The Mendelian rules

The Mendelian rules

In his cross-breeding experiments, MENDEL not only examined the characteristic color of pea seeds (green and yellow), but also different seed shapes (round or wrinkled), different segments of sprouts (short or long) and different shapes and colors of the fruits (pods). MENDEL's observations and results of the crossing experiments are summarized in the Mendelian rules.

Basic concepts for understanding the Mendelian rules

The genotype (hereditary picture) is the totality of the genetic make-up of an organism. The phenotype (appearance) is the outward appearance of an organism resulting from the totality of features. It arises as a result of the interaction of genetic material (genotype) with the environment.

A gene is a section on the chromosome that is responsible for the formation of a trait.

Each gene exists in two alleles.

Dominant (Latin: dominare = rule): An allele is more involved in the formation of one feature than the other. The trait-determining allele is dominant.

Recessive (Latin recedere = resign): The feature-deficient (suppressed) allele is referred to as recessive.

Identification of the homozygous red flower color gene

Gene: red flower color (phenotypically visible)
R: maternal allele for red flower color
R: paternal allele for red flower color

Labeling of the heterozygous red flower color gene
Gene: red flower color (phenotypically visible)
w: maternal allele for white flower color (recessive)
R: paternal allele for red flower color (dominant)

The hereditary factors can be represented in a crossing scheme.

Symbols for the representation of inheritance:
P = parental generation (parental generation)
F1 = 1st daughter generation (store generation)
F2 = 2nd daughter generation
x = crossing of 2 individuals

large letter = dominant (feature-determining) allele
small letter = recessive (feature-inferior) allele

Inferences with dominant-recessive trait development (dominant-recessive inheritance) and hereditary traits (intermediate inheritance) with intermediate trait development are distinguished.

Dominant recessive trait education occurs when, in individuals, the dominant allele of a gene alone determines the expression (phenotype).

An intermediate feature development occurs when in individuals both alleles of a gene are equally involved in the expression of the appearance. This lies between the two parental appearances.

1. Mendelian rule

MENDEL crossed green-seeded pea plants with yellow-seeded pea plants. All pea plants in the 1st generation (generation) had in their pods only yellow seeds. So you looked the same (uniform). The first Mendelian rule is therefore called uniformity rule.

If one crosses two individuals of one species, which are different in one trait, but each are homozygous, then the offspring in the 1st generation (generation) generation are all the same in this trait (law of uniformity). This also applies to reverse (reciprocal) crossing.

The results of crossbreeding experiments as well as the hereditary traits can be shown in a crossing scheme.

2. Mendelian rule

MENDEL crossed the mixed offspring of the 1st daughter generation (F1 generation) and examined the appearance of the offspring in the 2nd daughter generation (F2 generation). Again, he found legitimate results, which were summarized in a second Mendelian rule.

The two variants of the characteristics of the mixed first daughter generation split in the second daughter generation in a certain numerical relationship. The rule was therefore called fission rule.

If the individuals of the F1 generation are crossed with each other, the offspring in the F2 generation split according to the characteristics of the parents according to fixed numerical ratios. In dominant-recessive inheritance, the splitting occurs in the ratio 3: 1 (division law).

In dominant-recessive inheritance, for example, the splitting in the F1 generation takes place in the genotype in the ratio 1: 2: 1 and in the phenotype in the ratio 3: 1.

MENDEL also crossed pea varieties, which differed in two characteristic pairs, namely seed color (yellow / green) and seed form (round / wrinkled). The parents had only yellow, round and green, wrinkled seeds. In the F1 generation he found only yellow, round seeds. This confirmed the uniformity rule. In the F2 generation, he received 556 seeds, of which 315 were yellow and round, 101 were yellow and wrinkled, 108 were green and round, and 32 were green and wrinkled. This implies a cleavage ratio of all possible phenotypes of 9: 3: 3: 1 and again 3: 1 for each feature.

In order to verify his results and to ensure that the parents are also homozygous, MENDEL again performed backcrosses. The backcrossing again showed an average number ratio of the parent's homily

1: 1.

3. Mendelian rule

Looking at the combination square, it is noticeable that peas have emerged whose combinations of features did not occur in either the parent generation or the 1st generation of the daughter, namely yellow and wrinkled, and green and round. The genetic material (genes) must therefore be recombined and inherited independently. For this reason, the third Mendelian rule is also called the rule of independence or rule of the recombination of genes.

If two homozygous parents are crossed, which differ in several characteristics, then the genes are freely combined and independently inherited. In the generation, all feature combinations of the parent generation occur. Pure-born individuals with newly combined genes can be created.

The validity of the 3rd Mendelian rule is always limited if the facilities considered for the intersection are located on the same chromosome for certain characteristics, ie they form coupling groups. In these cases, the assets can not be independently distributed.