Mendelian Inheritance

Introduction image

Mendelian inheritance is an approach that explains the traits are the characters inherit from one generation to another by the discrete units, which later termed as genes. It also refers as “Mendelism” which was introduced by the botanist or an Austrian monk, Gregor Johann Mendel. Sir Mendel was honoured as “Father of Genetics” for his great efforts to explain the theory of inheritance, now popularly known as Mendelian inheritance or genetics.

To explain the theory of inheritance, Gregor Johann Mendel introduced three approaches, namely the law of dominance, law of segregation and law of independent assortment. Sir Mendel performed experiments by taking a plant species Pisum sativum or an ordinary garden pea with different traits.

Content: Mendelian Inheritance

  1. Definition
  2. Mendelian Traits
  3. Mendel’s Law
  4. Conclusion

Definition of Mendelian Inheritance

It can define as the theory of inheritance where Gregor Johann Mendel postulated three generalizations by giving a law of dominance, segregation and independent assortment after doing experiments in his monastery’s garden on a pea plant. According to Mendelian inheritance and genetics:

Traits: It can define as the features passed from the parent species to the new individuals by the carrier of discrete units.
Genes: It can define as the discrete units, where each unit expresses independently in the offsprings.
Alleles: It can define as the two alternative pairs of a gene, where one pair from each parent is transferred to the offspring. If a gene carry two identical forms of allele, then an individual is said to have “Homozygous genotype”. If a gene carry two different types (dominant and recessive) of allele, then an individual is said to have “Heterozygous genotype”.
Genotype: It can define as the specific arrangement of alleles for a given pair of genes. A  dominant gene (represents by an uppercase letter like ‘D‘), which will express its phenotypic characters significantly over a recessive gene (represents by a lower case letter like ‘d‘).
Phenotype: It can define as the visible traits of an organism like colour, length, shape etc. that results after a gene expression.

Mendelian Traits

Mendel took seven different traits with two various forms (dominant and recessive) in his experiment like:
mendelian traitsStem length: For such character, Mendel took tall stem (dominant form) and dwarf stem (recessive form).
Flower position: For such trait, he took flower on axial position (dominant form) and another on terminal position (recessive form).
Flower colour: For such trait, he took a violet coloured flower (dominant form) and a white coloured flower (recessive form).
Pod shape: For such trait, Mendel took one inflated pod (dominant form) and constricted seed (recessive form).
Pod colour: Here, Mendel took green coloured pod (dominant form) and yellow coloured pod (recessive form).
Cotyledon colour: Mendel took yellow coloured cotyledon (dominant form) and green coloured cotyledon (recessive form).
Seed form: For such trait, Mendel took some round seed (dominant form) and some wrinkled seed (recessive form).

Reasons for Choosing Pea Plant

The ideal reasons for experimenting with Pisum sativum include:

Easy cultivation: Mendel cultivated many generations of pea plant in his own monastery’s garden.
Distinct traits: Mendel observed two distinct varieties of trait, for example round and wrinkled form of seed, purple and white flower of pea plant etc.
Cross-pollination: Generally, a pea plant can be self-pollinated, but can also be cross-pollinated by transferring pollen from the flower’s anther of one plant to the flower’s stigma of another plant.
Hermaphrodite flowers: A pea plant contains bisexual flowers having male and female reproductive parts both together.
Germination time: Due to the short life cycle of a pea plant, they can grow within a short generation time that permit Mendel to upraise many generations of a pea plant.

Mendel Three Laws

Gregor Johann Mendel postulated some principles of inheritance, what we call as “Mendel’s Law”. He described three laws of inheritance like:

Law of Dominance

To explain the law of dominance, Mendel had given three postulates:

  1. Discrete factors or units direct phenotypic traits.
  2. The factors always comprise a couple of pairs that refer to as “Alleles”.
  3. In two dissimilar alleles, one will show dominant characters, and the other will show recessive traits. The dominant factor will always mask the recessive form.

Therefore, the law of dominance can define as the first law of inheritance that states the expression of only one form of a trait in the first filial generation, if there will be monohybrid cross between the heterozygous genes. In the first filial generation, one parental gene will be expressed in an offspring. In a second filial generation, both the parental characters will appear by giving a phenotypic ratio of 3:1.

