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Mendelian Inheritance

Monohybrid Cross: Cross between parents that are different only in a single characteristic.

  • Only a single trait is studied in monohybrid cross.
  • The first generation of a cross is the P (parental) generation.
  • The offspring from the parents in the P generation are the F1 (filial 1) generation. (Filial- Son)
  • When F1 plants are allowed to self-fertilize, producing a second generation called as F2 (filial 2) generation.
  • Mendel used the term ‘Factor’ for gene.

E.g. Cross is conducted between Homozygous Tall (TT) and Homozygous Dwarf (tt) parental generation. Cross is depicted by punnett square. F1 plants display the phenotype of only one parent, which was DOMINANT, i.e.Tall. But they inherit genetic factors from both parents ,i.e they all have genotypes (Tt).

Formation of F1 generation

Formation of F1 generation

On selfing two F1 plants, both tall and dwarf phenotype is observed in F2, because F1 plants have both tall and dwarf genetic factor in their genotypes, which MUST get separated/ segregated WITH EQUAL PROBABILITY during gamete formation by the process of meiosis, and only one allele is transmitted to a gamete.

Formation of F2 genenration

Formation of F2 generation

 

Complete Monohybrid Cross representation:

Representation of complete Monohybrid cross

Representation of complete Monohybrid cross

NOTE: Punnett square is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
It is constructed by drawing a grid, putting the gametes produced by one parent along the upper edge and the gametes produced by the other parent down the left side. All possible combinations are represented in boxes below in the squares, which generates a square output form.

  • Mendel proposed two general rules from his observation in monohybrid cross: Principles or Laws of Inheritance: the First Law or Law of Dominance and the Second Law or Law of Segregation.

Law of Dominance:   States that in a heterozygote, one allele may conceal the presence of another. Allele which shows its phenotype in heterozygote is called as dominant allele, and which get concealed is recessive allele.
(i) Characters are controlled by discrete units called factors.

(ii) Factors occur in pairs.

(iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).

The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.

Principle of Segregation: States that each individual organism possesses two alleles that can encode a characteristic. These alleles segregate when gametes are formed, and one allele goes into each gamete.

Alleles do not show any blending and that both the characters are recovered as such in the F2 generation.

NOTE: Founder of modern genetics:Gregor Mendel

Concepts of Inheritance: Two theories:

  • Pangenesis: It’s an early concept of inheritance which states that specific pieces of genetic information travel from various parts of the body to the reproductive organs, from where it is transferred to gametes and finally they are passed to the embryo
  • Germ-plasm theory: Weismann proposed the germ-plasm theory, which holds that the cells in the reproductive organs carry a complete set of genetic information that is passed to the egg and sperm.
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