Non-mendelian genetics involves the pattern of inheritance that does not follow Mendel’s laws. It describes the inheritance of traits linked to a single gene on chromosomes.

When scientists began exploring more and more test crosses, they observed that there are several traits that do not match up with Mendel’s laws.

Let us have a detailed look at the different types of non-mendelian inheritance patterns.

Variations Involving Single Genes

Incomplete Dominance

In incomplete dominance, the traits blend together producing an intermediate phenotype. If two flowers are crossed together, a hybrid will be produced that is in between both the parents. For eg., in snapdragon plant, if a homozygous white flower is crossed with a homozygous red flower, a pink flower is obtained.



Codominance

When the two alleles present, are simultaneously expressed, it is known as codominance. For eg., in some varieties of chicken, the alleles for black feathers are codominant with alleles for white feathers. If a black chicken is crossed with a white chicken, a chicken with both white and black feathers are obtained.

Types of Codominance

Multiple alleles: Some populations have multiple alleles of a given gene. For eg., the gene for coat colour in rabbits has four common alleles: C, cch, ch, and c.

Pleiotropy: When one gene affects multiple characteristics, it is known as pleiotropy.

For eg., Marfan Syndrome results in several symptoms such as very tall height, thin fingers, heart problems, dislocation of lens, etc. These symptoms are not related directly, but are caused by the mutation of a single gene.

Lethal alleles: A few genes have alleles that affect the survival of the organisms. For eg., lethal yellow occurs due to a mutation in mice that turns its coat yellow.

Mice with homozygous alleles die during embryonic development. Lethal alleles can be dominant or recessive and can be expressed in homozygous or heterozygous conditions.



Polygenic Inheritance

There are a few traits that are controlled by many genes. For eg., our height is controlled by more than 400 genes. Skin pigmentation is the result of several genes.

Extranuclear Inheritance

It is the transmission of genes that occurs outside the nucleus. Mitochondrial DNA is passed on from mother to offspring. It usually occurs in cytoplasmic organelles such as mitochondria and chloroplast.

It is a form of non-Mendelian inheritance discovered by Carl Correns.

The mitochondrial diseases are also passed on to the offspring. This does not follow Mendel’s laws.

Non-Mendelian Laws of Inheritance

There are several exceptions to Mendel's laws of inheritance, which are known as non-Mendelian laws of inheritance. These exceptions include:

1. Incomplete Dominance:
- In incomplete dominance, the heterozygous genotype results in a phenotype that is intermediate between the two homozygous genotypes.
- For example, in snapdragons, red-flowered plants crossed with white-flowered plants produce pink-flowered plants.

2. Co-dominance:
- In co-dominance, both alleles in the heterozygous genotype are fully expressed, resulting in a phenotype that shows characteristics of both alleles.
- For example, in humans, the ABO blood group system exhibits co-dominance. Individuals with the IAIB genotype have the AB blood type, which expresses both A and B antigens.

3. Multiple Alleles:
- Multiple alleles refer to the existence of more than two alleles for a particular gene in a population.
- For example, the ABO blood group system has three alleles: IA, IB, and i. Each individual can have two out of the three alleles.
- The expression of these multiple alleles results in the determination of different blood types.

4. Polygenic Inheritance:
- Polygenic inheritance occurs when a trait is controlled by multiple genes, each with additive effects on the phenotype.
- For example, human height is influenced by multiple genes. The combined effect of these genes determines the height of an individual.

5. Pleiotropy:
- Pleiotropy is the phenomenon where a single gene influences multiple traits or phenotypic characteristics.
- For example, sickle cell anemia is a genetic disorder caused by a mutation in the HBB gene. This mutation not only affects the shape of red blood cells but also leads to various other symptoms such as anemia and organ damage.

6. Epistasis:
- Epistasis occurs when the expression of one gene masks or modifies the expression of another gene.
- For example, in Labrador retrievers, coat color is determined by two genes. The expression of the B gene controls the production of black pigment, while the expression of the E gene determines whether the pigment will be deposited in the hair. If the E gene is recessive, no pigment will be deposited, resulting in a yellow coat color regardless of the genotype at the B gene locus.

In conclusion, these non-Mendelian laws of inheritance demonstrate exceptions to the classic Mendelian principles. They highlight the complexity of genetic inheritance, where traits can be influenced by multiple genes, exhibit incomplete dominance or co-dominance, and result in pleiotropic effects. Understanding these exceptions is crucial in comprehending the diverse patterns of inheritance observed in living organisms.