Sex chromosomes are the 23rd chromosome of the human genome and determine the sex of an individual. These are the X and Y chromosomes and carry the genetic information necessary to specify male or female. A female is defined as XX or having two X chromosomes. A male is defined as XY or having an X and a Y chromosome, with the Y chromosome being small in size.

One example of a small change in the sequence of DNA for a gene leading to a genetic disorder is sickle cell disease. In this case only a single base of DNA is changed causing a single amino acid substitution: valine -> glutamic acid. This change makes hemoglobin less soluble, leading to reduced blood oxygen level. The reduction in solubility leads to hemoglobin chains forming in blood cells, causing the hallmark shape of red blood cells in sickle cell disease.

One way to determine the function of a gene is to see what happens to an organism if that gene is missing. Since mutations can affect cellular processes, studying organisms which undergo mutations often hold the answer to all the questions about a gene function. For an instance, the genetic disorder called phenylketonuria (PKU) was better understood when the scientists have discovered that it was caused by an autosomal recessive allele on chromosome 12. This disease happens to people who lack an enzyme called phenylalanine hydroxylase. This enzyme is needed to metabolize phenylalanine, which is an amino acid that is usually found in milk. The accumulation of phenylalanine in the body can lead to mental disorders. Because of this discovery, scientists were able to figure out the function of one of the genes carried on chromosome 12.

To determine the possible genotypes of the children with regards to blood type, we must first determine the genotypes of the parents. As given, the man has type AB blood, while the woman has type O blood.