Many arguments on LBO have involved the idea that some human characteristic is or isn't genetic. In these arguments there is often a confusion between phenotype and genotype.
Phenotype. The realized expression of the genotype; the physical appearance or functional expression of a trait; the result of the biological activity of proteins or RNA molecules transcribed from the DNA.
Genotype. The total set of genes present in the cells of an organism, as contrasted with the phenotype, which is the realized expression of these genes; often also used to refer to the genetic constitution underlying a single trait or set of traits.
The steps and pathways or `expression' of an inheritable phenotypic trait that arises from its underlying genetic constitution can be extraordinarily complex.
(In addition, remember that natural selection can only directly effect the phenotypic expression.
However, in bacteria which reproduce asexually, have only one copy of each gene, only one large circular molecule of DNA, much less complex metabolic pathways---for example in protein synthesis---the relationship between phenotype and genotype is very much closer.)
Here are some complications in the relationship between phenotypic expression and its genotypic basis.
1) Multiple alleles. Although a diploid individual may posses no more than two alleles at one time, it does not follow there are only two alleles possible. Almost all genes exhibit several different alleles. For example, AB0 human blood types have three common alleles.
2) Gene interaction. Few phenotypes are the result of only one gene. Most traits are the expression of many genes that act sequentially or jointly.
3.) Epistasis. Epistasis is an interaction between the products of two genes in which one modifies the phenotypic expression of the other. Epistatic interactions between genes make patterns of inheritance very difficult to interpret. Human hair color is an example.
4.) Continuous variation. When multiple genes act jointly to influence a trait, the contribution caused by segregation of the alleles on one particular gene is difficult to monitor. Because all of the genes that play a role in determining a phenotype are segregating independently, the result is a graduation in degree of difference between individuals, when many individuals are examined. Human height and weight are examples.
5.) Pleiotropy. Often individual alleles will have more than one effect on the phenotype. Such alleles are called pleiotropic. A pleiotropic gene alteration may be dominant with respect to one phenotypic consequence and recessive for another. Pleiotropic relationships occur because the characteristics of organisms result from the interactions of products made by genes. These products often perform functions both known and unknown.
6.) Incomplete dominance. Not all alternative alleles are fully dominant or recessive in heterozygotes. Sometimes heterozygous individuals do not resemble one parent precisely. Some pairs of alleles instead, produce a heterozygous phenotype that a.) is intermediate between the parents (intermediate or incomplete dominance), b.) resembles one allele closely but can not be distinguished from it (partial dominance), or c.) is one in which both parental phenotypes can be distinguished in the heterozygote (co-dominance).
7.) Environmental effects. The degree to which many alleles are expressed depends on the environment. For example, some alleles encode an enzyme whose activity is more sensitive to conditions such as heat or light than are other alleles.
More definitions.
Allele. One of two or more alterative states of a gene.
Diploid. Having two sets of chromosomes.
Haploid. Having only one set of chromosomes, characteristic of eukaryotic (possesses nucleus) gametes.
Homozygous. A diploid individual that carries the same alleles on homologous chromosomes at one or more genetic loci.
Heterozygote. A diploid individual that carries two different alleles on homologous chromosomes at one or more genetic loci
(Adapted from, Biology, 3rd Ed, Raven B, Johnson GB, Mosby-Year Book, St Louis, 1992. A standard Intro Biology text.)
CG