Video Transcript
A man claiming to be a long lost
child of a recently deceased millionaire argues he has a claim to the
inheritance. The millionaire has blood group O,
and the child has blood group AB. Could the millionaire be the
father? Why? Use the blood group table provided
to help you. No, as a child would’ve inherited
one allele for group O from the father. No, as a man would have blood group
O if the father did. Yes, as a child could’ve inherited
the alleles for group AB from their mother alone. Or yes, because it is impossible to
tell paternity from the blood group.
Let’s remove the answer options so
that we can explore some of the key terms that are relevant to understanding the
inheritance of human blood groups to help us in answering this question. Human ABO blood grouping is a good
example of alleles showing codominance. Codominance is when two alleles for
a specific trait are both expressed in their observable characteristics
simultaneously if an organism is heterozygous for a trait.
Remember, heterozygous means they
have one of each allele for a certain trait. The set of alleles an organism
possesses for a specific trait is their genotype. You can see the genotypes that
correspond to each different blood group in the table here. The phenotype is the observable
trait that the genotype produces. In this case, the phenotype is a
blood group itself, shown in the top row of the table. The alleles for blood group are
denoted according to the antigens on the surface of red blood cells. If a person has the alleles IA and
IB in their genotype, then they have both type A and type B antigens present on the
surface of their red blood cells, giving them the blood type AB.
People with blood group AB have
inherited the IA allele from one of their biological parents and the IB allele from
their other biological parent, though we don’t know which allele came from which
parent. Instead of one allele being
dominant over the other, both alleles are codominant and are expressed
simultaneously in the phenotype to give them blood group AB. If a person has two IO alleles in
their genotype, this indicates that there are neither type A nor type B antigens on
the surface of their red blood cells. This gives them blood group O.
If we refer back to the question,
we can see that the child claiming to be an offspring of the millionaire has blood
type AB. He must have inherited one of these
alleles from his biological father and the other from his biological mother, though
we don’t know which came from which parent. This also means that to be the
father of the child, the millionaire must have a genotype that includes at least one
allele for blood group A, IA, or at least one allele for blood group B, IB. However, the millionaire has blood
group O.
Looking at the table, we can see
that the only time this phenotype is expressed is if the genotype for the blood
group allele is IOIO. And if this millionaire had
biological children, one of these IO alleles would be passed on to them and
therefore be present in their genotype too. Therefore, we can conclude that no,
the millionaire was not the father, as the child would’ve inherited one allele for
blood group O from him.
