Mendelian Genetics

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Key Questions

  • Gregor Mendel learned about heredity by conducting experiments on the heredity of seven true-breeding (homozygous) traits of pea plants. Fortunately Mendel had a good head for Mathematics, and through his studies he was able to deduce three laws of heredity; the law of segregation, the law of independent assortment, and the law of dominance.

  • The laws proposed to explain the inheritance of traits (characters) from one generation to another are known as Mendel's Laws of inheritance.

    Mendel's laws are as follows:

    Law of Dominance: When two alternative alleles of a trait (e.g.height of plant) are present together, either of the two only is able to express itself and is called dominant allele or factor and the other allele, though present, is not able to express itself is called recessive allele or factor.

    In Pisum sativum (edible pea) tallness is dominant and is written as (T) and dwarfness is recessive and is written as (t).

    Recessive allele is represented by small letter of the same alphabet that represents the dominant allele.

    A tall pea plant is either pure tall (TT) or impure tall (Tt). The allele of dwarfness (t) is not able to express itself when present along with dominant allele of tallness (T).

    Recessive allele can express only when both the alleles are (tt).

    Thus a tall plant can be pure (homozygous) or impure (heterozygous), but dwarf plant is always pure (homozygous).

    Law of Segregation: According to law of segregation two factors or alleles representing same expression or alternate expression of a trait segregate or separate at the time of gamete formation so that gametes contain only one allele or factor and are thus always pure.

    Law of Independent Assortment: According to this law segregation of the factors or alleles of two traits occurs independently of each other at the time of gamete formation.

    In other words the segregation of the factors of one trait does not affect the segregation of the factors of the other trait.

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