Tamil Nadu Board of Secondary EducationHSC Science Class 12th
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Monohybrid Cross

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Topics

description

  • Monohybrid cross
  • Reciprocal cross

notes

Monohybrid cross:

  • A cross between two pure parents differing in a single pair of contrasting character is called a monohybrid cross.
  • It involves the inheritance of two alleles of a single gene.

notes

The explanation for Monohybrid cross:

    1. The Tt plant is heterozygous for genes controlling one character (height), it is a monohybrid and the cross between TT and tt is a monohybrid cross.
    2. When a pure tall pea plant (TT) produces gametes by meiosis, each gamete possesses allele 'T'. Similarly, when pure dwarf pea plant (tt) produces gametes by meiosis, each gamete possesses allele 't'.
    3. During fertilization, the two alleles, 'T' from one parent say, through the pollen, and t from the other parent, then through the egg, are united to produce zygotes that have one T allele and one t allele. In other words, the hybrids have Tt. 
    4. Since these hybrids contain alleles which express contrasting traits, the plants are heterozygous

    5. In F2 generation, when F1 plants are self-pollinated the recessive trait reappears.
      This is because; The recessive parental trait is expressed without any blending in the F2 generation. While during gamete formation, these alleles segregate and each gamete receives only one allele from each pair of factors. This segregation of alleles is a random process and so there is a 50% chance of a gamete containing either allele.
      For example:
       
    6. When fertilization takes place, pollen grains with genotype 'T' have a 50% chance to pollinate eggs with genotype 'T', as well as genotype 't'. Similarly, pollen grains with genotype 't' have 50% chance to pollinate eggs with genotype 'T', as well as of genotype 't'. As a result of random fertilization, resultant zygotes formed by selfing of F1 generation have genotypes TT, Tt, or tt. (TT: Pure tall, Tt: Hybrid tall, tt: Dwarf).

      Monohybrid Cross

notes

Reciprocal cross:

  • In one experiment, the tall pea plants were pollinated with the pollens from true-breeding dwarf plants, and the result was all tall plants.
  • When the parental types were reversed, the pollen from a tall plant was used to pollinate a dwarf pea plant which gave only tall plants. The result was the same - All tall plants. Tall (♀) × Dwarf (♂) and Tall (♂) × Dwarf (♀) matings are done in both ways which are called reciprocal crosses.
  • The results of the reciprocal crosses are the same. So it was concluded that the trait is not sex dependent.
  • The results of Mendel’s monohybrid crosses were not sex dependent. The gene for plant height has two alleles: Tall (T) × Dwarf (t).
  • The phenotypic and genotypic analysis of the crosses has been shown by Checker board method or by Forkline method.
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