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Mendelian Inheritance - Mendel’s Law of Heredity - The Law of Independent Assortment

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Notes

The Law of Independent Assortment:

  • Mendel proposed the Law of Independent Assortment based on the dihybrid cross.
  • Mendel’s Law of Independent Assortment states that ‘when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters’.
  • Genes that are located in different chromosomes assort independently during meiosis.
  • Many possible combinations of factors can occur in the gametes.

    Dihybrid cross – Segregation of gametes
  • Independent assortment leads to genetic diversity.
  • If an individual produces genetically dissimilar gametes it is the consequence of independent assortment.
  • Through independent assortment, the maternal and paternal members of all pairs were distributed to gametes, so all possible chromosomal combinations were produced leading to genetic variation.
  • In sexually reproducing plants/organisms, due to independent assortment, genetic variation takes place which is important in the process of evolution.
  • The Law of Segregation is concerned with alleles of one gene but the Law of Independent Assortment deals with the relationship between genes.

Notes

Example:

  • Mendel considered a cross between a pure yellow-coloured round seeded Pea plant and a pure green-coloured wrinkled pea plant.
    We get, 9 (yellow round) : 3 (yellow wrinkled) : 3 (green round) : 1 (green wrinkled)
  • The Punnett square can be effectively used to understand the independent segregation of the two pairs of genes during meiosis and the production of gametes (eggs and pollen) in the F1 RrYy plants.
  • F2 generation thus has,
  • The genotypic ratio clearly shows that the inheritance of the yellow colour is unrelated to the round shape of the seed.
    Similarly, the colour green is unaffected by the wrinkled seed shape.
    This indicates that the two character pairs segregate independently.
  • Consider the segregation of one pair of genes R and r.
  • 50% of the gametes have the gene R and the other 50% have r.
  • The segregation of 50% R and 50% r is independent from the segregation of 50% Y and 50% y.
  • Therefore, 50% of the r bearing gametes has Y and the other 50% has y.
  • Similarly, 50% of the R bearing gametes has Y and the other 50% has y.
  • Thus, four genotypic combinations of gametes (four types of pollens and four types of eggs) formed are RY, Ry, rY, ry (each gamete has a frequency of 25% out of the total gametes produced).
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