Inheritance, mutation and selection


In sexual reproduction, one of these gametes is combined with the gametes of a sexual partner. Typically, one of the gametes is larger than the other and contains, in addition to the nucleus with the DNA, a larger amount of cytoplasm containing other cell components, e.g. mitochondria, the “power stations“ of the cells, which is responsible for cell respiration and energy supply. The larger of the two gametes is often referred to as the egg cell. The organism from which this cell and thus the mitochondrial DNA originates as the “mother organism“. The organism from which the smaller gametes originate is often classified as a “father organism“. The recombination of genes often leads to unpredictable results and a variety of expressions in traits that are determined by several genes. In humans, for example, several genes determine the hair color, so that all possible shades from all black to white blonde appear. The inheritance of traits which is determined by a single gene and not significantly dependent on external influences is more transparent.

Two inheritances

The monk Gregor(ius) Mendel (1822–1884) investigated the rules of inheritance for such genes in his famous experiments on peas and other plants. Two different hereditary traits are to be distinguished: the dominant-recessive inheritance, in which the gene variant of one parent dominates the trait and the intermediate inheritance, in which the gene variant of both parents influence the trait and the feature characteristic lies in the middle between the characteristics of both parents. The variants in which the genes can occur are called alleles. They often differ only in small details in the base sequence. However, the resulting proteins can clearly distinguish in their effect and thus lead to different feature characteristics.

Intermediate inheritance

The intermediate inheritance can be observed on the wonder flower. The flowers of the plant can be white or red. If one crosses plants with these flower colors, then all offspring develop pink-colored blooms and are among themselves alike (uniformity rule). Obviously, the plants each carry a red and a white allele of the gene that determines the color. If one crosses these offspring now among themselves, then there are four possibilities of the new combination: A plant of the third generation receives either from the mother plant and the father plant the allele for white flowers (→ white flowers) or from the mother plant the allele for white flowers and from the father plant the allele for red flowers (→ pink flowers). It is also possible that the allele for red flowers comes from the mother plant and the allele for white flowers from the father plant (→ pink flowers) or the allele for red flowers comes from the mother plant and the father plant (→ red flowers). The flower color of the offspring therefore divides in a relation of 1:2:1 (1 white, 2 pink, 1 red) (splitting rule).

Dominant-recessive inheritance

Mendel explored the dominant recessive inheritance, among other things, in the flower colors of the pea. If one crosses these plants with red and white flowers, all offspring form red flowers. Although they are identical in terms of flower color (uniformity rule), the characteristic corresponds to that of one parent, whereas the allele of the other parent, as we know today, is inherited, but has no influence on the feature characteristic. If one crosses the offspring now further, there are also four possibilities to combine the alleles for red and white, but with other effects on the feature characteristic. A third generation plant receives either from the mother plant and the father plant the allele for white flowers (→white flowers) or from the mother plant the allele for white flowers and from the father plant the allele for red flowers (→red flowers). It is also possible that the allele for red flowers comes from the mother plant and the allele for white flowers from the father plant (→ red flowers) or the allele for red flowers comes from the mother plant and the father plant (→ red flowers). The flower color of the offspring therefore divides in a relation of 1:3 (1 white, 3 red) (splitting rule).

Dominant alleles are indicated with capital letters (e.g. R for red), recessive alleles and alleles in intermediate inheritances with small letters (e.g. w for white).

There is probably no clear intermediate inheritance in humans. Although blood groups A and B are inherited intermediately, there is a third blood group variant, blood group 0, to which the rules of dominant-recessive inheritance apply. Therefore, the inheritance of blood groups in humans is somewhat complicated.

In order to get an impression of the effects of heredity in class, especially with younger students, two characteristics are suitable: The inheritance of earlobes that are grown-together and those of the ability to roll your tongue.

While introducing the thematic aspects in the classroom, it is too complicated for the students to use the term “allele”. Instead, one can speak of dominant and non-dominant genes.

Inheritance of grown-together earlobes

Earlobes can end with a free-hanging lobe. However, there are also people, which have not a lobe that does hang freely, but is attached to the head. If the lobes are free hanging, the dominant allele (L) is present. Grown-together earlobes are determined by the recessive allele (l). The rules of inheritance correspond to those of the pea plant. Therefore, when counting in class, there should be about twice as many children with free-hanging earlobes as with earlobes that are grown-together. If children have grown-together earlobes, their (bodily) parents usually have this kind of earlobes, too (unless the rare case of a mutation has occurred, see below). If one parent has the two alleles LL, all his children will have free-hanging earlobes, even if the other parent has ll or Ll as allele combination. If both parents have the allele combination ll, they also pass it on to their children. If both parents have the combination Ll (which cannot be seen from the outside, the earlobes are free hanging), then their children have a probability of 25% to get earlobes that are grown-together. A comparison with pictures of the parents should clarify the general heredity.

