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.
