Which law of Mendel is generally accepted

Mendel's Laws by Gregor Johann Mendel

Johann Gregor Mendel set up the so-called Mendelian laws of genetics and thus researched for the first time how characteristics are passed on from parents to children. In this article we explain to you what is behind this area of ​​biology.

Everyone knows this: Twins often look very similar. And many children appear optically to represent a "young version" of their parents or at least have some visual similarities to show. Why is that?
The answer to this question can be found in a great biological topic, the genetics. But what exactly is genetics concerned with? Well, the human genetic makeup, too DNA or other important genetic carriers in plant and human cells and bodies are dealt with in the section on genetics.


Before we go into more detail about DNA and chromosomes, however, let's first explain the basics of genetics to you. It is particularly important to take a close look at the following experiments. A man plays a very special role in the formative knowledge of inheritance rules: Johann Gregor Mendel. The monastery godfather, who made the genetic discovery of the transfer of genetic material in 1865, plays an important role in the following article.

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1. Mendel's rule: rule of uniformity

Johann Gregor Mendel was not, as many would suspect, a scientist, but a Bohemian Augustinian priest who carried out experiments with plants in the garden of his monastery. He used the biologically quite simple one Pea plant. The advantages of the pea plant: It is easy to care for as it only needs water and light to grow. It also carries a lot of fruits, here experiments can be carried out in a particularly representative manner.

He carried out experiments with this pea plant. He crossed pea plants with white flowers with pea plants that had red flowers. The blossoms (F1Generation or daughter generation) showed red flowers. If you look at the whole thing from the point of view of inheritance, the white pea plant inherited the information for white flowers and the red the information for red flowers. Normally one would have to say now: mixed red and white should result in pink. But why is this not the case here? The reason is that the red color prevails, it is dominant opposite the white color. The genetic information white has not prevailed and is called recessive designated. For this reason, one speaks here of a dominant-recessive inheritance.

Means: If pure red-flowered and pure-blooded white-flowered individuals are crossed, all members of the F1 generation have inherited a genetic make-up (a gene) for white (from one parent) and a genetic make-up for red (from the other parent); they are heterozygous. Nevertheless, they are all red-flowered, precisely because red is dominant over white. The different forms of a genetic makeup (in the example the gene forms for red and white) are also referred to as alleles. The following graphic shows this. Dominant properties are written in capital letters, recessive properties in small letters.

Dominant-recessive inheritance:

But now you can also cross plants with red and white flowers and get purple flowers. What's that supposed to mean now? Well, one characteristic has prevailed in dominant-recessive inheritance. This was the case with the flowering plant mentioned above. In other plants, e.g. the Mirabilis jalapa (miracle flower), there is no dominant-recessive inheritance, but an intermediate inheritance. So here no flower color prevails, but a mixed form is created. The red and white flowers of the parents become pink flowers of the offspring. The following graphic shows what this looks like. Note that all letters for the genetic information are written in lower case, as there is no dominant part.

Intermediate inheritance:

Both experiments show that the 1st daughter generation - also F1Generation always looks the same. In our case, the daughter generation always has the same color in both the dominant-recessive inheritance and the intermediate inheritance. The phenotype is therefore the same.

Mendel 1: rule of uniformity

If you cross two pure races of the same species with each other, the direct offspring show the same appearance.

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2. Mendel's rule: rule of division

As a reminder: In the case of a dominant-recessive inheritance, one characteristic prevails. Example: Red and white flowers on the parents and red flowers on the offspring. In the case of intermediate inheritance, no characteristic prevails. Example: Red and white flowers in the parent generation and pink colors in the offspring. In colloquial terms, we have now had parents and children. And now let's look at the grandchildren. We now cross the F1 generation (children) with each other and get the F2Generation (grandchildren).

In the case of dominant-recessive inheritance, the following happens: The F1Generation (children) have genetic information for white and red. Red-red can now be inherited, resulting in red. Red-white can be inherited, which also results in red, because red is dominant. If, on the other hand, white-white is inherited, white is created. So: If the inheritance is dominant-recessive, then a quarter of the F2-Individuals pure-breeding with two recessive genes and show a corresponding characteristic expression. The other three quarters show a characteristic like pure-bred individuals with two dominant hereditary factors. These three quarters are made up of pure inherited (one fourth) and mixed breeding (two quarters) individuals. Also here is a graphic for a better understanding:

Dominant-recessive inheritance:

In the case of intermediate inheritance, the picture looks a bit different: The F1-Generation (children) white genetic information for white and red. The F2Generation (grandchildren) can then look white, red or purple. What is happening here now? So 1st possibility: The F2Generation receives the genetic information "red" from both parents, then the offspring is also red. Possibility # 2: Red and white are inherited. In the case of intermediate inheritance, a mixture is created that is purple. And last possibility: White is inherited twice, then a white offspring also arises. The following graphic also shows this:

From this knowledge, Mendel's second law can be derived: the rule of division.

Mendel 2: rule of division

If you cross the half-breeds (daughter generation) with each other, the grandchildren will split up in a certain number ratio. The characteristics of the parents' generation reappear.

3. Mendel's rule: rule of independence

Mendel carried out another interesting experiment: he crossed plants with round and yellowish fruits with plants with green and angular fruits. He was now interested in what the color and appearance of the daughter generation looked like. The F1Generation (Daughter generation) all had round and yellow fruits. He crossed these again and received the F2Generation (grandchildren), which, however, looked very different: Both fruits with the shapes of the parents and two new breeds with yellow-angular and green-round appearance were created.

And that brings us to Mendel's 3rd law, which is also known as the rule of independence.

Mendel 3: Rule of Independence

If you cross two races that differ in several characteristics, the individual hereditary factors are inherited independently of one another. These genes can combine in new ways.

Note: Mendel's rules are not universally valid. There are a number of exceptions, but here you should first get a good overview of the experiments. A summary of the most important terms related to genetics can be found in our article "Basic Terms of Genetics Summary".

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