My parents have B +ve blood group in fact my brother also have B +ve blood group, then why my blood group is O -ve ?

  • $\begingroup$ Due to our being diploid, we have 2 genes for deciding blood type. the gene for blood type O is recessive and so is the Rh-. If your blood type is O- and your parents are B+ then your parent's genes are BO+/- for both of them. and you inherited O AND - from both of your parents. $\endgroup$
    – Kiny
    Mar 26, 2017 at 18:12

1 Answer 1


Humans are diploid organism. So humans have two copies of every chromosome and gene.

When a person has B blood. We mean that the blood cells of that person expresses the B antigen.

To produce the B antigen you will need at least one allele for the B antigen. Hence the B antigen phenotype can have a genotype of BB or BO.

Now if two people have the genotype of BO have children, a punnet square will show they they have a 25% of having a child with BB genotype, 50% of having a child with BO genotype and 25% of having a child with OO genotype.

Phenotypically, it means that there is 75% chance of having a child with B blood. And 25% chance of a child with O blood.

A similar system also occurs with rhesus +/- system. A person with +ve blood can have a genotype of ++ or +-. If two people with the genotype of +- have children together, there is a 25% chance that their child will have -- genotype and be rhesus -ve.

The two systems work together. So two people with genotype of BO blood type and +- rhesus antigen having children together, they can have a child with O blood and rhesus negative with a probability of 1/16 (6.25%)


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.