Genes and the Environment
The phenotype is the outward expression of a cell or organism. It is due to the interaction of the genotype and the environment.
1) Animal Hair Colour
In cats, the hair colour genes exert their effect by coding for enzyme production. Siamese cats have two copies of a recessive allele for one of these enzymes. This enzyme codes for the enzyme tyrosinase which is involved in the production of melanin. The enzyme is temperature sensitive (only active when temp is less than 31 degrees). This only happens on the extremities so the ears, paws and tail are dark when the rest is white.
2) Human Height
Even if a person inherited alleles of the genes that give the potential to grow tall, they will not grow tall unless they have the right amount of nutrients. Poor nutrition can limit the maximum height attained.
3) Monoamine oxidase A
Monoamine oxidase A (MAOA) is an enzyme found in the nervous system, liver and gut. It breaks down monoamines which are eaten as part of the diet. However the chemicals involved in transmitting nerve impulses across the synapses are monoamines. Once they have been released into the synapse they are broken down by MAOA.
If too much or too little of the enzyme is formed dangerous consequences can happen. Too much enzyme breaks down too many monoamines which can cause depression and addictive behavior and in some cases aggressive and violent criminal behavior. High levels can also cause depression. The amount of enzyme you have is determined genetically but environmental factors contribute to the problems this will cause.
For example, a study was done on children in New Zealand. People who had suffered maltreatment but had normal MAOA levels were no more antisocial then normal. 44% of the convictions for violent crime were done by people…