The Inheritance (rarely also: heredity , derived from Latin hereditas , heritage ' ., Cf. English heredity ) is the transfer of genetic material ( genes ) from a generation of living things to their offspring, similar in these characteristics and properties cause like the ancestors and bring forth. The material basis of the genetic make-up, the genetic material, is DNA .
While the family similarity that descendants show to their ancestors was unquestionably regarded as a direct effect of the parents in conception and reproduction, often in an unclear way as a property of the "blood", until the 19th century It became clear in the 20th century that inheritance is linked to discrete units of a special genetic material, the genes. It was only after the Second World War that it was proven that these are bound to a specific genetic material, the DNA (see Genetics # The genetic material ). The pioneers of genetics began research by comparing only living things and their properties with one another without knowing what a “gene” actually is. The most important tool of this research was the precise analysis of family trees . This research continues to this day, especially in the context of human genetics. The area of research is called formal genetics. The most important subject of formal genetics is the study of crossbreeds and inheritance .
In addition, two separate research programs developed within genetics. On the one hand, following the findings of formal genetics, the focus was on individual genes and their effects. This approach is particularly suitable for genes that each have a large effect so that their effects are easily identifiable and comparable. It is usually referred to as systematic genetics.
However, this method quickly reaches its limits when it comes to numerous traits because the relationships between traits and genes are often complex and difficult to identify. In most cases, a trait of interest is influenced by several, possibly hundreds, different genes, each to a small extent ( polygeny ), so that the influence of each individual gene is difficult to identify. In addition, each of these genes often has numerous, sometimes completely different, functions and effects ( pleiotropy ), which also interact with one another and with their environment in a way that is difficult to understand. Such traits, which are influenced by numerous genes, are researched in the context of quantitative genetics . An important concept in quantitative genetics is heritability or heredity.
Inheritance and heredity were originally legal terms that were only transferred to the area of the reproduction of organisms at the end of the 18th century . In the context of the preformation theory prevailing here , it was imagined at that time that all future offspring in the parental organism were already preformed and only had to develop. It was only on the basis of the detailed embryological studies by Christian Heinrich Pander (1817) and Karl Ernst von Baer (1828) that these ideas were overcome, and it was generally accepted that the organisms gradually develop from undifferentiated eggs or seeds ( epigenesis ). Now it was completely unclear what the similarity between parents and offspring is based on, i.e. what inheritance actually is in a biological sense .
Until the early 20th century, the prevailing view was that the entire parental organism influences the properties of the offspring and that this is mediated by a fluid (in humans, the blood). For example, offspring from mixed marriages or crossbreeds of different races were considered mixed breeds or hybrids and categorized accordingly. In addition, there was the idea that characteristics acquired in the course of a parental organism's life can be inherited (today referred to as Lamarckism ). These views were also represented by Charles Darwin with his pangenesis theory .
The Augustinian monk Gregor Mendel took a fundamentally different approach . In systematic cross-breeding experiments with plants, he examined individual characteristics, namely their sexual transmission and subsequent expression. His results, which he published in 1866, went almost unnoticed in science. Similarly revolutionary was the germplasm theory that August Weismann developed in the 1880s. Weismann rejected both the assumption that acquired properties were inherited and that the entire organism had an effect on inheritance. However, his postulates were initially very controversial.
Mendel's pioneering achievement did not become generally known in the professional world until 1900, when Hugo de Vries , Carl Correns and possibly also Erich Tschermak had independently reached results that confirmed the principles Mendel had obtained from peas. Another important step in the development of the concept of inheritance was the formulation of the chromosome theory of inheritance by Theodor Boveri in 1904.
The inheritance of characteristics of the external appearance of living things, including characteristics of behavior and metabolism, is essentially based on a long-chain macromolecule , deoxyribonucleic acid (DNA). Hereditary information is encoded in DNA by its nucleotide sequence . This means that the information content corresponds to a sequence of characters, a kind of alphabet made up of four letters, the so-called nucleotides (often simply referred to as bases), adenine A, guanine G, thymine T, and cytosine C, which lie one behind the other on the linear DNA strand. ( Viruses that do not count as living beings because they do not have their own metabolism are also inherited. Their genetic information consists of either DNA or RNA .)
The production of offspring is based on the cells and their division; most of the time (as in humans) each descendant goes back to a single cell. In normal cell division, the DNA of the mother cell is first doubled and then, half in each case, distributed between the two daughter cells. The genetic information itself remains unchanged, unless deviations occur due to rare errors in replication, known as mutations . However, the DNA is often recombined by special mechanisms during the inheritance process, so that if the basic sequence is unchanged, modified characteristics can occur (see below). In addition to sections that contain genetic information, DNA also consists of sections of which no information content is known; often it is a question of constantly repeated short sections with always the same sequence ( repetitive DNA ). Parts of the DNA without genetic information are only short in prokaryotes (e.g. bacteria ), but very long in eukaryotes , usually even longer than the information-containing sections. The remaining, information-carrying DNA sequences are the genes (including the sequence segments that serve to regulate them ).
