Why are they all different from each other
Inheritance is the passing on of genetic information to the following generations of a species. Only genetically determined properties are passed on, learned behaviors are not included. The transmission of genetic information is normally accomplished through sexual reproduction (cf. meiosis, fertilization).
The genotype describes the entirety of all genes in an organism. Together with environmental influences, they determine the appearance of a living being. Each individual has its own genotype which is fundamentally different from all others. Exceptions are clones (e.g. offshoots, identical twins).
In unicellular organisms such as bacteria, in plants and in a few lower animals, clones are used for asexual reproduction. More highly developed animals have lost this ability due to the increasingly complex structure of the organism as well as due to cells that specialize in a few functions (differentiated). A clone can only be created here through genetic engineering (the creation of a 'new' organism based on a few cells in a living animal). This is then identical to the donor. While cloning in bacteria enables them to have a virtually unlimited lifespan and plants can thus "rejuvenate" themselves, clones from higher animals are only viable to a limited extent.
Identical twins are formed when the fertilized egg cell divides in such a way that two embryonic cells are formed. The resulting embryos then have the same genotype. In plants there is a similar phenomenon called polyembryony. Here several genetically non-identical embryos from several zygotes develop (polyzygotic polyembryony), of which on average the strongest embryo survives. This in turn often divides into two genetically identical embryos (monozygotic polyembryony), of which only the strongest prevails. This ’archaic’ form of embryonic development occurs especially in the evolutionarily older gymnosperms (including conifers). There is a modified form of polyembryony in some angiosperms, e.g. B with the citrus plants, where in addition to the normally fertilized embryo, other embryos from somatic cells (cells that are not germ cells) develop (nucellar embryos). These have the genome of the mother plant and are therefore genetically identical to it, while the zygotic embryo has a different genotype.
The phenotype describes all morphological and physiological characteristics of a living being. It influences the appearance and is defined by the genotype, in that the information contained in the genes is read and implemented (expressed). During this development of the organism the environment also has an influence. These influences also shape the phenotype and are referred to as modification. In contrast to the genotype, they are not inheritable, but a direct reaction to environmental stimuli.
Depending on the living being, the phenotype is more or less strongly influenced by its environment. Environmental conditions such as climate, soil and altitude change the growth of plants and also their productivity. This is why identical twins also differ slightly from one another on closer inspection, although they are genetically identical.
In some living beings, the influence of the environment on the phenotype is more pronounced, in some it is less pronounced, or the genotype determines the phenotype to a greater or lesser extent. This measure is known as phenotypic plasticity. A high phenotypic plasticity occurs, for example, in amphibians.
Alleles are almost identical genes that are located at the same position on a chromosome (gene location or locus) and differ only slightly from one another in the base sequence. They have the same function (e.g. they are responsible for the flower color), but the slightly different base sequence means that gene expression changes (e.g. flower color red or white). So it is a gene that is present in different forms (two or more) that express themselves in a different phenotype.
Since most living things are diploid, i.e. have a double set of chromosomes, it can happen that two identical alleles or two different alleles can be found on the same gene location. In the case of the same alleles one speaks of homozygosity, in case of different alleles of mixed inheritance (heterozygosity).
In the case of inheritance, an attempt is made to trace the genetic transmission of a trait. For this purpose, family trees are created in order to track the occurrence of the characteristic. The hereditary pathway is researched particularly in the case of hereditary diseases, but also in the case of breeds (plants, livestock). The following information is important for this: On the one hand, the position of the gene on the chromosome (locus). It is also crucial here whether the trait is determined by only one gene (monogenic) or whether several genes are involved (polygenic). In the case of polygenic inheritance, a distinction is also made as to whether all genes are equally involved in inheritance (additive polygeny) or mutually restrictive influence (complementary polygeny).
The second factor is the type of chromosome on which the gene is located: In autosomal inheritance, the gene is on a chromosome that does not belong to the sex chromosomes. In gonosomal inheritance, the gene in question is on an X or Y chromosome (called a gonosome). Inheritance is then X-linked or Y-linked. This is important because the inheritance of certain traits is linked to gender.
The third factor concerns the expression of the characteristic in the next generation. A distinction is made between dominant, recessive and intermediate.
