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Mendel is the father of modern genetics, but there are some genetic characteristics that cannot be explained by simple Mendelian genetics. Such is the case with the human blood types in which there are 3 alleles for the same gene, A B, and o. A parent can pass allele A, B, or o to the offspring based on the parent’s genotype.

From these 3 alleles, there are 4 blood types (phenotypes): A, B, AB, and O, and there are six genotypes: AA, Ao, BB, Bo, AB, or oo. This is an example of codominance in which both A and B alleles are codominant to each other.

Blood types can be used in forensics to determine if blood is from the victim or criminal. Blood types can be used to determine parental source in situation where the father is unknown; however, blood types can only eliminate certain blood types. DNA fingerprinting is a better method that is used often in criminal and parental determination cases.

Punnett squares

Punnett squares such as the one shown above are used to determine the probabilities (percentages) for genotypes of offspring given specific genotypes for the parents.


In the example above, the Punnett Square represents a cross (mating) between a male (on the left side) with blood type AB, and a female, (top of square), with blood type B, genotype BB.
Fill out and turn in the Punnett square for AB x BB above. And, answer the following questions for the cross represented above. Make sure you understand the difference between phenotype (blood type) and genotype. The Punnett Square shows the possible genotypes. When answering the questions, percent (probability) calculations and your answers should be in terms of the phenotypes (the blood types) and NOT the genotypes.

1. What are the possible blood types for the offspring?

2. What are the ratios or percentages for each possible blood type from this cross?

3. What blood type is not possible from this cross?

  • Biology -

    I cannot draw the Punnett square online. One parent can give either an A or a B, while the other can only give a B. With this information, you should be able to answer all the questions.

    The phenotypes for blood are A, B, AB and O. Which are not possible from this cross?

    We do not do your homework for you, but the above information should help. Thanks for asking.

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