Living material is unique in that its composition and form is represented in two radically different formats, called phenotype and genotype .
The physical material itself - an arm, cell, chunk of meat or thymus gland - constitutes its phenotype. It is more than what is visible by eye or microscope however, as it includes the full complement of behaviours, the developmental dynamics, as well as the chemical compositionof the organism.
Genetic representations are inscribed in a code in DNA molecules infused throughout all biological tissues. Every cell contains a complete genetic description of not only its own phenotype but of the whole organism encompassing it. A fat cell in the belly contains the complete genetic information for brain neurons as well as itself. Every speck of tissue visible to the eye carries hundreds or thousands of these genetic representations in the chromosomes of its cells. Moreover, they do not merely describe but are part of an elaborate cellular machinery to cause a body part to develop the form of those descriptions.
The genotype is the full complement of the genetic information repeated exactly in every body cell. It is a major determinant of the phenotypic attributes of the organism, which is why an egg laid by a hen hatches as a chick instead of a duckling. But, genes are not exclusively responsible for a person's phenotype. The environment also plays an essential role. A baby can be born with fetal alcohol syndrome because the mother abused alcohol during her pregnancy. Also, Japanese children grow up speaking Japanese and British children English, due exclusively to their environment. In general phenotypic traits are specified or "determined" by a combination of genetic and environmental factors.
The way genes interact with the environment is complex. One popular conceptualization is that the genotype specifies potential for a person's traits or abilities and the environment determines how fully that potential is reached. Another view is that genes and the environment have additive effects; the final phenotype reflecting the sum of a myriad of genetic and phenotypic influences. In reality, the situation is more complicated than either model.
The relationships between genetic and environmental causes
vary, and the degree to which genes specify phenotype differs
from trait to trait. The more complex the trait the less people
agree on the relative importance of genotype vs environment (or
nature vs nurture) for it. It is fairly obvious that the age at
which a child's six-year molars appear depends primarily on genes
and that environmental conditions largely determine when they
disappear. On the other hand, the relevance of genes to
criminality or to intelligence is bitterly contested. The
underlying difficulty is that the science of genetics simply has
not matured enough to be able to resolve such questions.
At the moment we understand our phenotype far more completely
than our genotype. This is true even for traits which we
understand poorly. Our detailed knowledge of the anatomy of the
brain may be rudimentary, but it still vastly exceeds that of the
genes for the nervous system. The goal of the Human Genome
Project is to bring our knowledge of the anatomy of the human
genotype to a level equal to that of the gross and microscopic
anatomy of our bodies. Such knowledge would likely reveal the
functions, normal variations and pathology of the genotype, and
make our understanding of them comparable to our knowledge of
human physiology, anatomical diversity. and disease. In short,
the human genome project promises to reveal the other half of our
biological being; if it delivers, it will transform biology and
medicine..