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Pisum sativum: smooth versus wrinkled seeds, yellow versus green seeds, grey versus white seed coat, tall versus short plants, etc. In the plants he used, he found no intermediate versions of these traits. In addition, these traits were independent, the presence of one trait did not influence any of the other traits he was considering. Each was controlled (as we now know) by variation at a single genetic locus (position or gene). The vast majority of traits, however, do not behave in this way. Most genes play a role in a number of different traits and a particular trait is generally controlled (and influenced) by many genes. Allelic versions of multiple genes interact in complex and non-additive ways. For example, the extent to which a trait is visible, even assuming the underlying genetic factor is present, can vary dramatically depending upon the rest of an organism’s genotype. Finally, in an attempt to established the general validity of his conclusions Mendel examined the behavior of a number of other plants, including hawkweed. Unfortunately, hawkweed uses a specialized, asexual reproductive strategy, known as apomixis, during which Mendel’s laws are not followed196. This did not help reassure Mendel or others that his genetic laws were universal or useful. Subsequent work, published in 1900, led to the recognition of the general validity of Mendel’s basic conclusions197.
Mendel deduced that there are stable hereditary "factors" - which became known as genes - and that these genes are present as discrete objects within an organism. Each gene can exist in a number of different forms, known as alleles. In many cases specific alleles (versions of a gene) are associated with specific forms of a trait or the presence or absence of a trait. For example, whether you are lactose tolerant or intolerant as an adult is influenced by which allele of the MCM6 gene you carry. The allele that promotes lactose tolerance acts to maintain the expression of the LCT gene; the LCT gene encodes the enzyme lactase, which must be expressed for an organism to digest lactose198. When a cell divides, its genes must be replicated so that each daughter cell receives a full set of genes (a genome). The exact set of alleles a cell inherits determines its genotype (note, words like genomes and genotypes are modern terms that reflect underlying Mendelian ideas). Later it was recognized that sets of genes are linked together in a physical way, but that this linkage is not permanent - that is, processes exist that can shuffle linked genes (or rather the alleles of genes).
In sexually reproducing (as opposed to asexual or clonal) organisms, like the peas that Mendel originally worked with, two copies of each gene are present in each somatic (body) cell. Such cells are said to be diploid. During sexual reproduction, specialized cells (known as germ cells) are produced; these cells contain only a single copy of each gene and are referred to as haploid (although monoploid might be a better term). Two such haploid cells (typically known as egg and sperm in animals and ovule and pollen in plants), derived from different parents, fuse to form a new diploid organism. In a population there are typically a number of different alleles for each particular gene, and many thousands of different genes. An important feature of sexual reproduction is that the new organism reflects a unique combination of alleles inherited from the two parents. This increases the genetic variation within the population, which enables the population (as opposed to specific individuals) to deal with a range of environmental factors, including pathogens, predators, prey, and competitors. It leaves unresolved, however, exactly how genetic information is replicated and how new alleles form, how information is encoded, regulated, and utilized at the molecular, cellular, and organismic levels.
- It is perhaps worth reading Evolution in Four Dimensions (reviewed here: www.ncbi.nlm.nih.gov/pmc/articles/PMC1265888/) which reflects on the factors that influence selection.
- Apomixis in hawkweed: Mendel's experimental nemesis: http://www.ncbi.nlm.nih.gov/pubmed/21335438
Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.