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Question Video: Identifying the Principle of Mendelian Inheritance Not Met in a Non-Mendelian Trait Biology

The inheritance of ABO blood groups is considered non-Mendelian. If we cross a female homozygous for blood group A and a male homozygous for blood group O, which principle of Mendelian inheritance will this particular cross break?

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Video Transcript

The inheritance of ABO blood groups is considered non-Mendelian. If we cross a female homozygous for blood group A and a male homozygous for blood group O, which principle of Mendelian inheritance will this particular cross break? (A) Complete dominance, (B) segregation of genetic factors, (C) independent assortment of genes, or (D) none of the answers are correct.

This question asks us about ABO blood groups, which are found in humans. You might have learned that humans can have one of four different blood groups or types: A, B, AB, or O. These are determined by an individual’s two alleles or variations in the gene for the blood group. One allele for this gene is inherited from the individual’s biological mother and the other from the biological father.

In our question, we are specifically asked about the reproductive cross between a female who is homozygous for blood group A and a male who is homozygous for blood group O. Homozygous means that they possess two of the same alleles, since homo- means the same. This means that the female will possess two A alleles, while the male possesses two O alleles. When these alleles are separated into gametes, or sex cells, to prepare for reproduction, only one copy of each allele will be allocated to each gamete. This is called Mendel’s law of segregation. This principle will not be broken in this, or any other, cross.

In addition to this, an individual’s different genes will undergo independent assortment during the formation of gametes. This just means that particular alleles for eye color, blood type, and height will not be predictably inherited together. They would be assorted in various, random combinations. Since this question is only asking about the inheritance of a single gene, it does not show an exception to the rule that different genes are inherited separately.

So, assuming that gamete formation follows the rule of segregation, can you determine what kind of gametes the female and male can produce? The female can only produce gametes containing the A allele since this is the only allele she possesses in her own genome. Similarly, the male can only produce gametes containing the O allele. Therefore, all of the potential offspring from this reproductive cross will contain one A allele from the biological mother and one O allele from the biological father.

Here, the A allele is dominant to the O allele. A dominant allele will always be expressed if it is inherited. On the other hand, the O allele is a recessive allele. So blood group O will only be expressed if two O alleles are inherited. Therefore, the blood group gene shows complete dominance and does not break the principle of complete dominance.

However, since there are three possible alleles for this gene, A, B, and O, blood groups are in fact an example of multiple alleles since there are more than two possible alleles for the trait. However, only two of these alleles are ever present in a single person.

We should now be able to answer our question correctly. We are asked which principle of Mendelian inheritance will this particular cross break. And the correct answer is (D); none of the answers are correct.

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