How does a blood test connect two siblings with the same mother?

Blood type is inherited from both parents. A child receives one blood group from each parent. If two people have the same blood group, it is possible that they are related, but they may not be able to conclude they have the same mother just by blood type.

However, if the two people have different blood groups, they cannot have the same mother. This can be due to:

- One parent being homozygous dominant for the blood type and the other being homozygous recessive.

- Both parents being heterozygous and sharing a recessive allele.

Even if these conditions are met, the people may still not be siblings, as another woman with the same blood type could be their mother. There are more sophisticated tests that can be done, such as DNA tests, which can accurately establish whether or not two siblings share the same mother.