A close-up shot of the nurse holding a tray in which surgical equipment is kept, including a blood sample.
Blood type inheritance causes more confusion in Indian families than almost any other genetics question. Partly because people don't get taught it in school in a way that sticks. Partly because a child with an unexpected blood type can trigger unwarranted suspicion. And partly because the rules, once you understand them, are genuinely interesting.
Here is everything you need to know.
Blood type is determined by the gene you inherit from each parent. For the ABO system, there are three possible versions of this gene (alleles): IA (produces A antigen), IB (produces B antigen), and i (produces neither).
Everyone inherits two copies of this gene — one from their mother, one from their father. The combination determines blood type:
| Gene Combination | Blood Type | |---|---| | IA IA or IA i | A | | IB IB or IB i | B | | IA IB | AB | | ii | O |
The key insight: IA and IB are codominant — both are expressed if you inherit one of each. The i allele is recessive — it is only expressed when you have two copies (ii).
This is why blood type is not always predictable from looking at the parents' expressed blood type. A person with blood type A might be IA IA (no hidden O gene) or IA i (carrying one hidden O gene). From the outside, both look the same. But what they pass to their children differs.
| Father's Blood Type | Mother's Blood Type | Possible Child Blood Types | |---|---|---| | O | O | O only | | O | A | A, O | | O | B | B, O | | O | AB | A, B | | A | A | A, O | | A | B | A, B, AB, O | | A | AB | A, B, AB | | B | B | B, O | | B | AB | A, B, AB | | AB | AB | A, B, AB |
Note: The "possible" column shows all outcomes that can occur depending on the specific gene variants each parent carries. In some combinations, not all listed outcomes are equally likely.
Separately from ABO type, the Rh factor (positive or negative) is inherited. Rh-positive (having the D antigen) is dominant over Rh-negative.
| Father's Rh | Mother's Rh | Possible Child Rh | |---|---|---| | Rh+ | Rh+ | Rh+ or Rh– (if both parents carry a recessive Rh– gene) | | Rh+ | Rh– | Rh+ or Rh– | | Rh– | Rh– | Rh– only |
If both parents are Rh-negative, all children will be Rh-negative — there is no Rh-positive gene available to pass down.
Completely possible. If both parents are blood type A but each carries one hidden recessive O allele (IA i genotype), each parent has a 50% chance of passing the i allele to the child. Two parents each passing an i allele produces an O child (ii genotype).
Statistically, two IA i parents have:
So roughly one in four children from two A parents can be blood type O. This is not rare. It is Mendelian genetics.
Same mechanism. B blood type can be IB IB or IB i. Two IB i parents each have a 1 in 4 chance of producing an O child. Nothing unusual about it.
This one is genuinely impossible. An O parent can only pass the i allele. An AB child requires one IA and one IB allele. An O parent cannot contribute either. If a child is tested as AB and one confirmed parent is O, there are three possibilities: a lab error, a very rare genetic mutation, or the parentage assumption is incorrect.
Also genuinely impossible. Two O parents (both ii) can only produce O children (ii). An A or B child would require an IA or IB allele, which neither O parent can supply. Again — lab error, or the most unusual genetic scenario (cis-AB phenotype, which is extremely rare).
This gets said carefully because it matters.
Blood type inheritance can exclude paternity — certain parent combinations cannot produce certain child blood types. If a child is blood type O and both parents are AB, that child cannot be biologically related to both those parents.
But blood type cannot confirm paternity. Many blood type combinations are possible from multiple different parent combinations. Two B+ parents could be the biological parents of a B+ child, an O child, or a B– child. That doesn't tell you who the father is — it only tells you what blood types are consistent with those parents.
If paternity confirmation is genuinely needed, DNA testing is the only reliable method. Blood typing is not a substitute.
For most Indian families, blood group inheritance is academic — useful to know, occasionally interesting, rarely medically significant.
Rh-negative mothers: If a woman is Rh-negative and her partner is Rh-positive, their child may be Rh-positive. As covered in the marriage compatibility blog, this Rh mismatch during pregnancy requires medical attention (anti-D injection protocol) to prevent haemolytic disease of the newborn.
Parents who are blood group carriers for rare types: In communities with higher rates of rare blood groups (Bombay phenotype, Kell-negative blood, and others), inheritance analysis helps identify which family members might carry rare alleles and therefore be rare type donors.
Through blood donation: When a young adult (18+) donates blood for the first time at any blood bank, their ABO and Rh type is tested automatically. Free.
Through diagnostic testing: Any diagnostic lab offers ABO and Rh group testing for ₹50–200. No medical referral needed.
Through the government blood donation system: Register on eRaktKosh or TheBloodApp after a donation and your blood type is recorded in your profile.
Knowing your child's blood type — and yours — is straightforward preventive health knowledge. It takes one test. Store the result somewhere you'll find it.
Register on TheBloodApp with your blood type noted. If you haven't donated blood before, a blood donation at any licensed blood bank includes free ABO and Rh typing. To find donation camps near you, call the number listed in the app.
Sources: NBTC India — Blood Group Genetics | Genetics Society of America — ABO Inheritance | Wikipedia — ABO Blood Group System | GeeksForGeeks — Blood Group Pregnancy Problem | eRaktKosh MoHFW | Fortis Healthcare — Rh Factor Guide
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