Make a two row by two column table.
There are four different blood types: type-O, type-A, type-B and type-AB. Type-O, the most common, is known as the universal donor because any person can receive a blood transfer of type-O blood. Type AB is known as the universal receiver because type-AB can receive a blood transfer of any type of blood. You can only find possible blood types from knowing your parents' blood types; you cannot say with certainty which blood type you have just based on your parents.
Label the two columns based on your mother's blood type. If your mother has type-A blood, enter "A" over the first column and "O" over the second column.
Label the two rows based on your father's blood type. For example, if your father has type-AB blood, enter "A" to the left the first column and "B" to the left of the second column.
Combine the column with the row to find the possible blood types. For this example, in the upper left box, you will get "AA." For the upper right box, you will get "AO." For the lower left box you will get "AB." For the lower left box you will get "BO."
Drop the "O" from "AO" or "BO," if applicable. In this example, drop the "O" from "AO" to get "A" and the "O" from the "BO" to get "B." Therefore, you could have type-A blood, type-B, blood or type-AB blood.
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