There is a relatively rare birth defect call phenylketonuria (PKU.) It is an autosomal recessive trait inherited in a classic Mendelian pattern. A person may have inherited the PKU gene from one parent but not both parents; that means the person is a carrier for the trait, but will not have the disease and will not manifest any symptoms. YOU could be a carrier for PKU and not even know it. If you mate with someone who is NOT a carrier, none of your children will have PKU but there is a 50% chance that each one of them will be a carrier for the disease.
If a carrier mates with another carrier, there is a 25% chance that each child will inherit the gene from both parents and, therefore, develop the disease.
Phenylalanine is an "essential" amino acid. There are approximately 20 amino acids that naturally combine into chains to form polypeptides and proteins. Most proteins do NOT contain all 20 of the essential amino acids. When your body processes proteins, it breaks the protein down into its amino acids components and eventually those amino acids are further metabolized by the liver and excreted in the urine.
A person with PKU lacks the ability to metabolize and excrete the phenylalanine metabolites and they build to toxic levels in the body. That is what causes most of the symptoms of PKU. It does not compromise the life expectancy of its victims but people with PKU may have a life of marginal existence and dependence, at best.
The symptoms of PKU may include
-- A musty odor in the breath, skin or urine, caused by too much phenylalanine in the body
-- Neurological problems, including seizures
-- Skin rashes (eczema)
-- Fair skin and blue eyes (because people with PKU can't convert phenylalanine into melanin, the pigment responsible for hair and skin tone)
-- Abnormally small head (microcephaly)
-- Intellectual disability
-- Delayed development
-- Behavioral, emotional and social problems
-- Psychiatric disorders
I have told you about PKU because it is a relatively simply and easily understood example of the interplay between "nature and nurture." The nature vs. nurture issue arises frequently when we discuss human behavior; does a person behave a certain way because they were born with that behavior "programmed" in their genetic complement, or because they were exposed, early in life, to an environment which promoted that behavior. The answer is usually not easily discerned.
E.g, suppose I identify a certain behavior that is much more common in people of Chinese descent. Certainly they have a different and unique genetic blueprint that manifests in some obvious ways, but most of them were also raised in China and they were exposed to certain behaviors in their parents, grew up eating a certain diet which varies from a Western or African diet, and we can't really say that the "Chinese" behavior under study is caused by Chinese genetics or Chinese nurturing.
Back to PKU. (While it is clearly an inherited condition, it is interesting that it occurs less frequently in people of African descent.) If you are born with PKU, are you doomed to a lifetime of seizures, rashes, microcephaly, hyperactivity, intellectual disability, etc.?
No. In the US, every child is screened shortly after birth for the presence of PKU. If a child is positive, they are immediately started on a low protein diet, because that is the sole source of phenylalanine in our diet. Take away phenylalanine and the body can't produce the dangerous phenylalanine metabolites that harm the body. People with PKU are also given a protein supplement composed only of proteins which do not contain phenylalanine. Finally, there is a relatively new drug - sapropterin (Kuvan) - which reportedly makes phenylketonurics more tolerant of phenylalanine, but its long term efficacy is not well established.
If treated from infancy with the above regimen, most phenylketonurics will not develop symptoms of the disease and they will develop normally both physically and mentally. It is controlled extremely well by these dietary restrictions.
So, this is a disease that is inherited but the expression and severity of the disease is controlled quite effectively by modifications of diet. So, nature and nurture both work together for phenylketonurics.
Aspartame is the artificial sweetener used in Diet Coke and many other diet soft drinks. Aspartame contains phenylalanine so anyone with PKU should avoid drinking Diet Coke and any of the other diet drinks sweetened with aspartame! Next time you see a can of Diet Coke, I'll bet you look at the label for the PKU warning.