Dominant and Recessive Traits


 
 
Concept Explanation
 

Dominant and Recessive Traits

Notations used in Mendel’s experiments:

The dominant trait is usually represented by a capital letter. For example, tallness is represented by ‘T’ and dwarfness is represented by the corresponding small letter ‘t’. If tallness is due to a pair of dominant alleles, it is written as TT. If tallness is due to only one dominant allele, it is written as Tt. If both the alleles are recessive, making the organism dwarf, then it is written as tt. A homozygous condition is one in which both the alleles are of the same nature, e.g., TT or tt. A alleles are of different nature, e.g., Tt. When two heterozygous condition is one in which the characters are taken into account, the notation for the homozygous dominant could be AABB, and for the homozygous recessive it could be aabb. He performed following two experiments:

1. Inheritance of Traits for one Contrasting Character:

Mendel took pea plants with different characteristics such as height (tall and short plants).

  • The progeny produced from them (F1 -generation plants) were all tall. Mendel then allowed F1 progeny plants to undergo self-pollination.
  • In the F2-generation, he found that all plants were no three quarter were tall and one quarter of them were short. This observation indicated that both the traits of shortness and tallness were inherited in F1 -generation, but only the tallness trait was expressed in F1 -generation. This is known as the law of dominance.
  • Two copies of the traits are inherited in each sexually reproducing organism. TT and Tt are phenotypically tall plants, whereas tt is a short plant. For a plant to be tall, the single copy of ‘T’ is enough. Therefore, in traits Tt, ‘T’ is a dominant trait while ‘t’ is a recessive trait. In F2 - generation, both the characters are recovered, though one of these is not seen in F1 stage. During gamete formation, the factor or allele of a pair segregate from each other. This is known as the segregation.
  • 2. Inheritance of Traits for Two Visible Contrasting Characters:

    Mendel took pea plants with two contrasting characters, i.e. one with a green round seed and the other one with a yellow wrinkled seed.

  • When the F1 progeny was obtained they had round and yellow seeds, thus establishing that round and yellow are dominant traits.
  • Mendel then allowed the F1 progeny to be self-crossed (self-pollination) to obtain F2 progeny. He found that seeds were round yellow, round green, wrinkled yellow and some were wrinkled green.
  • The ratio of plants with above characteristics was 9:3:3:1, respectively (Mendel observed that two new combinations had appeared in F2).
  • In F2-generation, all the four characters were assorted out independent of the others. Therefore, he said that a pair of alternating or contrasting characters behaves independently of the other pair, i.e. seed colour is independent of seed coat. This is known as the law of independent assortment.
  • Sample Questions
    (More Questions for each concept available in Login)
    Question : 1

    ___________________ took pea plants with different characteristics to study dominant and recessive traits.

    Right Option : A
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    Explanation
    Question : 2

    Mendel took _______________ to perform the experiment to understand inheritance of traits.

    Right Option : A
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    Explanation
    Question : 3

    Eye color in the fruit fly is said to be sex-linked. This simple means that the gene for eye colour is ______________________

    Right Option : D
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    Explanation
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