Genotype: Definition, Example & Determination

In classical genetics, a distinction can be made between the so-called genotype and phenotype. The two terms are particularly important in connection with Mendel’s rules. Frequently used examples to explain the genotype are the flower color of plants or the blood groups.

Definition genotype

When examining living things for a specific trait, as is often the case in connection with Mendel’s rules, the combination of alleles of the trait being examined is called a genotype.

If an individual has two of the same alleles of the gene that determines the trait under study, the individual’s genotype is referred to as homozygous (homogeneous). If there are two different alleles of the gene, it is called one heterozygousin (mixed) Genotype.

Genotype – difference phenotype

As already mentioned, the genotype is the genetic composition. In other words: It is about the exact genetic combination of hereditary information, which then leads to a specific characteristic (e.g. eye color).

The phenotype, on the other hand, includes these externally visible characteristics (e.g. eye color = blue). It is possible to draw conclusions about the phenotype from the genotype – but this is not possible the other way around, or only to a limited extent.

Phenotypic Plasticity

If there is a strong imprint from prevailing environmental influences, we speak of a high phenotypic plasticity. In this case, the phenotype is less easy to predict. With low plasticity, i.e. little to no environmental influences, the phenotype can be determined relatively reliably from the genotype.

Genotype – gene frequency and genotype frequency

The concept of gene frequencies or allele frequencies comes from population genetics. A gene frequency indicates how often a gene occurs in a population. Gene frequencies can change due to evolutionary factors. With the help of Hardy-Weinberg rulealso known as Hardy-Weinberg equilibrium, the allele frequencies can be determined.

The genotype frequency results from the allele frequencies. If the allele frequency does not change, the genotype frequency also remains unchanged.

Genotype – examples

The expression of characteristics of an individual is often characterized by a interaction determined from genotype, environmental factors and epigenetic influences. Other traits are solely dependent on the genotype. In the following, you will learn how the genotype influences the appearance of an individual using two different examples.

flower color

With regard to the flower colors of plants there is two forms: Red (R) and white (w). The allele that causes the flower to turn red is dominantwhich is the allele of white coloration recessive. This is represented by capitalization of the abbreviations.

Homogeneous plants with red or white flowers are represented as follows:

phenotype: Red | White

genotype: Rw

The combination of alleles (in this case: alleles that affect flower color) is called a genotypethe color of the flowers themselves is called the phenotype.

dominant alleles are always written with a capital letter (here: R for red). Recessive alleles, on the other hand, are marked with a lower case letter (here: w for white).

In the case of Figure 1 there is also a heterozygous genotype because the genotype has both the dominant red alleles (R) and the recessive white alleles (w). If the combinations RR or ww would result in the genotype, it is one homozygous Feature.

The example also clearly shows that the phenotype can be deduced from the genotype. Of course, this is only possible to a limited extent. In this example, one would assume that at least one parent must be red. However, what about the second cannot be said exactly. It is possible that the second plant is also red or has another recessive color (white in this case).

Depending on how complex a phenotypic trait is, it is not possible to infer the exact genotype – simply because there are far too many ways in which the alleles can be combined. In the present example, however, there are only two different alleles. Since this is a dominant-recessive inheritance process, the genotype of the F1 generation can only be the combination «Rw».

Blood groups as a genotype

Another example of genotype is the inheritance of blood types. Blood group is inherited from parents and is not mutable. The blood group inheritance follows the principle of Mendel’s rules.

When inheriting blood type, there are three possible alleles (A, B or 0). A and B are both dominant, while 0 is recessive. Altogether result from the combination of the AB0 system six various genotypes such as four possible phenotypes.

Possible phenotypes: Blood group A, B, 0 or AWAY

Possible genotypes: AA, BB, 00, AB, A0 or B0

The blood group system AB0 is also an example of a so-called codominance. We speak of codominance when several alleles are equally dominant and are therefore equally likely to occur in a phenotype.

Genotype pedigree analysis

In contrast to the phenotype, the genotype cannot be read directly from external characteristics. By a pedigree analysis However, the genotype can be inferred from the phenotype. It does this by looking for patterns within inheritance that explain the formation of the phenotypic trait under consideration.

As mentioned above, a pedigree analysis also distinguishes between dominant and recessive traits. In addition, between a autosomal and one gonosomal inheritance distinguished. In an autosomal inheritance, the alleles are on the chromosomes, in a gonosomal inheritance on the gonosomes (sex chromosomes).

Example family tree analysis

An example of family tree analysis follows for clarification. In Figure 2 is a autosomal dominant inheritance shown. Again between homozygous (aa or AA) and heterozygous (Aa) characteristics can be distinguished.

In an autosomal dominant inheritance process, the trait carriers can be both heterozygous and homozygous. Someone who does not have the trait in the phenotype does not have it in the genotype either.

Figure 2: Autosomal dominant inheritance.

In the 1st generation, both carriers have the trait in a heterozygous form. Among her children (2nd generation), two also have the trait and one does not. In turn, the woman with the trait has children with a man who doesn’t have the trait. Your children (3rd generation) may or may not inherit the trait.