Inheritance of “tongue rolling“

Some people can stick out their tongue and roll it up from the sides to form a kind of tube. Although several genes are involved in the development of tongue rolling, but that’s not enough – you also need a lot of practice to learn tongue rolling. The inheritance of tongue rolling is so high that the example can be used well as an observation task for elementary school children. Of the people in whom the genetic conditions for tongue rolling are present, about half of them learn to roll their tongue until the age of 7 years, another 22% until they are 12 years old. So some of these people don‘t learn it, despite their genetics. For people who do not have the appropriate genetic prerequisites, practice is of no use. However, one does not know if the students have the necessary genes. So it doesn’t matter if you let them practice for a while.


Mutations are changes in the genetic material that occur during cell division. They arise in the (prevenient) process of DNA duplication. If these mutations occur in body cells, this is partly without consequences, but can also have clearly negative effects and e. g. cause cancer. The mutation can only be passed on to the next generation if the change takes place during the production of gametes (or their precursor cells).

Gene mutations: Single or a few base pairs are replaced, removed or inserted by a reading error in the duplication of the DNA strand. The exchange of a single base pair can have no effect. During translation, a different amino acid is incorporated into the resulting protein at the respective place, but if the new amino acid is similar to the original amino acid, the protein changes only slightly. If, however, an amino acid is incorporated that has significantly different properties than the original one, the consequences are considerable. A single amino acid can prevent the protein from unfolding in the necessary way so that it can no longer fulfil his function. The insertion or removal of base pairs, if their change is relevant for the protein formation, usually also has a significant effect. Because the entire base sequence shifts behind the place of the mutation and completely different amino acids are installed. If in the last two cases the protein is important for life and the deficit cannot be compensated by an unchanged gene copy of the other parent), then the cell dies or the fertilized egg cannot develop. Certain diseases are also triggered in this way. For example, the mucoviscidosis, in which the lungs produce viscous phlegm (that damages the lungs), is due to the loss of some base pairs on chromosome 7.

Chromosomal mutations: They occur when chromosome pieces are incorrectly exchanged during cross-over (see above). Then it can happen, that on one of the resulting chromosome strands some pieces are missing, but can be found twice on the other. Missing genes often impair the functionality of the cell. A doubling can have positive or negative effects.

Genome mutations: In this process, entire chromosomes are distributed unequally among the daughter cells, so that they are for example completely absent or exist as triple or even more frequent, instead of double (the normal case). Surprisingly, this can have a positive effect on plants. For example, the larger garden strawberries have the same chromosomes as wild strawberries, but you can found a duplicated genetic set (four instead of two versions) there, which makes the strawberries larger and stronger. In humans, a gene mutation leads for example to the Down‘s syndrome, in which the chromosome 21 is present three times. Thus, this mutation has the name “trisomy 21”


However, mutations are not necessarily detrimental and lead to illness or death. Sometimes mutations can even be useful. A mutation is not targeted; its result is caused by chance. Moreover, sometimes this happens receiving a positive result. An example for such a “positive” result is the lactase gene. Lactase is a protein that acts as an enzyme to break down lactose and digest it. All healthy humans (and mammals) have this gene because as infants they depend on drinking breast milk and digesting lactose. However, this gene was originally switched off, as the organism grew older. That was “resource-saving”. After all, it did not make sense to put energy into the production of an enzyme that is no longer needed.

However, when people started keeping cows etc. several thousand years ago and consequently began drinking milk in adolescence and adulthood, too, those who had a defective switch-off mechanism due to a mutation had a survival advantage. They were able to drink milk and survive food shortages better than people who did not tolerate milk. The advantage as a cattle breeder to digest milk was so great that the mutation has spread and today 80-95% of adults in Central Europe can digest lactose. Regardless of the mutation in Europe, mutations have also occurred in people of different ethnic groups East Africa that prevent the gene from being switched off. If a mutation is profitable, unfavorable or without any consequences also depends on the requirements and possibilities of the respective environment

Changes in the genome are therefore an important mechanism of the evolution. If a mutation suddenly makes a living creature more successful than its fellows do, it has better chances of survival and produces more offspring. We call this mechanism selection. The inheritance of positive mutations can change a species or even create a new species.