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Question Video: Identifying That Blood Group in Humans in an Example of Codominance Biology

The table provided shows the different genotypes of the four blood groups in humans. The table shows that if one allele for A is inherited and one allele for B is inherited, both are expressed simultaneously in the blood type. What trait is this demonstrating?

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Video Transcript

The table provided shows the different genotypes of the four blood groups in humans. The table shows that if one allele for A is inherited and one allele for B is inherited, both are expressed simultaneously in the blood type. What trait is this demonstrating? (A) Codominance, (B) incomplete dominance, (C) complete dominance, or (D) recessive alleles.

This question asks about non-Mendelian modes of inheritance. Mendelian inheritance refers to the inheritance of alleles that result in completely dominant or recessive phenotypes. You may remember that when a purple flowering pea plant is crossed with a white flowering pea plant, the purple allele is dominant to the white allele, resulting in more purple-flowered offspring. However, more complex forms of inheritance exist. These include codominance, incomplete dominance, and multiple alleles.

Codominance refers to the expression of two different alleles for a particular gene at once. This is true for blood type in humans. When both an A and B allele are inherited, they are expressed together. So an individual with both an A and B allele will have the blood type of AB. Neither A nor B is dominant over the other, so they are expressed simultaneously.

Incomplete dominance refers to the mixing of alleles. For instance, if a red flowering plant were crossed with a white flowering plant and this cross only produced pink offspring, the red and white alleles would exhibit incomplete dominance. Notice how alleles exhibiting incomplete dominance will result in a phenotype that is different from both parents.

The last non-Mendelian mode of inheritance is called multiple alleles. Multiple alleles refers to any gene that has three or more alleles. This is also true for blood type in humans, which is determined by A, B, and O alleles. While these three alleles exist for that particular trait, an individual will only ever possess two of these alleles, one inherited from their biological mother and one inherited from their biological father.

Our question asks us about the AB genotype in blood. When an individual possesses the A and B alleles, neither exhibit dominance, so they are simultaneously expressed. This mode of inheritance refers to codominance because both of the alleles are expressed at the same time, with neither exhibiting dominance over the other. Therefore, the correct answer to our question is (A), codominance.

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