II.3 The gene is the basic unit of genetic information.
The phenotype of an individual can be understood to consist in the set of all the properties that the particular individual exhibits. Some of these properties are genetically determined, i.e. their occurrence is determined by the presence of specific genetic information in the genome of the particular individual. Other properties, such as behaviour in an environment with a temperature of 3000 oC, are not genetically determined but follow from the properties of the substances from which the bodies of organisms are composed. A great many properties are simultaneously determined genetically and by the environment. In this case, the occurrence of certain properties is dependent, e.g., on the character of the external environment in which the particular individual occurs, or on the internal environment of the particular organism, which parameter is, once again, frequently determined by the overall genotype of the given individual.
The gene is the natural unit of genetic information. A gene is traditionally understood to be genetic information that affects a discernable property of the individual, i.e. the occurrence of a certain trait or its particular form. Part II.3.1 will present a different conception of the gene. A trait can consist in the presence or absence of certain morphological structures, just as it can correspond to the presence or absence of a certain pattern of behaviour. If two organisms differ in a particular gene, i.e. if they have a different variant of a particular gene, i.e. a different allele, then they can differ in the relevant trait. In some cases, the carriers of different alleles differ under all circumstances; in some cases the differences are manifested only in a certain specific environment, either external or internal – under conditions with the presence of quite specific variants of the other genes in the genome of the individual. It is necessary to be aware that the relationship between the gene and the trait that it determines is actually quite the opposite than would follow from the above definition. The existence of a genetically determined phenotype difference between the individuals of the particular species is the primary feature. Only on the basis of identification of this difference is it possible to identify the specific trait and to postulate the existence of the particular gene. The relevant methods, whether genetic methods (search for the gene for the known phenotype manifestation through crossing) or the reverse genetic method (looking for the phenotype manifestation of a known gene through targetted introduction of the DNA section into the genome of the individual or targetted removal or damaging of this section) can then be employed to locate the particular gene in the overall genome of the studied organism, i.e. to determine the locus at which this gene is located.
Although the gene is delimited through its functional manifestations, this does not mean that it evolved in evolution precisely because of selective pressure on this function. A gene is apparently very frequently defined and named according to a phenotype manifestation of a mutation that the particular gene inactivates. This type of phenotype manifestation need not have any connection with the actual function of this gene and can be a quite random side product of its damage. The familiar joke about how pulling all the legs off a flea leads to its becoming deaf (because it then fails to react to the instruction “jump, flea!”) could be retold as an excerpt from a concluding grant report. “We have fulfilled the main target of the project, we have discovered the gene controlling the function of hearing in fleas. Using the technique of gene targetting, we have unambiguously demonstrated that the gene Hear 1 is responsible for the ability to hear acoustic signals in fleas. Fleas with both copies of this gene inactivated ceased to react to acoustic stimuli. Footnote in small print: An additional result of the project was the discovery that limbs are not formed in these fleas. However, research on the ontogenesis of limbs was not part of the original grant plan and thus this potentially interesting phenomenon was not studied further.”
The combination of the specific alleles in all the loci present in the genome of the particular individual is called the genotype. If we are interested in alleles present at a specific locus or in several particular loci in the given individual, in this connection we can also use the term genotype for the particular combination of alleles. If we are interested in a combination of alleles located in a single continuous section of one DNA molecule in the given individual, for example in a certain section of a pair of homologous chromosomes, then this is termed a haplotype. The term haplotype can also be employed in the sense of the combination of all the alleles located in the genome of a haploid cell, for example in the genome of a sex cell (gamete). The genotype of a haploid individual thus contains two haplotypes of each DNA section; new haplotypes can be mixed from these two haplotypes through recombination during meiois The sum of the alleles in all loci for all individuals of a certain population is called the gene pool of the population and, for all the individuals of a certain species, this is called the gene pool of the species.