Rules for the Inheritance

Mendel’s Contribution

Gregor Johann Mendel referred to as the ‘Father of genetics, was an Australian Monk. He framed this law of Inheritance using his scientific and mathematical knowledge. Mendel did this experiment to understand the concept of heredity, His work laid the foundation of modern genetics. He used pea plants for his experiment as he found them easy to grow and they had a greater number of visible characteristics like tall/short, inflated/constricted pod shape, violet/white flower, and round/wrinkled seeds.

During his experiment, Mendel found that genes are the factors that control the expression of traits. Genes are present in pairs for a specific trait and they are termed alleles. Depending on the expression of traits the genes could be either dominant or recessive. 

Dominant Traits: The traits that express themselves in the offspring in every possible combination and can be seen by the naked eye are called Dominant traits.

In Mendel’s experiment, the tall trait in the pea plants tends to express more than the small trait. Therefore, the tall trait of plants is said to be dominant over the small trait.

Recessive Traits: A trait which is not expressed itself in the presence of a dominant allele is known as a Recessive trait. 

A recessive trait is expressed in an offspring when it is contributed by both parents. So, the recessive trait is present in an organism but cannot be seen due to the presence of the dominant trait.

The morphological expression of a single character which is observable physically is termed a phenotype. Examples – are tall or short, round or wrinkled seeds, the colour of a flower, etc.

The genetic constitution of the allele pair for a specific trait is termed as the genotype. Example – Tt or tt or TT.

Mendel was a mathematician so he used statistics to record the traits in each generation by using a statistical method known as Punnett square for predicting the possible genotype and phenotype in the offspring.

Monohybrid cross

A monohybrid cross is the hybrid of two individuals with homozygous genotypes. Only one character is considered. In this cross, Mendel showed the inheritance of dominant and recessive characters. 

E.g. If a Round seed (RR) is crossed with a wrinkled seed (rr), we get 3 Round seeds and 1 wrinkled seed at the end of the F2 generation. The ratio of characters, arising after the cross at the end of the F2 generation is called the monohybrid ratio. In this case, the monohybrid ratio is 3:1. For the cross below the shape of the seed is considered.

Dihybrid cross

A dihybrid cross is a cross between two individuals with two observed traits that are controlled by two distinct genes. In this cross, two characters are considered. The ratio of characters arising at the end of the F2 generation is known as the dihybrid ratio.

E.g. If a plant with a Round green pea (RRyy) is crossed with a plant having a wrinkled and yellow pea (rrYY), the first generation would have all-around yellow peas (RrYy). While crossing the same for an F2 generation, the result had greater variations and the new combinations were Green Round, Yellow round, yellow wrinkled and green wrinkled. Thus, the dihybrid ratio is 9:3:3:1.

The transmission of particular characters from parents to offspring is termed Heredity and Evolution. When a living organism is developed or gradually progressed from a simple life form to a complex organism it is termed Evolution. In all living organisms, the rules of inheritance determine the pattern of inheritance. Evolution is due to the inheritance of these variations in a particular organism. When there is reproduction between two genetically diverse organisms it gives rise to variations in any living organisms. Sexual reproduction causes more successful variations as compared to asexual reproduction.

Human beings can be distinguished from one another based on the inherited characteristics that children get from their parents. They reproduce sexually and hence various heredity traits are passed on, which has resulted in such dynamic changes in the later generations over the years. The human body and animals show a variety of changes that occur due to Heredity and Evolution.