Research based on the so-called ENCODE project on the human genome, for example , has shown that gene regulation is far more complex than previously assumed. For example, genes on the DNA strand can partially overlap and result in different proteins through alternative splicing . Some proteins, whose transcription units are far apart in the DNA strand, are put together afterwards. Other DNA segments encode RNA sequences that help regulate distant genes through RNA interference .
The entirety of the genetic information contained in the DNA of an organism is called the genome . In eukaryotes - and thus in all higher organisms - most of the DNA is organized in chromosomes , which are located in the cell nucleus . In addition, the mitochondria and plastids contain their own genetic information. In these organelles as well as in the prokaryotes (e.g. bacteria), the DNA is mostly present as a ring-shaped molecule.
Depending on the point of view, a “gene” corresponds to different facts, which are, however, logically related.
- A separate (discrete), individual “hereditary disposition” for a certain trait (or a combination of traits). (Level of consideration of formal genetics.)
- A specific, information-carrying section of the genetic material DNA. In this section, a fixed sequence of nucleotides (bases; e.g. AATCAGGTCA ...) encodes the genetic information. Each gene is characterized by a specific nucleotide sequence.
- The genetic code requires a group of three bases (a base triplet ) for a specific amino acid in a protein . That is why the information-carrying DNA units are organized as an open reading frame . Each gene corresponds to a transcription unit that leads to a protein.
- However, the protein-coding DNA sequence is only part of the actual genetic unit. Long sections, which can be much longer than the coding sequence itself, determine when this gene is read (transcribed). So-called cis elements are an important example . They lie on the coding strand and switch its transcription on / off when a certain cell signal arrives. According to this definition, regulatory DNA sequences belong to the gene. They are important for the inheritance process because they mutate independently of the coding sequence and can thus change characteristics.
From genotype to phenotype
The phenotype of a living being is largely determined by the activity of enzymes, which in turn is determined by the information contained on the DNA - this is called the genotype . The interaction of enzymes and regulatory proteins with the environment during the development of the individual gives rise to the phenotype. The connection between the genotype, the environment and the resulting phenotype is the reaction norm. In the form of the regulatory mechanisms of genetic expression, the reaction norm represents the conversion function R between environment U and phenotype P: P = R (U).
Among the numerous genes of a higher organism ( eukaryotes ) there are only a few individual ones that cause an equally single characteristic in the phenotype. Only such relatively rare cases mendel : they immediately show the so-called Mendelian rules . This circumstance illuminates the genius of its discoverer, who had looked for the sole cause of the pea genome.
Genomes do not have to be passed on unchanged through all generations. Errors can occur in the duplication of genomes and in the distribution of DNA during cell division. The resulting changes in the genome can affect the phenotype. Such changes are called mutations and the individuals who deviate from the previous generation as a result are called mutants. Mutations are one of the prerequisites for the evolution of living things.
Transmission of genetic material
Transmission with asexual reproduction
In unicellular organisms , which usually multiply by division , the DNA is distributed to the daughter cells in the form of identical copies. To do this, it must be available in at least two identical copies. Cell division is therefore preceded by a duplication of DNA. In the case of eukaryotic unicellular organisms, the number of chromosomes remains constant, and each chromosome then consists of two identical “ chromatids ” placed next to one another . These sister chromatids are allocated to two daughter cell nuclei in a strictly regulated manner through the process of mitosis , and both daughter cells each receive one of the genetically identical nuclei.
In a corresponding manner, all cells are equipped with identical genetic material when multicellular organisms grow. During reproduction by splitting off a cell or a multicellular stage of development ( asexual reproduction ), all offspring are therefore genetically identical.
Transmission during sexual reproduction
With sexual (sexual) reproduction , parts of the genomes of two individuals (parents) are recombined ( recombination ). Each offspring receives half of its genome from one of the parents and therefore has (at least) two homologous sets of chromosomes. This doubling of the number of chromosomes is balanced in the course of the life cycle by a corresponding halving in the case of a reduction division ( meiosis ); both processes together are called a core phase change . In the simplest and most common case, it is a change between a haploid phase with one set of chromosomes and a diploid phase with two homologous (but usually not genetically identical) sets. However, there can be more than two sets (particularly in the case of cultivated plants) ( polyploidy ).
In humans and generally in vertebrates , only the sex cells ( gametes ) are haploid, and they combine to form a diploid zygote , from which the diploid offspring also emerges. In other organisms, such as mosses , ferns or coelenterates , diploid and haploid generations alternate ( generation change ), and still others, e.g. B. many primitive algae , are normally haploid and only form diploid zygotes, from which haploid offspring emerge again after meiosis.
In all of these cases, homologous chromosomes are randomly distributed to the daughter cells during meiosis, and parts of homologous chromosomes are usually exchanged ( crossing-over ), which means that genes on homologous chromosomes can also be recombined.
