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
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.