What are Inheritance and Mendelian Genetics?

Inheritance means the passing of traits to offspring from parents. These traits could be passed either through asexual reproduction or sexual reproduction. The offspring receives the genetic material from the parents. Mendelian inheritance is a certain biological inheritance that follows the rules and principles laid down by Gregor Mendel in 1865. The initial hypothesis was considered wrong, but other inheritance theories modified the concept of inheritance and thus genetics.

Further, these principles were combined with Fisher's theory of natural selection, putting evolution into the picture using the mathematical stage for population genetics within evolutionary synthesis. These observations laid the basis of genetics in biology. They further contributed to understanding the concept of the transmission of mutations and diseases from parents to children. Some mutations are fatal, while some can be beneficial. This could also help study evolution as the changes in the genes or mutations can be observed after millions of years, and the connection between two different species can be learned. 

History of Mendelian Genetics

Gregor Mendel gave the principles of inheritance through his experiments on Pisum sativum. He cultivated around 5000 plants for this experiment and deduced certain principles that laid down the basis of genetics. Initially, Mendel's work was completely ignored, but his work was rediscovered over time by Carl Correns, Vries, and Tschermak. The terms like alleles were coined by another botanist named William Bateson. Mendel's true breeding concept included discrete characters such as color and shape rather than variable characters. His work was statistically analyzed, and the results were quite informative. The backcrossing was done to reveal the presence of recessive characters. A test cross was done to confirm whether or not the recessive characters are passed to the next generation. The dominant characters are easily observed in the phenotypes, but the recessive traits are observed only by back or test cross.

Mendel's Genetic Discoveries

The following discoveries by Gregor Mendel were divergent from the other common theories:

• Characters are discrete; that is, they are unitary.
• Genetic features have alternative forms, and each feature is inherited from one of the two parents. These are called alleles.
• One allele is dominant over the other allele. The dominant allele is reflected in the phenotype.
• Random segregation creates the gametes. The heterozygotic individuals produce gametes with an equal frequency of both alleles.
• Genes are unlinked; that is, different traits arise from the independent assortment.

The above discoveries are known as Mendel's principles or rules. There are certain exceptions to these rules that are mentioned under non-Mendelian inheritance.

Mendel performed an experiment for which he selected certain traits to be studied in the pea plant. These characters are:

• Colour of seed coat- green or yellow
• Length of the stem
• Colour of the unripe pod- yellow or green
• Flower color- yellow or white
• Position of flower- terminal or axial
• Form of ripe pods- wrinkled or inflated
• Form of ripe seeds- round or wrinkled

Mendel crossed a pure wild breed of white flower with a purple flower of a pea plant. This cross formed the parent generation. Later, artificial pollination was performed. The resulting flower of the pea plant had the dominant trait that is purple color in the F₁(first) generation. Mendel called this trait to be the dominant trait. When F₁ generation was self-fertilized, mixed results were obtained with a purple: white ratio of 3:1.

He then noticed the hereditary units, which he named as factors. These units are now called genes which are forms of factors. The alternative forms are called normal alleles. For one trait, an organism has two normal alleles, which is one from each parent. When an individual has two same alleles, they are said to be homozygous for the trait. Alternatively, when the organism has different alleles for a gene, it is heterozygous for that gene. 

The genotype of an organism is reflected by the alleles it possesses. The phenotype is the result of the expression of all characters associated with the alleles. The one normal allele that determines the organism's phenotype is the dominant allele, while the other which does not affect the organism's phenotype is the recessive allele. The recessive is a condition that appears only when it is present in the homozygous state. The recessive allele is expressed even if only a single dominant allele is present. For example, blue eye color is a recessive trait expressed only when each of the alleles from both parents is recessive that is blue.

Mendelian Inheritance Laws 

1. Law of dominance and uniformity
2. Law of segregation
3. Law of independent assortment

Law of dominance and uniformity

It states that in a heterozygote situation, the dominant trait will mask the recessive one. The recessive trait will only be displayed if the individual is homozygous for the recessive allele.

Law of segregation

When two heterozygous individuals are crossed, the F₂(second) generation will have a different phenotype and genotype resembling that of the parent (P) generation. The genotypic ratio and phenotypic ratio, in this case, will be 1:2:1 and 3:1, respectively. 

Law of independent assortment

This law states that alleles for different traits will be passed independently. The biological selection of one allele does not interfere with the allele for another trait.

Mendelian Traits

A trait controlled by a single locus in an inheritance pattern is the Mendelian trait. A mutation in a single gene can lead to a genetic disorder. The recessive alleles are inherited unnoticeably by genetic carriers sometimes. Certain examples following this inheritance pattern are sickle cell anemia, xeroderma pigmentosum, and cystic fibrosis. 

Non-Mendelian Inheritance

There were certain exceptions in the Mendelian principles. These laws were only applicable for sexually reproducing organisms and failed to explain certain inheritance patterns even in the sexually reproducing organisms. One such inheritance pattern is codominance. When a phenotype is produced reflecting both the parents, it is called codominance. 

Common Mistakes

• The recessive trait is displayed only when it is in the homozygous state.
• Gregor Mendel's principles are not true for all the organisms on this Earth.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biology
• Master of Science in Botany
• Master of Science in Biotechnology

Related Concepts

1. Evolution
2. Genetics
3. Codominance