No, twins do not always have the same blood type.
Understanding Twin Types and Blood Type Inheritance
There are two main types of twins: identical (monozygotic) and fraternal (dizygotic). This distinction significantly impacts the likelihood of sharing a blood type.
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Identical Twins: These twins develop from a single fertilized egg that splits, resulting in two genetically identical individuals. Therefore, identical twins almost always share the same blood type.
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Fraternal Twins: These twins develop from two separate eggs fertilized by two separate sperm. Genetically, they are no more similar than any other siblings. As a result, fraternal twins may or may not have the same blood type. They could have the same blood type or different blood types, just like any other siblings. As stated in the provided reference, "Fraternal twins may have the same blood type, or they may have different blood types. They are no more genetically alike than siblings from different births."
Examples
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Scenario 1: Two identical twins will almost certainly have the same ABO blood type (A, B, AB, or O) and Rh factor (positive or negative).
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Scenario 2: Two fraternal twins could have different ABO blood types and Rh factors (e.g., one might be A+ and the other O-).
Conclusion
The possibility of twins sharing a blood type hinges on whether they are identical or fraternal. While identical twins usually have matching blood types, fraternal twins can have the same or different blood types.
