In the beginning was the pea, then came the rules. You may have heard about Mendel’s rules in biology class. But what do they say and what meaning do they have? And what do they have to do with peas?
Mendelian rules – definition and history
Everything started with Gregory Mendel (1822-1884). The Augustinian monk experimented with it in his monastery garden pea plants. He led crossing experiments through, so tried to produce offspring from pea plants with different characteristics. He evaluated these experiments statistically – in concrete terms: he counted peas.
The first thing he noticed was that pea plants whose offspring were all identical in terms of a certain trait, homozygous (=homozygous) had to be for this feature. So if all the offspring of a plant produced green seeds, he concluded that the plant was homozygous for the green seed color. He meticulously documented his observations.
This helped him to discover universal rules or laws behind the supposedly random transmission of certain plant characteristics. Based on his crossing experiments, he three rules on:
As is so often the case, the importance of Mendel’s discovery was not fully appreciated until after his death.
Mendel’s Rule 1 – The Uniformity Rule
Mendel’s first rule applies when two for a trait homozygous individuals be crossed with each other. The resulting offspring are – hence the name uniformity rule – all same (uniform) in relation to the corresponding feature.
It is irrelevant whether the inheritance is dominant-recessive or intermediate. As soon as the Parents (P generation) are homozygous for a particular trait become their Offspring (F1 generation) be uniform among themselves.
You can present such inheritance clearly by assigning the first letter of the dominant trait to a trait encoded by a gene.
You take as an example a plant with red flowers and a plant with white flowers. You know that the red flower color is the dominant feature. So you give the gene for the red color the capital letter «R». The gene variant for the white color is the lowercase «r». That way you know it’s the same type of trait. Since eukaryotic organisms are diploid, i.e. have a double set of chromosomes, you always indicate characteristics with both alleles, for example «RR» for a homozygous plant with a red flower.
In the figure you can see a possible inheritance that shows you the rule of uniformity as an example. In the P-generation you recognize the two homozygous individuals. They have the following genes:
Allele for «red flower» (dominant): R (red circle)
Allele for «white flower» (recessive): r (white circle)
Since offspring always receive one chromosome each from one of the parents during sexual reproduction, each trait has two gene loci (alleles). The genotype of the plants consists of two alleles:
Genotype of the homozygous red plant: RR
Genotype of the homozygous white plant: rr
In genetics you have to choose between the genotype and the phenotype differentiate:
Of the genotype denotes the genetic endowment of an individual. This means all genes. If one refers to a specific gene or trait, the genotype states whether an individual pure or mixed for this feature.
Of the phenotype describes that appearance of an individual. This designation also refers to the totality of all characteristics. A phenotype can be characterized by several genotypes.
As you can see in the picture, all the offspring have a red flower – so the gene for the red flower color is dominant. The offspring of the F1 generation are all heterozygous for the flower color characteristic. You can also do this in a so-called combination square display clearly:
Mendel’s Rule 2 – The Cleavage Rule
If you now cross the uniform individuals of the F1 generation among themselves, a certain characteristic expression is obtained in the next generation of offspring (F2 generation). This is described in Mendel’s second rule.
Figure 2: Schematic representation of the splitting rule Source: wikipedia.de
In the figure you can see an example of inheritance, which explains why the second Mendel rule also applies splitting rule means: The offspring in the F2 generation are no longer the same, because their genotypes and phenotypes are split. You can also see this in the combination square.
The offspring in the F2 generation now produce both red and white flowers. While that is relationship of individuals with red flower too such with white blossom 3 : 1. However, this ratio only affects the phenotype. For the genotypes, a ratio of RR : Rr : rr = 1 : 2 : 1 watch. The originally homozygous genotypes of the P generation reappear in the F2 generation.
Mendel’s Rule 3 – The Independence Rule
In the previous examples came only a trait before that was inherited (monohybrid inheritance). But what if two characteristics be passed on to the offspring at the same time (dihybrid inheritance)?
For that, there’s Mendel’s third rule, too independence rule called. However, it only applies if the two characteristics independently be inherited from each other. That means the genes for it different chromosomes lie.
