With the rediscovery of MENDEL's laws began a phase of intense research in countries like Germany, Denmark, Great Britain, France, Sweden or the USA. Its aim was to outline their validity and the interpretation of possible deviations. In his autobiography from 1978 ['Vier Jahreszeiten miterlebter Genetik', ('Four Seasons of Experienced Genetics')], H. KAPPERT lists four complexes of questions that on one hand had to bear the close examination of many researchers at the beginning of the 20th century and on the other hand presented the starting point of new experiments.
clusions drawn from them and did they allow such a far-reaching generalization, that they could be spoken of as the laws of inheritance ? And is especially the assumption of the invariability of the factors of heredity in the hybrids that is fundamental for the explanation of the segregation phenomenons, justifiable?
Does the segregation of maternal and paternal characteristics indeed take place with constant ratios, which presupposes the formation of maternal and paternal hereditary factors in the ratio 1:1 and their chance combination?
Does the behaviour of plant hybrids of different plant varieties of secondary taxonomical levels generally allow to draw conclusions concerning the behaviour of higher taxonomical levels like species or genera? Is especially a transfer of the conditions found in plants to animals or humans possible?
Are the rules that seem to apply for the features of colour and shape also valid for important characters that influence the life of an organism or may even govern it?
In 1909, the British researcher W. BATESON did accomplish a list (that was incomplete even then) of more than 100 analyzed examples of the kingdoms of plants and animals, that proved that the inheritance of the most different characters follows MENDEL's laws and that they have therefore to be regarded as universal. But the closer evaluation of the details showed a number of seeming and real divergences, for which it was not easy to find an explanation. At the same time, the list enabled further research into genetic deviations. The influence a gene exerts on the expression of a character, that a feature can be influenced by several independent genes, that one gene may exert an influence on several features at the same time and that single genes act at different times in the development of an organism was begun to be understood.
BATESON and the Dane W. JOHANNSEN coined a number of terms thus generating the basis of modern genetic terminology. The following terms are essential for the understanding of the principles of inheritance. We will therefore give some short definitions before advancing further.
A gene is a hereditary factor and the fundamental unit of inheritance. On the molecular basis, a gene is a piece of DNA that controls a discrete hereditary character. It corresponds usually to a single protein or RNA. The term gene includes the entire functional unit: coding DNA sequences, noncoding regulatory DNA sequences and introns. It is characterized by the existence of different alleles.
An allele is the state of a gene. A haploid organism contains just one copy of a gene (= one allele) per gene location, a diploid organism contains two. If they are identical, then the respective organism is said to be of a homozygous condition, if they are different of a heterozygous condition.
A genotype is the specific combination of the alleles of a cell. The term means either the whole genome or (the sense it usually has) certain genes.
The genome is the totality of all genes of an individual.
The gene pool is the totality of all alleles of a population.
A gene mutation is a change in a hereditary factor that is based on a structural change of the respective gene.
The phenotype is the appearance of an individual that is based on an underlying genotype and on the influence that the environment exerts.
A dominant allele is an allele that determines the phenotype in a heterozygous condition.
A recessive allele displays no influence on the phenotype in heterozygous individuals.
|
|