Now that we understand the biochemistry behind the ABO blood types [ABO Blood Types] it's time to look at the genetics. Recall that the human ABO gene on chromosome 9 has three common variants of the gene. Different variants are called alleles. The A allele encodes N-acetylaminogalactosyltransferase and this enzyme makes the A antigen that confers blood type A. The B allele encodes a variant enzyme that makes B antigen and gives rise to blood type B. The O allele encodes a defective enzyme that doesn't make either antigen. In the absence of both A antigen and B antigen your blood type will be O.
Everyone has two copies of chromosome 9 so you have two ABO genes. If both of them are the A alleles then your genotype (genetic makeup) will be AA. Your blood type will be A. If one of your ABO genes is the A allele and the other is the O allele then your genotype will be AO. In this case your blood type will be A as well since there are only three possibilities: (1) you have A antigen, (2) you have B antigen, (3) you have no antigen (you can have both antigens, see below). As long as you have one A allele, you will produce the A antigen on the surface of your blood cells.
Now let's look at the possibilities for inheritance of blood types. If someone has blood type A then their genotype might be AA or AO. Imagine that they mate with someone having blood type O. That person must have genotype OO. The manifestation of the genes is called the phenotype. The phenotype is related to the genotype but you can't always predict the genotype from the phenotype.
We can examine the possible genotypes and phenotypes of the children by constructing a Punnett Square where the alleles from one parent are listed on the side and the alleles from the other parent are listed on the top. You can think of this as being two different kinds of sperm or two different kinds of eggs; keeping in mind that sperm and eggs have only one copy of chromosome 9.
The results for the AA parent are shown on the left. All four of the possible combinations are shown in the matrix. There is only one genotype that will show up in the children (AO). All of the children will have blood type A, shown as purple boxes.
If the blood type A parent has the AO genotype then the Punnett square calculations look like the diagram on the right. In this case, there are two possible combinations; AO and OO. Half the children will have blood type A and half will have blood type O.
Since the O phenotype is masked by the presence of the A phenotype, we say that O is recessive to A and A is dominant with respect to O. The only way to see the O phenotype is when the genotype is OO. We refer to this as the homozygous recessive state. AO individuals are heterozygous because they have two different alleles.
Now let's look at combinations with the B allele. If you only express the B allele then you will only have B antigen on the surface of your cells and you will be blood type B. If you are heterozygous for the A and B alleles then you express both A antigen and B antigen and your blood type is AB. Note that the A phenotype is not recessive to the B phenotype or vice versa. The A and B alleles are co-dominant. This result is very common even thought we don't hear about it as often as dominant and recessive alleles.
A mating between an AB father and an OO mother will produce children who are either blood type A or blood type B as shown in the Punnett square on the left. None of the children will be blood type O.
A mating between two AB individuals will result in 50% of the children with AB blood type like their parents, 25% with blood type A, and 25% with blood type B. The percentages are derived from the matrix. Every combination of sperm and egg is random with respect to the genotype so the matrix represents the probabilities and not the definitive outcome whenever you have four children. It's quite possible for two AB parents to have four children with blood type A although the probability of this happening is low (0.254=0.4%).
The concept of probabilities is extremely important in genetics and it's also important in understanding evolution. The fact that every allele isn't necessarily passed on to the next generation gives rise to fluctuations in the frequency of alleles in a population. Over time, these fluctuations can result in the loss of an allele by chance. This is evolution by random genetic drift.
Now lets try a more difficult case that will test your understanding of genes and genetics. This example is based on an actual paternity case from several years ago. The mother has blood type AB and so does the child. The man who claims to be the father has blood type O. All genetic tests indicate that he really is the father of the child. How do you explain the blood type results?
UPDATE: Simple Mendelian genetics accounts for the vast majority of blood type inherited by children from their parents. However, in a population as large as humans with multiple alleles segregating independently, there will always be anomalies. Some are due to strange alleles that don't exactly conform to the recessive and dominant phenotypes and some are due to various recombination events. Some children can even show new, spontaneous mutations that were not present in either parent. The important lesson is that you can't rule out parental relationships based on ABO blood types alone.
Check Online Mendelian Inheritance in Man for some examples of rare alleles.