II.5 The way genes are transferred from generation to generation is described by Mendel’s genetic laws.
All the genes are inherited simultaneously in organisms that do not reproduce sexually. Consequently, the offspring exhibits exactly the same genotype, i.e. exactly the same combination of alleles in all loci as its parents. This statement is, of course, valid only if no mutation has occurred in the particular individual; however, because of the relative rarity of this phenomenon, the possibility of the occurrence of a new mutation will not be considered here. Some types of asexual reproduction can involve the process of recombination and segregation, so that the progeny can differ from one another. However, during asexual reproduction, the genetic variability present in the genome decreases constantly, as the alleles forming the genotype of the parent organism can decrease but not increase in number in subsequent generations. It does not always happen that one copy of each allele is passed on to the next generation; in half of cases, the progeny inherits two copies of a single allele. Thus, after several generations of asexual reproduction, the progeny will then be genetically uniform even in types of asexual reproduction with recombination and segregation.
The vast majority of eukaryotic organisms reproduce sexually. In the typical case, the germ of a new organism is formed by the combination of two haploid sex cells, each of which brings one complete chromosome set to the future individual, containing one allele from each gene. Each chromosome is thus located in the nucleus of the cell in two specimens, i.e. the genome of the cell is formed by pairs of homologous chromosomes. Homologous chromosomes contain the same loci; however, they can differ in the alleles located on these loci. In an adult diploid individual, i.e. an individual that has at least two sets of chromosomes in its cells, originally derived from two gametes, and thus two alleles from each gene, new haploid sex cells are then formed. In the creation of sex cells, reduction division entails not only repeated reduction in the number of chromosome sets to one, but also random mixing of the chromosomes originally derived from one gamete or the other (segregation) and also exchange of mutually corresponding DNA sections in the framework of pairs of homologous chromosomes (recombination). As a consequence of segregation and recombination, the formation of gametes is accompanied by the mixing of alleles derived from the two parents and, because of the large number of possible combinations, each gamete basically acquires a unique combination of alleles, a unique haplotype. Understandably, if two gametes with unique haplotypes meet in the germ during fertilization, the final genotype and thus the consequent phenotype will be a new organisms that is unique and unrepeatable.
Some laws follow from the manner in which genetic information is transferred during the life cycle. These laws were derived in the middle of the 19th century by the Brno abbot Johann Gregor Mendel (1822-1884). He came to these conclusions without any knowledge of the mechanisms of transfer of genetic information, solely on the basis of the results of his hybridization experiments. These rules were later designated as genetic laws. The first law is generally termed the law of segregation and can be formulated roughly as follows using our contemporary terminology: two alleles of any gene present in a parent individual segregate into independent gametes in each generation without undergoing a change and thus without affecting one another. This law could seem trivial to us in the light of contemporary knowledge of the mechanism of storage and transfer of genetic information. In actual fact, however, its discovery meant a fundamental break-through in the thinking of biologists. On the basis of empirical observation of the heredity of phenotype traits, up to that time biologists automatically assumed that heredity is “soft”, that the predisposition for the formation of the individual traits, today we would say genes, that an individual acquires from both parents, mutually interact and mostly are basically “averaged” and are passed on to further generations in this altered form. The law of segregation basically states that, without regard to the mutual interaction of phenotype manifestations of the individual genes, the genes themselves do not affect each other in any way and are transferred from one generation to the next in unaltered form.
Amongst other things, this discovery eliminated one of the basic problems of Darwin’s theory of evolution. A serious argument of some of the opponents of the theory of evolution was that an evolutionarily advantageous new trait cannot be selected through natural selection simply because, amongst sexually reproducing organisms, it is gradually “dispersed” after several generations as a consequence of crossing of individuals with the new trait with the far greater number of individuals bearing the original variant of the particular trait. Henry Charles Fleeming Jenkin (1833-1885) graphically described this problem. Imagine that a white man is shipwrecked on a tropical island. Because of his excellent psychological and physical qualities (in all probability he was an English gentleman) he rapidly excels in competition with the local black men and becomes the head of their tribe He would certainly win out in competition for women and would leave the greatest number of progeny. All his descendants will, however, be half black and thus only half as good as the original shipwrecked man. If the population on the island is sufficiently numerous, only a few generations after the presence of the shipwrecked white man there will be only a minimal genetic trace, probably in the form of occasionally emerging blue eyes amongst the otherwise dark inhabitants of the island.
The whole problem can be described mathematically in a somewhat more politically correct manner. It can be derived that, if the predispositions from the father and the mother were actually averaged, exactly half of all the genetically determined variability present would disappear in each generation. After a few generations, only variability determined by the environment would remain and natural selection would not be able to make any choices. It is an interesting paradox that the greatest problem associated with Darwin’s theory of evolution was resolved by Mendel who expected to overthrow Darwin's theory through his experiments. It is quite possible that he was basically very lucky that his work remained unnoticed during his lifetime, safely buried in the local Brno bulletin, and that its importance for the theory of evolution was not understood, e.g. by Darwin, who is said to have owned a copy of the work. The abbot would apparently have received recognition and fame from evolutionary biologists, but the reaction of his superiors would probably have been less enthusiastic. I am not very well informed about the organization of church life, but I would guess that abbots are named and recalled more frequently by church dignitaries than by evolutionary biologists.
The law of independent assortment of predispositions is the second law of genetics. According to this law, the individual pairs of alleles of various genes segregate into the gametes independently of one another and the manner of distribution of one pair of alleles in no way affects the distribution of another pair. The result is that the predispositions and the corresponding traits freely combine and the occurrence of the individual combinations of predispositions and traits is controlled only by the laws of combinatorics (Fig. II.12). If two genes are located on two different chromosomes, then, in accordance with the second law of genetics, the relevant alleles are freely combined. If one of the pairs of homologous chromosomes bears allele a1 in locus A and the second has allele a2 and one of the chromosomes of a different pair of homologous chromosomes has allele b1 in locus B and the second of this pair has allele b2, then 4 types of gametes bearing alleles a1 and b1, a1 and b2, a2 and b1, and a2 and b2 will be formed with the same probability. If two individuals with this genotype were to reproduce together, then 9 types of progeny would be created, with genotypes a1a1b1b1, a1a1b1b2, a1a1b2b2, a1a2b1b1, a1a2b1b2, a1a2b2b2, a2a2b1b1, a2a2b1b2 and a2a2b2b2 in a ratio of 1:2:1:2:4:2:1:2:1. It is apparent that this law is valid only for a pair of alleles from genes that are located on different chromosomes so that, during segregation of chromosomes, they segregate independently into different sex cells and also for genes that, while they are present on the same chromosome, are so far apart that crossing-over and thus genetic recombination will most probably occur in each meiosis in the section between them.