This time, the flowers of the plants in the example can be not only red or white, but also smooth (G) or curly (g). As you can probably tell right away, «smooth» is the dominant feature here.
Also with the independence rule you go in the P generation of homozygous individuals out:
P generation phenotypes: smooth red flower × ruffled white bloom
P generation genotypes: GGRR gggrr
Germ cells of the P generation: RG rg
Can you imagine why the germ cells of the P generation suddenly only have two letters instead of four? You can find the answer at the end of the article.
With this information you can set up the combination square:
germ cellsGRGRbigGgRrGgRrbigGgRrGgRr
As you may notice, the genotypes – and therefore the phenotypes – of all offspring in the F1 generation are the same. They have red, smooth flowers. Mendel’s first rule also applies when two traits are inherited.
But what happens when the individuals of the F1 generation are crossed with each other? The following germ cells exist in the F1 generation: GR, size, gr and big
You can use this to set up the combination square:
As you can see, there are quite a few possibilities. Incidentally, the genotypes highlighted in italics on the diagonal are all homozygous for the respective trait. If you look closely, you will also see that now two new homozygous genotypes have arisen: GGrr (smooth white bloom) and ggRR (ruffled, red bloom). The genes were thus – in comparison to the P generation – recombined. That is why it is also called Mendel’s third rule recombination rule.
The phenotypes of the F2 generation always appear in a certain ratio:
red-smooth red-ruffled White-smooth White-ruffled
9 : 3 : 3 : 1
Do you remember the question from above? Here’s the answer: As you may already know, germ cells only have half a set of chromosomes because they undergo meiosis (mature division). Therefore, they only have one allele that expresses a certain characteristic.
Mendelian rules – blood types
Each person has only one blood group. Which one it is depends on the blood groups of the parents, because the inheritance of blood groups can also be explained using Mendel’s rules. One allele from the mother and one allele from the father is inherited by the child. Exactly which allele is inherited is random.
In general, every human being has a diploid set of chromosomes. This is made up of two alleles, the allele of the father and that of the mother. Together these alleles make up the genotype. This in turn can either be homozygous or heterozygous:
- In the homozygous genotype, the child has two of the same alleles. Possible combinations are therefore: AA, BB or 00.
- In the mixed, heterozygous genotype, the two alleles differ from each other. Therefore only AB, A0 or B0 are possible combinations.
Total there four different blood groupswhich can be distinguished by their genotype:
- Blood group 0 (genotype: 00)
- Blood group A (genotype: AA or A0)
- Blood group B (genotype: BB or B0)
- Blood group AB (genotype: AB)
The alleles A and B represent the dominant alleles in the inheritance of the blood group. They prevailed over the allele 0. For example, blood group A can still have the mixed genotype «A0». However, the «A» allele is dominant over the «0» allele. The allele 0 can therefore be described as recessive. In addition, the alleles A and B can be described as codominant. When they are combined (AB), both antigens are formed in the same ratio.
But the blood groups can also be distinguished phenotypically by their antigens. More specifically, blood groups are differentiated by identifying the antigen that covers the erythrocyte membrane. Blood group A also has antigen A. Blood group B has antigen B. Blood group AB has both antigen A and antigen B. On the other hand, blood group 0 has no antigens.
Using the combination square, you can work out which blood group is inherited. You can find out all possible genotypes of the child from the blood group of the first and second parent.
Figure 3: Blood group combination square
Accordingly, the following possible blood groups of the child can be deduced from the blood groups of the parents:
If one parent has blood group «0» and the other parent has blood group «B», then there is no possibility that the child might have blood group «A» or «AB». Instead, only «B» and «0» are possible blood types for the child. In some cases, paternity can also be ruled out by means of the blood group, since in this case, for example, no man with blood group «A» could be the father of the child.
In Germany, most people have blood group A, which is the most common blood group at 43%. This is closely followed by blood group 0. Blood group 0 occurs in 41% of people in Germany. The blood group AB is the rarest with about 5%.
Mendelian Rules – Eye Color
The inheritance of eye color can also be explained using Mendel’s rules. That…