PKU is an autosomal recessive condition. What does that mean? Well, that means that people that have PKU have two recessive genes for the disorder. In scientific terms they are homozygous recessive for the PKU trait. The recessive gene which characterizes PKU is inherited from both parents.
If neither of a PKU child's parents have the disorder themselves, they are carriers of the gene, or heterozygous for the condition. Heterozygous is the biological term for a person that has one dominant gene for a trait and one recessive gene. In individuals that are heterozygous for PKU, the dominant gene completely masks the recessive one and the person displays no characteristics of the disorder.
Say that "P" designates the dominant gene for the PKU condition, and "p" designates the recessive gene. The heterozygous parent of a PKU individual has a genotype, or genetic makeup, of "P p", or one dominant gene and one recessive. However, their phenotype, or outwardly displayed characteristic for the trait, is of a normal non-PKU individual. Approximately one in 60 people in the United States is heterozygous for the PKU trait. For a person with PKU, their genotype is "p p", two recessive genes, and their phenotype is PKU.
A Punnett square is a diagram of the possible offspring which could result from parents of designated genotype. The above figure displays a Punnett square of two parents which are heterozygous for PKU. The heterozygous father's genotype is displayed on the left, and the heterozygous mother is on the top of the diagram. In the upper two squares the father supplies the offspring with a dominant "P" gene, and in the lower two squares he supplies the recessive "p". In the left two squares the mother gives the offspring her dominant "P" gene, and in the right squares the recessive "p" gene.
The result is three possible genetic combinations. In the upper left square, both parents supply the offspring with a dominant gene and the result is a child that has two dominant genes, homozygous dominant, and does not have PKU. In the upper right box the father gives a dominant gene and the mother a recessive. The result is a heterozygous child who can pass PKU on to their offspring. In the lower left box the father supplies the child with a recessive gene and the mother supplies a dominant gene. The result is another heterozygous child. In the lower right box the offspring inherits a recessive gene from both the father and the mother and the result is a homozygous recessive child with PKU.
There is a 1 in 4 chance, or 25%, that the child will have a homozygous dominant genotype, a 2 in 4 chance, or 50%, that the child will have a heterozygous genotype and carry the PKU gene, and a 25% chance that the child will be homozygous recessive and have PKU. However, there is a 3 in 4 chance, or 75%, that the child will not have PKU.
So, per pregnancy, there is a 25% chance of a PKU baby being born to heterozygous parents. In statistical terms, the chance of two heterozygous parents having one PKU child out of four is 42%. Many people believe that the odds of having one PKU child out of four is 25%. This is actually not the case because there is a 25% chance *per pregnancy*. The odds are greater of having at least one child with PKU if heterozygous parents have four children.
Since I have PKU my parents are obviously heterozygous for the trait. Statistically speaking, the odds were completely against them because out of their four children, three of us have PKU. The chance of that happening is only 4.6%.
Similar Punnett squares can be done for parents of varying genotype. For example, working out the Punnett square of a heterozygous parent (P p) with a homozygous recessive PKU parent (p p) shows that there is a 50% chance of having a heterozygous child, and a 50% chance of having a PKU child.
If a homozygous recessive PKU parent has children with a homozygous dominant (P P) parent there is no chance of having a PKU child. The offspring can only inherit a "p" gene from the PKU parent. This recessive gene will be masked by the "P" gene from the homozygous dominant parent. Hence, the resulting children will all be heterozyous for PKU.
I hope this page has been helpful and has cleared up some questions that you might have had regarding the genetics of PKU!