Extra chromosomal inheritance
The extrachromosomal or cytoplasmic inheritance is based on the fact that some cell organelles , the mitochondria and plastids , have their own small genome, which is inherited independently of the chromosomes. These organelles are called semiautonomous because some of the genes required for their formation and function are not located in the cell nucleus, but in the organelles themselves. The endosymbiotic theory gives a generally accepted explanation of this special case .
Since the female germ cells always have significantly more cytoplasm than the male germ cells (the female sex and the male sex are defined by the size difference of the germ cells), the cell organelles integrated in the cytoplasm, and thus also their genetic material, are wholly or at least predominantly via the maternal (maternal) line passed on. Thus the extra-chromosomal inheritance obeyed not the Mendelian rules .
The extrachromosomal inheritance is relevant for some rare hereditary diseases (see also inheritance of mitochondriopathy ).
Examples of inheritance
Dominant recessive inheritance
In the dominant - recessive form of inheritance, the dominant allele prevails over the recessive allele. The fur color of the house mice is z. B. dominant-recessive inheritance, whereby the allele for gray fur is dominant and the allele for white fur is recessive. If a young mouse receives the genetic information for white fur from one parent and the genetic information for gray fur from the other, it will have a gray fur. The genetic information for the recessive allele (here "white coat color") can, however, be passed on to the next generation.
In a diploid organism, the splits described in Mendel's rules can be observed. In the case of dominant-recessive inheritance, the offspring often completely resemble one parent, since only the dominant gene prevails - the characteristics of the recessive gene are indeed present in the genetic make-up (carrier), but are not expressed in this generation.
Hereditary diseases are usually inherited recessively, including albinism , cystic fibrosis and sickle cell anemia . The few dominantly inherited diseases include night blindness , cystic kidney disease (ADPKD), short fingers , skeletal deformations ( cleft hand , cleft foot , polydactyly , syndactyly ), the nervous disease Huntington's disease and Marfan's syndrome .
In the case of intermediate inheritance, a hybrid of the two genes is formed. For example, the flower color of the Japanese miracle flower ( Mirabilis jalapa ) is inherited as an intermediary: If a specimen has a system for red and one for white petals, it will develop pink petals.
Intermediate inheritance is the rarer variant of inheritance.
To a large extent, inheritance does not follow Mendel's rules. A very common deviation is gene coupling , in which different genes are not inherited independently of one another, but are coupled with one another because they are on the same chromosome. Each chromosome in the haploid chromosome set thus forms a coupling group. However, the coupling is not absolute either, but is partially canceled by the crossing-over during meiosis . Therefore, the closer they are to each other on the chromosome, the more strongly genes are inherited, while genes located far away from each other are inherited independently, because there is certainly at least one crossing-over between them.
Another exception is cytoplasmic inheritance, which is based on genes that are not in the chromosomes, but in the mitochondria or plastids . Since these organelles are only passed on in the female gender, inheritance takes place here only in the female line (maternal).
Various other deviations from Mendelian inheritance are summarized as Meiotic Drive . The issue here is that certain genes or chromosomes get into the gametes more often than their homologues ( non-random segregation in meiosis) or are preferentially passed on to the offspring in some other way.
Epigenetic inheritance, which is treated in the next section, does not follow Mendel's rules either.
In addition to the inheritance based on the transmission of genes, there are also various forms of inheritance of properties independent of the base sequence in the DNA. They are called epigenetic and are the subject of epigenetics . While epigenetics primarily examines processes in the differentiation of cells and tissues within an organism, epigenetic inheritance in the narrower sense is the transmission of epigenetic modifications over several generations.
The most common epigenetic modification is the methylation of certain bases of the DNA, whereby the base sequence remains unchanged, but the gene expression is changed. The histones , proteins associated with DNA, can also be chemically modified, which can also affect gene expression. Thirdly, there are different variants of gene silencing in which short pieces of RNA mediate the recognition of homologous DNA or RNA sequences and transcription or translation is specifically inhibited. All of these epigenetic effects can last for generations.
Another possibility are prion- like proteins that occur in different folds. When these folds are stable and the presence of one shape triggers the refolding of the other, information can be inherited. This inheritance has been proven, for example, in fungi such as yeast.
Epigenetic inheritance is particularly common in plants and also in the nematode Caenorhabditis elegans and the fly Drosophila melanogaster , while it occurs only rarely in mammals (including humans). The latter is related to the fact that in mammals epigenetic programming is reset after fertilization and again in the germline , whereby the cells in question become totipotent , i.e. H. can differentiate into all more specific cell types. Plants, on the other hand, do not have a separated germline and can reproduce vegetatively or be reproduced artificially by cuttings.
Inheritance outside of biology
The ability to inherit and evolve is not limited to systems of biological origin. Synthetic polymers with information-storing properties are also capable of this.
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