Mendelian Genetics

Terminology of Mendelian Genetics

Gene: Gene is a segment of DNA that determines a biological character of an organism. It is also called the basic unit of heredity.

Genome: The total genetic content of an organism is called genome.

Allele or allelomorph: Allele or allelomorph is an alternate form of the same gene in which one is paternal and the other is maternal. For illustration, a gene for the plant height has two alleles - a gene for tallness (T) and a gene for dwarfness (t). 

Locus: The point on a chromosome where an allele is located is called locus (pl. loci).

Homozygous: A diploid individual that carries two identical (similar) alleles of the same gene is known as homozygous individual or homozygote. For illustration, A pea plant having both the alleles of tallness (TT) is homozygous individual for tallness.

Heterozygous: A diploid individual that has two different alleles of the same gene is called heterozygous individual or heterozygote. For example, an individual pea plant with an allele of tallness (T) and an allele of dwarfness (t) for the plant height is a heterozygous individual.

Dominant allele: Between two alleles, when one allele for a character can express itself by suppressing its contrasting allele is known as the dominant allele. For example, the allele of tallness (T) expresses over dwarfness (t). This allele of tallness is the dominant allele and is represented by T (uppercase).

Recessive allele: Out of two alleles, the allele which is suppressed by its alternate allele is called the recessive allele. For example, the allele of dwarfness (t) is suppressed by the allele of tallness (T). This allele of dwarfness (t) is the recessive allele and is represented by t (lowercase).

Genotype: The genetic expression or the genetic constitution of an organism is called genotype. For plant height, TT, Tt, and tt are genotypes of an individual plant.

Phenotype: The physical or the observable character of an organism is called phenotype.

Character: The general feature of an organism is called character such as plant height, seed texture, seed colour, and eye colour, etc.

Parental generation: The generation of plants in which plants are used as parents in a cross is called parental generation. It is designated by ‘P’.

Hybrid: The product of a cross between the two dissimilar parents having contrasting characters is called hybrid.

Monohybrid cross: A cross between two parents that differ in one pair of contrasting characters is called a monohybrid cross. For example, a cross between a tall and a dwarf plant.

Dihybrid cross: A cross between two parents that differ in two pairs of contrasting characters is called a dihybrid cross. For example, a cross between two pea plants considering seed shape and seed colour (round yellow versus wrinkled green).

Filial (F) generation: The generation of a progeny, which is obtained by a cross between parents, is a filial generation.

Pure line: A breed or a variety of plant or animal that is homozygous for a particular trait is called pure line. It is also called a true breeding variety.

Pedigree: The history of the descent of a person or family.

Backcross: A cross between a F1 hybrid with one of its parents (P1 or P2) is known as backcross. It is done for the analysis of the genetic constitution of offspring.

Test cross: When the individual of an unknown genotype is crossed with homozygous recessive parents is a test cross. It is done in order to determine the genotype of the unknown individual. 

Reciprocal cross: The cross of the same genotype where gametes from the parental generation are reversed is called reciprocal cross. The purpose of reciprocal cross is to find out whether both parents are making equal contribution or not.

Punnett square: It is a checker board to study all possible results of various crosses. It was devised by a geneticist R. C. Punnett (1906). All gametes from female parents are entered on the left hand vertical side and all gametes from male parents are entered on the top horizontal side. Squares are filled by combining gametes from both the parents.

Selection of pea plants by Gregor John Mendel

Gregor John Mendel (1822-1884) was an Austrian geneticist. He did the hybridization of pea (Pisum sativum) plants to study various characteristics of plants. From those hybridization experiments on the garden pea, he explained the process of inheritance of characters and formulated laws. He published those laws in ‘The Annual Proceedings of Natural History Society’ of a local natural history society.

No importance was given to his work till 1900, when Mendel’s laws were rediscovered simultaneously by a Dutch biologist Hugo de Vries, a German botanist Carl Corens, and an Austrian botanist Erich von Tschermark. Trios found Mendel’s explanation to be correct. Finally, Mendel’s laws were widely accepted. Now, he is called the ‘Father of Genetics’.

