Difference Between Mendelian and Non Mendelian Inheritance

Main Difference – Mendelian vs Non Mendelian Inheritance

The patterns of inheritance in sexual reproduction are described by means of Mendelian and non Mendelian inheritance. A set of characters or traits passes from parents to offspring during reproduction. These characters pass through generations by the inheritance of genetic material through sex cells. Each character is determined by a particular gene in the genome. The alternative forms of a gene are referred to as alleles. The main difference between Mendelian and non Mendelian inheritance is that Mendelian inheritance describes the determination of traits by means of dominant and recessive alleles of a particular gene whereas non Mendelian inheritance describes the inheritance of traits which does not follow Mendelian laws.  

Key Areas Covered

1. What is Mendelian Inheritance
      – Definition, Features, Fundamental Laws of Inheritance
2. What is Non Mendelian Inheritance
      – Definition, Features, Examples
3. What are the Similarities Between Mendelian and Non Mendelian Inheritance
      – Outline of Common Features
4. What is the Difference Between Mendelian and Non Mendelian Inheritance
      – Comparison of Key Differences

Key Terms: Codominance, Incomplete Dominance, Law of Dominance, Laws of Inheritance, Law of Independent Assortment, Law of Segregation, Mendelian Inheritance, Multiple Alleles, Non Mendelian Inheritance, Offspring, Phenotypic Plasticity, Polygenic Inheritance, Sex-Linked Inheritance, Traits

What is Mendelian Inheritance

Mendelian inheritance describes the way in which genes and their corresponding traits are passed from parents to their offspring by means of dominant and recessive alleles. The modes of Mendelian inheritance are autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive.  The fundamental laws of inheritance were first described by Gregor Mendel in 1865. According to Mendel, genes can be found in pairs and they are inherited in distinct units. Thus, offspring receives one gene from each parent. The alternative forms of a gene are called alleles. Depending on the appearance of genes in the offspring, two types of alleles can be identified as dominant alleles and recessive alleles.

Figure 1: Mendelian inheritance of dominant and recessive phenotypes

Three Fundamental Laws of Inheritance

Law of Segregation

Any inheritable trait comes in an allele pair. During the production of sex cells, each allele of the pair segregates into one sex cell. Once they are united during fertilization, the offspring has inherited an allele from each parent.

Law of Independent Assortment

The different alleles of genes are sorted independently from each other in such a way that the inheritance of one allele is independent of the inheritance of another allele.

Law of Dominance

In the presence of two alleles, only the dominant form of the allele is expressed. 

What is Non Mendelian Inheritance

The traits which do not follow the Mendelian inheritance follows the non Mendelian inheritance. Typically, the genes with multiple alleles are inherited in non Mendelian patterns. Multiple alleles do not show true dominance/repressiveness. The phenotype of the offspring highly depends on the environment. In humans, almost all the traits are determined by the non Mendelian inheritance. Examples of the non Mendelian inheritance include multiple alleles, incomplete dominance, codominance, polygenic inheritance, phenotypic plasticity, and sex-linked traits.

Examples of Non Mendelian Inheritance

Multiple Alleles

For the determination of a particular trait, multiple alleles comprise more than two alleles in the population. Human blood type is determined by three alleles, A, B, and O.

Incomplete Dominance

In incomplete dominance, a heterozygous individual contains a phenotype halfway between the dominant and recessive phenotypes. Different flower colors in snapdragons arise by the incomplete dominance.

Codominance

In codominance, both dominant and recessive alleles independently show up in the offspring. As a result of codominance, roan cattle show both red and white hair on their skin. Codominance during the inheritance of flower color in Mirabilis jalapa is shown in figure 2.

Figure 2: Codominance in Mirabilis jalapa flower color

Polygenic Inheritance

In polygenic traits, more than one gene is involved in the determination of a particular trait. These genes can be found in different loci of different chromosomes. Weight, height, skin color, and most of the other human traits are polygenic.

Phenotypic Plasticity

In phenotypic plasticity, the phenotype affects the environment. Skin color, personality traits, weight, and height are phenotypic traits.

Sex-Linked Inheritance

The inheritance of genes on the X chromosome is different in males and females. In order to show the recessive phenotype in males, only a single recessive allele is required. Therefore, males get the recessive condition in diseases such as hemophilia and red/green color blindness.

Similarities Between Mendelian and Non Mendelian Inheritance

• Mendelian and non Mendelian inheritance describe the patterns of the inheritance of a particular trait during sexual reproduction.

• Both Mendelian and non Mendelian inheritance can be used to describe the genetic basis of the phenotypes in the heterozygous individuals for a particular trait.

Difference Between Mendelian and Non Mendelian Inheritance

Definition

Mendelian Inheritance: Mendelian inheritance is the way in which genes and their corresponding traits are passed from parents to their offspring by means of dominant and recessive alleles.

Non Mendelian Inheritance: Non Mendelian inheritance is the patterns of inheritance which do not follow the Mendelian inheritance.

Number of Alleles

Mendelian Inheritance: Only two alleles of a particular gene are involved in Mendelian inheritance.

Non Mendelian Inheritance: Multiple alleles or polygenes are involved in the non Mendelian inheritance.

Dominant/ Recessive

Mendelian Inheritance: The two alleles of a gene which follows the Mendelian inheritance are either dominant or recessive.

Non Mendelian Inheritance: The alleles in non Mendelian inheritance are neither dominant nor recessive.

Phenotypic Proportions

Mendelian Inheritance: The phenotypic proportions of Mendelian inheritance can be pre-determined theoretically.

Non Mendelian Inheritance: The phenotypic proportions in non Mendelian inheritance differ from the theoretic proportions.

Examples

Mendelian Inheritance: Phenotypic traits in Mendel’s pea plants is an example of Mendelian inheritance.

Non Mendelian Inheritance: Many human traits follow non Mendelian inheritance.

Conclusion

Mendelian and non Mendelian inheritance are the two methods that describe the genetic basis of phenotypes in heterozygous individuals for a particular trait. Mendelian inheritance describes the inheritance of phenotypes, determined by only two alleles. One of the two alleles is dominant and the other is recessive for the phenotype. Non Mendelian inheritance describes how multiple alleles and polygenes are involved in the determination of phenotypes. Therefore, the main difference between Mendelian and non Mendelian inheritance is the influence of the number of alleles or number of genes that are involved in the determination of a particular trait.

Reference:

1.” Concept 1 Children resemble their parents.” DNA from the Beginning. N.p., n.d. Web. Available here. 27 July 2017. 
2. “Non-Mendelian Genetics.” [email protected] N.p., n.d. Web. Available here.  27 July 2017. 

Image Courtesy:

1. “Mendelian inheritance” By Benutzer:Magnus Manske – (Public Domain) via Commons Wikimedia
2. “Mendelian inheritance 1 2 1” By Magnus Manske – (CC BY-SA 3.0) via Commons Wikimedia

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