Genetics deals with the inheritance of characteristics of a living being by its descendants. Both the visible characteristics of a trait and the underlying genetic composition are considered.
Although genetics is a young but also an essential part of biology, after all it forms the basis for current topics such as research into hereditary diseases and genetic engineering.
Definition and basics of genetics
The biological subfield genetics describes the theory of hereditary processes, which deals with the storage, transmission and realization of genetic information.
Genetics encompasses many basic terms, processes, and concepts. You can find out more about the most important of these below.
Mendelian rules
Gregor Mendel’s (1822-1884) crossing experiments with pea plants and the resulting Mendel’s rules form the basis of genetics and the beginning of research on the inheritance of traits.
the Mendelian rules allow a description of the inheritance of traits whose expression is only influenced by one gene.
Mendel crossed pea plants and observed the inheritance of some specific traits. He statistically evaluated how the characteristics were inherited. Referring to this, Mendel established three rules of inheritance that are still valid today:
1. Mendel’s rule (uniformity rule)
become two for one trait homozygous individuals (P-generation) crossed together, the phenotypes and genotypes of the resulting offspring (F1 generation) are all related to the corresponding trait same (uniform) – therefore the name uniformity rule. This applies to both dominant-recessive and intermediate inheritance.
2. Mendel’s rule (splitting rule)
If you cross them uniform individuals of the F1 generation among themselves, the phenotypes and genotypes of the following F2 generation split in a certain ratio, from which the name splitting rule comes. The genotypes of the P generation reappear.
3. Mendel’s rule (independence rule)
the independence rule applies to dihybrid, dominant-recessive inheritance. In other words, those in which two independent traits (not on the same chromosome) are inherited, of which there is a dominant and a recessive variant.
If such individuals are crossed with one another, uniform offspring are again obtained in the F1 generation. If the offspring of the F1 generation are crossed with each other, the phenotypes of the F2 generation split in the ratio 9:3:3:1. This leads to the formation of new genotypes and phenotypes. Therefore, this rule will also recombination rule called.
On you will find extensive articles on the individual Mendelian rules – the uniformity rule, division rule and independence rule.
DNA
the DNA is the carrier of genetic information and thus the central element of genetics, more precisely of molecular genetics. All information about the structure of an organism is stored in its DNA.
To understand how DNA can contain this information, you need to understand the structure of DNA. In short, DNA consists of so-called nucleotides.
Each nucleotide consists of a phosphate residue, the sugar deoxyribose, and one of four organic bases. These bases are adenine, guanine, cytosine and thymineabbreviated A, G, C, and T. Many nucleotides form a chain, a so-called polynucleotide strand.
DNA consists of two oppositely parallel polynucleotide strands that are connected to one another by hydrogen bonds between the bases. These bonds can only arise between adenine and thymine and guanine and cytosine (complementary base pairing). Ultimately, the DNA is still twisted, you can imagine its shape like a twisted rope ladder. One speaks of the helically wound double helix.
Figure 1: Structure of DNA
You can find more information about the molecular basis of heredity in the article on DNA!
gene
A gene is a unit of genetic information that determines the expression of a trait.
More precisely, a gene is a specific section of DNA whose base sequence corresponds to the Structure of an enzyme protein coded. Enzyme proteins control various reactions in the organism, which in turn influence the expression of characteristics.
allele
A gene determines the expression of a trait. That allele on the other hand, describes how the characteristic is expressed. Alleles could therefore be described as «variants» of a gene.
In biology class you will mainly deal with creatures whose genes each have two alleles (e.g. humans).
In organisms with double sets of chromosomes, each gene is present in two alleles. The question now arises as to which of the two alleles ultimately determines the expression of the trait. Come here dominant and recessive alleles in the game. If an organism has a dominant and a recessive allele of a gene, i.e. is heterozygous, the characteristic is expressed after the dominant allele.
A dominant allele prevails over a recessive allele. This means that the characteristic of a recessive allele is only expressed if the organism has two recessive alleles of a gene, i.e. is homozygous.
If alleles occur in dominant and recessive form, one speaks of one dominant/recessive inheritance. However, there are also intermediate inheritance, where neither allele is dominant. There is a mixture of both forms here.
