Principles of Inheritance and Variation

Laws of Inheritance

Gregor Johann Mendel was a scientist who is recognized as the Father and Founder of genetics. Mendel conducted many experiments on the pea plant (Pisum sativum) between 1856 and 1863. He studied the results of the experiments and deducted many observations. Thus, laws of inheritance or Mendel’s laws of inheritance came into existence. Before learning about Mendel’s laws of inheritance, it is important to understand what the experiments performed by Mendel were.

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Mendel’s Experiments on Pea Plant

Mendel after carefully study selected the pea plant for many reasons:

  • The pea plants were easy to grow and maintain
  • It has many clearly distinct and contrasting characters.
  • The pea plant is an annual plant and so many generations of the plant can be studied in a short period of time.
  • Peas are naturally self-pollinating but can also be cross-pollinated.

Mendel made a list of contrasting characters which he studied:

Laws of Inheritance

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Mendel structured his experiments in a way that he would observe one pair of contrasting characters at one time. He began his experiments using purebred lines for contrasting characters.

He cross-pollinated two pure lines for contrasting characters and the resultant offsprings were called F1 generation(also called the first filial generation). The F1 generations were then self-pollinated which gave rise to the F2 generation of second filial generation.

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Results of Mendel’s Experiments

Let us look at the results of Mendel’s experiments on crossing a pure tall pea plant with a pure short pea plant.

  • In the F1 generation, Mendel observed that all plants were tall. there were no dwarf plants.
  • In the F2 generation, Mendel observed that 3 of the offsprings were tall whereas 1 was dwarf.
  • Similar results were found when Mendel studied other characters.
  • Mendel observed that in the F1 generation, the characters of only one parent appeared whereas, in the F2 generation, the characters of the other parent also appeared.
  • The characters that appear in the F1 generation are called dominant traits and those that appear for the first time in the F2 generation are called recessive traits.

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  • The genes that are passed from the parents to the offsprings exist in pairs. These pairs are called alleles.
  • When the two alleles are the same, they are called homozygous. When both the alleles are different, they are called as heterozygous.
  • Dominant characters are described using capital letters and recessive using small letters. For example, the dominant genes for tallness in a pea plant are written as TT and recessive genes as tt. The heterozygous genes are written as Tt where the plant appears tall has the recessive gene which might express itself in the future generations.
  • The appearance of the plant is known as the phenotype whereas the genetic makeup of the plant is called the genotype. So, a plant with Tt genes appears tall phenotypically but has a recessive gene.
  • During gametogenesis, when the chromosomes become half in the gametes, there is a 50% chance of either of the alleles to fuse with that of the other parent to form a zygote.

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Based on these observations, Mendel proposed three laws.

Laws of Inheritance

Mendel proposed three laws:

  • Law of Dominance
  • The Law of Segregation
  • Law of independent assortment

Law of Dominance

Laws of Inheritance

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This law states that in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele and the characters of this dominant allele are called dominant characters. The characters that appear in the F1 generation are called as dominant characters. The recessive characters appear in the F2 generation.

Law of Segregation

Laws of Inheritance

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This law states that when two traits come together in one hybrid pair, the two characters do not mix with each other and are independent of each other. Each gamete receives one of the two alleles during meiosis of the chromosome.

Mendel’s law of segregations supports the phenotypic ratio of 3:1 i.e the homozygous dominant and heterozygous offsprings show dominant traits while the homozygous recessive shows the recessive trait.

Law of Independent Assortment

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This means that at the time of gamete formation, the two genes segregate independently of each other as well as of other traits. Law of independent assortment emphasizes that there are separate genes for separate traits and characters and they influence and sort themselves independently of the other genes.

This law also says that at the time of gamete and zygote formation, the genes are independently passed on from the parents to the offspring.

Solved Example for You

Q1: What is the genotype of an individual?

  1. Physical appearance
  2. Genetic makeup
  3. Nature of the individual
  4. Blended characteristics of the individual

Sol. The correct answer is the option ”b”. The genetic makeup of the individual is known as the genotype whereas the physical appearance of the individual is known as the phenotype.

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