Law of Segregation

Mendel introduced the law of segregation after performing experiments on different traits of a pea plant through monohybrid cross to explain the law of dominance. The law of segregation is an explanation or the advance view of the law of dominance. It states that an individual carry two factors for a particular trait which separates during the gamete formation, after which a gamete will take only a single factor.

The discrete units of inheritance are the separate and distinct units that do not blend if present together. This law is formulated for the diploid organisms that reproduce sexually by producing haploid gamete. To explain the law of segregation, Mendel postulated that:

  1. A gene carry a couple of alleles that are generally two in number.
  2. Gametes are produced after meiosis cell division that will cause segregation of gene which remains each gamete with one member of an allele.
  3. An organism always inherited with the two factors (dominant and recessive) for a single trait.
  4. If the two allelic pairs of the gene are different like (Tt), then the dominant factor (T) will be expressed over the recessive factor (t). If the two allelic pairs of the gene are similar by possessing two dominant alleles, then it will inherit homozygous dominant traits. Similarly, if the two allelic pairs of the gene possessing two recessive alleles, than it will inherit homozygous recessive traits.

Example

Let us take a case by performing a monohybrid cross between purple coloured (PP) and white coloured flower (pp) of a pea plant.

law of segregation

At the time of crossing over, a gene separates into distinct alleles where one factor from each of the parent will inherit into the F-1 progeny. ‘Pp’ will form in a first filial generation where a dominant factor (P) will conceal the phenotypic characters of a recessive allele (p). On the self-pollination of F-1 progeny, we will observe the production of both purple coloured and white coloured flower in a phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1.

Law of segregation gives a brief on the law of dominance, by focussing on some other approaches by explaining the law of incomplete dominance.

Law of Incomplete dominance

After performing repeated experiments on pea plant, Mendel experimented on other plants by taking different traits, where he found that the progeny formed in F-1 generation did not show any features of the P-generation.

Example
To explain the law of incomplete dominance, Mendel performed crossing over between the red coloured (RR) and white coloured (rr) flowers of Antirrhinum species.

law of incomplete dominance

The two breeds ‘RR’ and ‘rr’ after crossing over produced pink coloured flowers in the first filial generation with a genotype ‘Rr’. On self-pollination of ‘Rr’, the genotype ratio (1:2:1) will be same as of the law of dominance, but the phenotypic ratio (3:1) will change into 1:2:1.

Mendel concluded that the dominant allele of ‘Rr’ gene would not dominate entirely over the recessive allele that resulted in the formation of pink snapdragon flower in place of red.

Law of Independent Assortment

Mendel proposed a law of independent assortment by experimenting a dihybrid test cross of two independent traits. According to this law:

  1. The assortment of a single paired gene is independent of the other pair at the time of gametogenesis.
  2. Each pair of a gene will express their phenotypic characters differently and independently.
Example

Test cross between the pea plants having round, yellow seed with the wrinkled, green seed.

law of independent assortment

After crossing over, all round, yellow seeds will produce in the first filial generation. When the F-1 hybrid plants are allowed for self-fertilization, then we could see the inheritance of independent seed colour characteristics from the original varieties. The following phenotypic variations will obtain: Round, yellow (9): Round, green (3): Wrinkled, yellow (3): Wrinkled, green (1).

The genotypic ratio obtained in the second filial generation is RRYY (1): RRYy (2): Rryy (1): RrYY (2): RrYy (4): Rryy (2): rrYY (1): rrYy (2): rryy (1).

After performing such dihybrid cross, Mendel concluded that the segregation of the seed colour is independent of the seed shape, which will result in the evolution of new characters and some parental characters in the offsprings.

Conclusion

Initially, the theory explained by Sir Mendel faced many controversies and rejections and was not accepted by many scientists until the turn of the 20th Century. The idea given by Mendel became successful when T.H. Morgan and his co-workers compared the Mendelian model with the chromosomal model of inheritance. They concluded that the chromosomes are the actual carriers of such discrete factors what we now call as genes. Mendelism is one of the popular theory in genetic science that has introduced three approaches to explain the factors of inheritance.

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