This is the case, for example, when alleles for red and white flowers meet in a flower and together produce pink flowers.
genotype
as genotype is the term used to describe the totality of genes that form the genetic information of an organism, or a combination of alleles that leads to a specific external appearance.
When examining living things for a specific trait, as is often the case in connection with Mendel’s rules, the combination of alleles of the trait being examined is called a genotype.
phenotype
as phenotype is the expression of a certain characteristic in the appearance of an individual.
The phenotype is the opposite of the genotype.
When examining plants for the flower color trait, the phenotype is the apparent color of the flowers. The genotype, on the other hand, is the combination of alleles that gives the plant its flower color.
You can learn even more about these interesting topics with the Genotype and Phenotype articles on !
DNA replication
Replication is a prerequisite for cell division and reproduction. Every cell of an organism contains all of its DNA. If the genetic information is to be passed on, it must first be copied.
The process of replication describes the duplication of DNA. The DNA is copied according to the principle of semi-conservative replication. This means that a DNA double strand becomes two new DNA double strands, each consisting of an «old» and a «new» single strand.
DNA replication occurs in three steps. First, the DNA double strand is unwound and split. The enzyme does this job helicase. An RNA primer is synthesized by the primase to provide a starting point for replication.
Then the enzyme synthesizes DNA polymerase a new, complementary single strand on each of the two single strands of DNA. This runs continuously in one of the two individual strands.
In the other, the DNA strand is discontinuously replenished. This is because DNA polymerase can only read DNA in one direction. In the case of the discontinuous strand, so-called Okazaki fragments synthesized later by the enzyme DNA ligase be linked.
Figure 2: Semiconservative DNA replication. Source: Wikipedia.org
cell cycle
During the interphase, the cell carries out its actual task and gets ready for the next cell division. The cell cycle ultimately describes these two phases, which a cell goes through alternately until it is no longer capable of division and is differentiated.
mitosis
Mitosis, or division of the cell nucleus, is fundamental to biology. Together with the interphase, it forms the cell cycle.
In the mitosis Two equally diploid daughter cells arise from a diploid cell. The daughter cells are identical to each other and to the mother cell in terms of their genetic and chromosomal makeup.
Figure 3: Mitosis process Source: wikipedia.org
meiosis
the meiosis is a process in which the diploid (double) set of chromosomes in primordial germ cells becomes a haploid (single) set of chromosomes.
This means that meiosis is an important prerequisite for fertilization, in which the cell nuclei of the egg cell and des sperm merge. Since the chromosome sets of both cells are combined when the egg cell and sperm fuse, a diploid chromosome set is created again.
If meiosis did not occur before fertilization, two diploid sets of chromosomes would combine and result in a quadruple set of chromosomes.
Meiosis consists of reduction division and equivalency division. Equation division takes place according to the principle of mitosis.
Mitosis and meiosis are very complex processes. To understand them better, you can read the related articles on .
protein biosynthesis
Protein biosynthesis describes how the genetic information is realized, i.e. how the proteins are synthesized using the information stored on the DNA. So it is the key to understanding how genes actually affect traits of an organism. Protein biosynthesis consists of two steps that you should know how to do: transcription and translation.
transcription
the transcription is the «transcribing» of the genetic information of the DNA into its transport form, the mRNA. This is necessary because no proteins can be synthesized directly on the DNA.
The information in the DNA is thus copied to the mRNA and can thus be transported out of the cell nucleus.
The transcription process is very similar to that of replication. The DNA double strand is also unwound and opened, but only one of the two single strands is read during translation, since only one RNA single strand is formed.
Furthermore, not the entire DNA is read during translation, but only one (eukaryotes) or a few genes (prokaryotes) are transcribed. In eukaryotes, transcription produces only the pre-mRNA. At the subsequent processing it becomes the mRNA.
translation
the translation is the «translation» of the base sequence of the mRNA into the amino acid sequence of a protein.
Translation takes place on the ribosomes in the cell plasma. The ribosomes form a site where the tRNA can “dock” to the mRNA. The tRNA transports («transfer RNA») amino acids to the mRNA and based on the base sequence of the mRNA, the amino acids are linked together in the correct order.
This creates a chain of amino acids, a so-called polypeptide chain….