Monohybrid Cross - Inheritance Of One Gene

The mystery of genetics was unlocked during mid-nineteenth century by Gregor Mendel. He conducted an experiment on pea plants. By cultivating the pea plants and observing the pattern of inheritance in stages of generation.

Mendel’s Experiment

Mendel is the father of genetics. The Mendel’s laws are Law of Independent Assortment, Law of Dominance, and Law of Segregation. These laws came into existence by experiments on pea plants with a variety of traits.

Mendel investigated the pairs of pea plants with one contrasting trait. Mendel studied on the following seven characters with contrasting traits:

  • Flower color: Violet/white
  • Flower position: Axial/terminal
  • Pod color: Green/yellow
  • Pod shape: Inflated/constricted
  • Seed color: Yellow/green
  • Seed shape: Round/wrinkled
  • Stem height: Tall/dwarf

Monohybrid Cross

For monohybrid cross, Mendel began with a pair of pea plants with two contrasting traits i.e., one tall and another dwarf. The cross-pollination of tall and dwarf plants resulted in tall plants. All the hybrid plants were tall. He called this as a first hybrid generation (F1) and offspring were called Filial1 or F1 progeny. He conducted an experiment with all the seven contrasting pairs. He observed that the entire F1progeny showed one pattern in their behavior i.e., they resembled either one of the parents. Another parent character was completely absent.

Monohybrid cross generation

He continued his experiment with self-pollination of F1 progeny plants. Surprisingly, he observed that one out of four plants were dwarf while other three were tall. The tall and the short plants were in the ratio of 3:1. He also noted that no progeny was in intermediate height i.e., no blending. The result was same for other traits of plants too. And he called them second hybrid generation and offspring were called Filial2 or F2 progeny.

Monohybrid cross generation

Mendel observed that traits were absent in F1 generation had reappeared in F2 generation. He called such suppressed traits as recessive traits and expressed traits as dominant traits. He also concluded that some ‘factors’ are inherited by offspring from their parent over successive generations.

Later, these ‘factors’ were called genes. Genes are responsible for the inheritance of traits from one generation to another. Genes consist of a pair of alleles which code for different traits. If a pair of alleles is same i.e., TT or tt, such alleles are called homozygous pair while those that are different or non-identical (e.g. Tt) are called heterozygous pair.

The cross between two monohybrid traits (TT and tt) is called monohybrid cross. Monohybrid cross is responsible for the inheritance of one gene. It can be easily shown through a Punnett Square.

Practise This Question

Sex chromosomes in a boy are identical.