Mendel selected garden pea as his model organism due to following reasons:

1. Pea plants possess many varieties with well defined characters.
2. Their flowers are bisexual. Sex organs are completely enclosed within a corolla and the flowers are normally self pollinated. 
3. They can easily be cross pollinated or crossed by hands.
4. They require relatively small space to grow each plant and are easy to cultivate.
5. They have a very short lifespan; many generations can be formed in only a single growing season.
6. Hybrid pea plants are fertile enough to carry out further cross.       

Mendel’s Experiments

Mendel did hybridization experiments on the garden pea to study the inheritance of seven different traits. Some of them are the height of the plant (stem), colour and position of the flower (flower), colour and shape of the seed (seed), and colour and shape of the fruit (pod). Further, he conducted experiments in two ways - monohybrid cross and dihybrid cross.

Mendel’s monohybrid cross

A cross between the two pure or homozygous parents differing in only one pair of contrasting characters is called monohybrid cross. In this cross, a cross between pea plants is done by considering a plant height i.e. tallness and dwarfness.

Mendel selected a pair of pea plants with two contrasting characters or traits - tall (TT) and dwarf (tt). He made cross-pollination of tall plants (TT) with dwarf plants (tt). He called the plants raised by sowing seeds obtained after such cross-pollination  as the hybrid plants. This generation he called a first filial generation (F1 generation) and offsprings as F1 progeny. Then, he self-pollinated the F1 progenies. He observed the phenotypic ratio of tall and dwarf plants 3:1 but their genotypes were TT, Tt, and tt with the ratio of 1:2:1.

Mendel’s dihybrid cross

A cross between the two pure (homozygous) parents differing in two pairs of contrasting characters is called dihybrid cross. Two characters or traits such as seed colour and seed shape of pea plants are selected.           

Mendel crossed round-yellow seeded (RRYY) with wrinkled-green seeded (rryy) pea plants. In the F1 generation, pea plants were with round-yellow (RrYy) seeds. When these F1 progeny were self-pollinated, the pea plants, in the second filial generation (F2 generation),  were with four different combinations of seeds. Those combinations were round-yellow, wrinkled-yellow, wrinkled-green, and round-green with a phenotypic ratio of 9:3:3:1. However, the genotypic ratio was 1:2:1:2:4:2:1:2:1.

Mendel’s laws of inheritance

Mendel postulated three laws of inheritance relying on the monohybrid and dihybrid crosses. Those laws are called Mendel’s laws of inheritance. Mendel’s laws of inheritance are principle of dominance, principle of segregation or purity of gametes (first law), and principle of the law of independent assortment (second law).

1. Principle of dominance

The phenomenon of being only one trait dominant over another in heterozygous plants is called the principle of dominance. For illustration, when a homozygous tall (TT) pea plant is crossed with a homozygous dwarf (tt) pea plant, in the first filial generation (F1), all the plants are tall. Here, the F1 plants are heterozygous (Tt) with tallness (T) being dominant over dwarfness (t). The tallness (T) is the dominant trait and the dwarfness (t) is the recessive trait in pea plants.

2. Law of segregation (First law of inheritance)

The law of segregation states that when two contrasting alleles of a character, which remain together in a heterozygote individual, are not mixed but they separate or segregate during the gamete formation. This law is also called the ‘law of purity of gametes’. For illustrations, in Mendel’s F1 pea plants, the allele for tallness (T) and allele for dwarfness (t) are two contrasting alleles of the plant height. They remain together and produce only tall characters in heterozygous F1 progenies (Tt). Soon, those contrasting alleles, in F1 progenies, separate or segregate during gamete formation resulting in the the homozygous tall (TT), heterozygous tall (Tt) and homozygous dwarf (tt) in F2 progenies.

3. Law of independent assortment (Second law of inheritance)            

The law of independent assortment states that the factors or genes of different characters located in different pairs of chromosomes are independent of one another during gamete formation. For instances, the Mendel’s dihybrid cross between pea plants of two different characters with round seeds and yellow cotyledons (RRYY) and  wrinkled seeds and green cotyledons (rryy) provides all the pea plants with round seeds and yellow cotyledons in F1 generation. However, when F1 pea plants are self-crossed, four phenotypes and nine genotypes are developed in F2 generation as these factors tend to remain independent of one another during the gamete formation. During the gamete formation, the F1 pea plants make four types of gametes RY, rY, Ry, and ry and produce four phenotypes with the ratio of 9:3:3:1 and nine genotypes with the ratio of 1:2:1:2:4:2:1:2:1.