is a detail your counsellor uses to work out risk for your children.
Who can explain what my zygosity result means for me?
A genetic counsellor or clinical geneticist can connect the word on your report to the specific gene and your family history. Call the helpline if you are not sure who to ask.
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)
Medical Oncologist
Medical Oncologist
Want a specific doctor for your case? Mention them when booking.
Book Free ConsultationBook an appointment with our specialist
Share your name and number — we'll call you back within 30 minutes to schedule your consultation.
Sources
- MedlinePlus Genetics — Is the inheritance pattern always the same for a given disease?
- GeneReviews (NCBI) — GeneReviews Overview
- National Cancer Institute — Genetic Testing for Inherited Cancer Susceptibility Syndromes
- Cancer Research UK — Inherited cancer genes and increased cancer risk
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
Biallelic findings and why they are differentWhat it means when both copies are affectedHow to read a germline genetic test reportWhere the zygosity word sits on the full reportA negative result: what it does and does not rule outThe other side of a zygosity findingReading variant nomenclature: c. and p. notationThe notation that sits beside your zygosity result
Talk to us
Not sure what your zygosity result means?
Tell us the word your report uses and which gene it names, and we will help you find the right specialist to explain it. One helpline serves every CION centre.