Non-Mendelian Inheritance

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.

Also Read:Incomplete Dominance


When 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.

Also Read: Codominance and Multiple alleles

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.

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