Genetic differences between animals are a result of differences in their DNA. If a trait is determined by only a single gene, such as for example in the case of having horns in cattle or not, then the phenotype depends on the combination of alleles for that single gene. Having horns only occurs in case of homozygosity for the recessive allele h. Being polled (having no horns) is a dominant trait, so both Hh and HH animals are polled, and only hh cattle have horns. Especially in dairy cattle the calves are de-horned when they are still very young. However, also in the Holstein-Friesian some animals are naturally polled because they carry the H-allele, derived from another breed.
Some aspects of coat colour color are also determined by only a single gene with two alleles. But that may result in more phenotypic variation than in the example of having horns or not in cattle. In chestnut horse, for example, horses with no dilution factor (DD) are chestnut (brown coat and brown mane and tail). But if they have a single dilution allele (Dd) they are palomino (lighter manes and tail, slightly lighter coat), and with a double dilution allele (dd) they are cremello (almost white in coat and mane and tail, and also lighter eyes). So this dilution gene results in 3 different phenotypes, whereas the polled gene only produced 2.
It may seem that monogenic traits are never influenced by the environment, so P = G. This is not always the case. An example of a gene that is influenced by the environment is the gene that causes phenylketonuria or PKU in humans. It is a rare recessively inherited metabolic disorder. Only if two carriers have a child, that child can have PKU. In the Netherlands 1 in 18,000 children has it, but all are tested for it as newborn baby in the blood collected with the heel pinch. In people with PKU the enzym Phenylalaninehydroxylases is not present or not functioning, so that it cannot break down the amino acid Phenylalanine. This is thus accumulating in the blood and the spinal fluid, which causes damage to nerve cells, eventually resulting in brain damage. Patients with untreated PKU are usually mentally retarded with behaviour behavior problems, and often suffer from skin diseases. The treatment is very basic: a life-long diet with very little protein and no intake of aspartaam as that contains phenylalanine. Patients do get amino-acid additives to prevent deficiencies. Thus, the expression of this monogenic traits can be influenced by the environment, by the diet in this example.
...