Tamil Nadu Board of Secondary EducationSSLC (English Medium) Class 10

Inheritance of One Gene (Monohybrid Cross)

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Topics

  • Inheritance of one gene
  • Representation of a gene

Inheritance of one gene:



Steps in the cross-pollination of pea flowers

  • Consider the hybridisation experiment carried out by Mendel, where a tall pea plant (TT) was crossed with a dwarf pea plant (tt) to study the inheritance of one gene.
  • The seeds produced from this cross were collected and used to make the first hybrid generation. 
  • This generation is known as the First Filial Progeny or the F1.
  • Mendel noticed that all first-generation plants were tall, with no dwarfs plants in F1.
  • He made similar observations about the other pairs of traits. He concluded that the F1 generation resembled either of the parents and that the trait of the other parent was not seen in them.
  • Mendel then self-pollinated the tall F1 plants.
  • The second generation produced by crossing the F1 generation is called the Second Filial generation or F2.
  • The F2 generation progeny showed characteristics of both the parents, i.e. some were dwarf while others were tall. However, the F2 generations were similar to their parents and did not show any blending of traits (i.e. intermediate height). The character that was not seen in the F1 generation was now expressed.
  • The proportion of plants that were dwarf was 1/4th of the F2 plants, while 3/4th of the F2 plants were tall.
  • Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F1 generation, while at the F2 stage, both the traits were expressed in proportion 3:1. The contrasting traits did not show any blending at either F1 or F2 stage.
  • Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’.which are now known as 'genes'.
  1. Genes, therefore, are the units of inheritance.
  2. They contain the information required to express a particular trait in an organism.
  3. Genes that code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.

Representation of a gene:

  • Generally, a gene is represented by alphabetical symbols.
  • The capital letter is used for the trait expressed at the F1 stage, and the small alphabet for the other trait. For example, in the case of the character of height, "T" is used for the Tall trait and 't' for the ‘dwarf’,
  • Both T and t are alleles of each other.
    Hence, in plants, the pair of alleles for height would be TT, Tt, or tt. 
  • Mendel also proposed that in a true-breeding, tall or dwarf pea variety, the allelic pair of genes for height are identical or homozygous, TT and tt, respectively.
  • Whereas, when an individual possesses dissimilar alleles (Tt) for a particular trait, it is known as 'Heterozygous' or 'Hybrid' for a specific trait.
  • Thus, the genetic constitution for a particular trait (i.e. 'TT', Tt, or 'tt') is known as the 'Genotype' of the individual and the type of character it represents (i.e. Tallness and dwarfness) is known as 'Phenotype'.
  • As Mendel found the phenotype of the F1 heterozygote Tt to be exactly like the TT parent in appearance, he proposed that in a pair of dissimilar factors, one dominates the other (as in the F1) and hence is called the dominant factor while the other factor is recessive. For example, T (for tallness) is dominant over t (for dwarfness), which is recessive.
  • It is convenient to use the capital and lowercase of an alphabetical symbol to remember this concept of dominance and recessiveness.
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