StemCell Therapy

The term molecular genetics is now redundant because contemporary genetics is thoroughly molecular. Genetics is not made up of two sciences, one molecular and one non-molecular. Nevertheless, practicing biologists still use the term. When they do, they are typically referring to a set of laboratory techniques aimed at identifying and/or manipulating DNA segments involved in the synthesis of important biological molecules. Scientists often talk and write about the application of these techniques across a broad swath of biomedical sciences. For them, molecular genetics is an investigative approach that involves the application of laboratory methods and research strategies. This approach presupposes basic knowledge about the expression and regulation of genes at the molecular level.

Philosophical interest in molecular genetics, however, has centered, not on investigative approaches or laboratory methods, but on theory. Early philosophical research concerned the basic theory about the make-up, expression, and regulation of genes. Most attention centered on the issue of theoretical reductionism. The motivating question concerned whether classical genetics, the science of T. H. Morgan and his collaborators, was being reduced to molecular genetics. With the rise of developmental genetics and developmental biology, philosophical attention has subsequently shifted towards critiquing a fundamental theory associated with contemporary genetics. The fundamental theory concerns not just the make-up, expression, and regulation of genes, but also the overall role of genes within the organism. According to the fundamental theory, genes and DNA direct all life processes by providing the information that specifies the development and functioning of organisms.

This article begins by providing a quick review of the basic theory associated with molecular genetics. Since this theory incorporates ideas from the Morgan school of classical genetics, it is useful to sketch its development from Morgan's genetics. After reviewing the basic theory, I examine four questions driving philosophical investigations of molecular genetics. The first question asks whether classical genetics has been or will be reduced to molecular genetics. The second question concerns the gene concept and whether it has outlived its usefulness. The third question regards the tenability of the fundamental theory. The fourth question, which hasn't yet attracted much philosophical attention, asks why so much biological research is centered on genes and DNA.

The basic theory associated with classical genetics provided explanations of the transmission of traits from parents to offspring. Morgan and his collaborators drew upon a conceptual division between the genetic makeup of an organism, termed its genotype, and its observed manifestation called its phenotype (see the entry on the genotype/phenotype distinction). The relation between the two was treated as causal: genotype in conjunction with environment produces phenotype. The theory explained the transmission of phenotypic differences from parents to offspring by following the transmission of gene differences from generation to generation and attributing the presence of alternative traits to the presence of alternative forms of genes.

I will illustrate the classical mode of explanatory reasoning with a simple historical example involving the fruit fly Drosophila melanogastor. It is worth emphasizing that the mode of reasoning illustrated by this historical example is still an important mode of reasoning in genetics today, including what is sometimes called molecular genetics.

Genes of Drosophila come in pairs, located in corresponding positions on the four pairs of chromosomes contained within each cell of the fly. The eye-color mutant known as purple is associated with a gene located on chromosome II. Two copies of this gene, existing either in mutated or normal wild-type form, are located at the same locus (corresponding position) in the two second-chromosomes. Alternative forms of a gene occurring at a locus are called alleles. The transmission of genes from parent to offspring is carried out in a special process of cellular division called meiosis, which produces gamete cells containing one chromosome from each paired set. The half set of chromosomes from an egg and the half set from a sperm combine during fertilization, which gives each offspring a copy of one gene from each gene pair of its female parent and a copy of one gene from each gene pair of its male parent.

Explanations of the transmission of traits relate the presence of alternative genes (genotype) to the presence of alternative observable traits (phenotype). Sometimes this is done in terms of dominant/recessive relations. Purple eye-color, for example, is recessive to the wild-type character (red eye-color). This means that flies with two copies of the purple allele (the mutant form of the gene, which is designated pr) have purple eyes, but heterozygotes, flies with one copy of the purple allele and one copy of the wild-type allele (designated +), have normal wild-type eyes (as do flies with two copies of the wild-type allele). See Table 1.

To see how the classical theory explains trait transmission, consider a cross of red-eyed females with purple-eyed males that was carried out by Morgan's collaborators. The offspring all had red eyes. So the trait of red eyes was passed from the females to all their offspring even though the offspring's male parents had purple eyes. The classical explanation of this inheritance pattern proceeds, as do all classical explanations of inheritance patterns, in two stages.

The first stage accounts for the transmission of genes and goes as follows (Figure 1): each offspring received one copy of chromosome II from each parent. The maternally derived chromosomes must have contained the wild-type allele (since both second-chromosomes of every female parent used in the experiment contained the wild-type allele -- this was known on the basis of previous experiments). The paternally derived chromosomes must have contained the purple allele (since both second-chromosomes of every male parent contained the purple allele -- this was inferred from the knowledge that purple is recessive to red eye-color). Hence, all offspring were heterozygous (pr / +). Having explained the genetic makeup of the progeny by tracing the transmission of genes from parents to offspring, we can proceed to the second stage of the explanation: drawing an inference about phenotypic appearances. Since all offspring were heterozygous (pr / +), and since purple is recessive to wild-type, all offspring had red eye-color (the wild-type character). See Figure 1.

Notice that the reasoning here does not depend on identifying the material make-up, mode of action, or general function of the underlying gene. It depends only on the ideas that copies of the gene are distributed from generation to generation and that the difference in the gene (i.e., the difference between pr and +), whatever this difference is, causes the phenotypic difference. The idea that the gene is the difference maker needs to be qualified: differences in the gene cause phenotypic differences in particular genetic and environmental contexts. This idea is so crucial and so often overlooked that it merits articulation as a principle (Waters 1994):

Here is the original post:
Molecular Genetics (Stanford Encyclopedia of Philosophy)

This entry was posted on May 22, 2015 at 4:48 pm and is filed under Molecular Genetics. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed. |