Dominant recessive and intermediate inheritance
These allele combinations are expressed differently in the phenotype: Often in heterozygosity (mixed inheritance) one (dominant) allele dominates the other (recessive). Hence, the dominant allele is ultimately expressed in phenotype. This inheritance is called dominant-recessive. A gene can be present in two forms: as a dominant (conditionally e.g. red flower color) and as a recessive (conditionally e.g. white flower color). If a plant has two dominant alleles for flower color, the flowers are red. If there are two recessives, the flower color turns white. Once each allele is present, the flower color turns red because the dominant allele prevails.
Dominant means that this allele always prevails if it is present (homozygous or heterozygous). The recessive allele can only prevail if it is homozygous (pure hereditary).
Codominance occurs when both alleles are equally pronounced in heterozygous individuals. An example is the formation of blood groups: One allele results in blood group A, the second blood group B. If both are present at the same time (heterozygosity), blood group AB arises with the complete characteristics of both blood group types.
In the intermediate mode of inheritance, the characteristics are expressed differently. Here the flower color is mixed with heterozygosity (i.e. one allele for red color, one allele for white color), resulting in pink.
The inheritance of traits that are only determined by one gene was first formulated by Gregor Mendel (1822 - 1884). He conducted most of his research on the garden pea (Pisum sativum) and the Japanese miracle flower (Mirabilis jalapa) and formulated its rules based on the statistical results. Most of them are still valid today, even if a lot of new knowledge has been added and the general applicability of the rules has been restricted. For this reason, the sentences previously known as Mendel's laws are now ’only’ referred to as rules.
1. Uniformity rule (reciprocity rule)
If two parents (parental generation or P) who differ in only one trait, for which they are both homozygous (pure breeding), are crossed with each other, the individuals of the first daughter generation (1st branch generation or F1) are uniform ( equal) and heterozygous (mixed-breed) for this trait. There are therefore two options: either the entire daughter generation has the characteristics of one parent (in the case of dominant-recessive inheritance, for example, the flower color of the dominant gene, i.e. red) or all individuals of the daughter generation have a mixture of both characteristics of the parents (in the case of intermediate inheritance e.g. flower color pink). The rule of uniformity does not apply if traits are inherited that are based on sex chromosomes. The F1 generation is then not uniform.
2. Rule of division (segregation rule)
If two individuals of the F1 generation are crossed (or two individuals that are similarly mixed with respect to one trait), the offspring (F2 generation or 'grandchildren') are no longer uniform. Here come the different traits of the P generation reappear.
At the dominant-recessive inheritance then three quarters of the F2 generation would have the flower color of the dominant gene, red, and a quarter the flower color of the recessive gene, white. The ratio is 3: 1, with the white flowers and a quarter of the red flowers being homozygous, the other two quarters being mixed.
At the intermediate inheritance There are a quarter of red flowers, a quarter of white flowers (both pure-blooded) and two-quarters of pink flowers (mixed-blooded). So here the ratio is 1: 2: 1.
3. Independence rule (recombination rule)
The rule of independence refers to the independent inheritance of two characteristics (dihybrid inheritance). Since the features can be freely combined with one another, new feature combinations appear from the F2 generation onwards. Back then, Mendel crossed plants that differed in two characteristics: one had predominantly large, red flowers, the other recessively small, white flowers. In the first inheritance (i.e. in the F1 generation) all flowers were red and large. With the second inheritance (F2 generation), the result split in a ratio of 9: 3: 3: 1. It resulted in 9 red, large flowers, 3 small red flowers, three large white flowers and one small white flower. Of these, only one red large flower and the small white one are pure breeding in both characteristics. In the phenotype, the distribution of the rule of independence only applies to the (by far more frequent) dominant-recessive inheritance; in the genotype it applies to both.
This rule says that different characteristics are passed on separately from one another. To do this, however, they have to be far enough apart on the chromosome so that they can be inherited separately by crossing over or they have to be located on different chromosomes. If this is not the case, the rule of independence does not apply, as these genes are inherited together (in so-called coupling groups). For the validity of Mendel's 3rd rule, genes must therefore be decoupled.
Extra chromosomal inheritance
In the more highly developed organisms there are semi-autonomous (semiautonomous) organelles in the cytoplasm of the cells, such as the mitochondria and in plants the chloroplasts. They have their own genome, which they pass on independently, but not through new combinations, but through simple DNA replication. There are already organelles in the cytoplasm of the germ cells that are passed on to the F generation. There are significantly more in the large female germ cells than in the small male ones. Therefore, the genome of these organelles, which hardly changes, is usually passed on from the mother's side and is therefore not based on Mendel's rules. The genome of these organelles is used to create family trees.
See also: heterozygosity / homozygosity, F2 generation.
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