Karnataka Board PUCPUC Science 2nd PUC Class 12

Mendelian Inheritance - Mendel’s Laws of Heredity - The Law of Dominance

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Notes

The Law of Dominance:

Law of Dominance states that “When two homozygous individuals with one or more sets of contrasting characters are crossed, the characters that appear in the F1 hybrid are dominant and those that do not appear in F1 are recessive characters".

According to the law of dominance:

  1. 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 generation. It also explains the proportion of 3:1 obtained at the F2.
  2. The characters are controlled by discrete units called factors, which are now called alleles.
  3. Factors occur in pairs.
  4. In a dissimilar pair of factors, one member of the pair is dominant and the other is recessive.
    For example - Tallness in pea plants is a dominant character, while dwarfness is a recessive character.
  5. In a cross between pure tall and pure dwarf pea plant, the only tall character is expressed in all the individuals of F1 generation. As a result, tallness is the dominant character in pea plants, while dwarfness is a recessive character.
  6. Tallness in F1 hybrid is determined by genotype Tt, in which the dominant allele 'T' suppresses the recessive allele 't', thereby suppressing its expression in the phenotype.
  7. The Law of dominance is significant and true but it is not universally applicable.

Notes

Why Mendel’s pea plants are tall and dwarf?

The plant height is controlled by a single gene with two alleles. The reason for this difference in plant height is due to the following facts:

  1. the cells of the pea plant have the ability to convert a precursor molecule of gibberellins into an active form (GA1)
  2. Tall pea plants have one allele (Le) that codes for a protein (functional enzyme) which functions normally in the gibberellin-synthesis pathway and catalyzes the formation of gibberellins (GA1). The allele is dominant even if it is two (Le Le) or single (Le le), it produces gibberellins and the pea plants are tall. Dwarf pea plants have two recessive alleles (le le) which code for non-functional protein, hence they are dwarf.

    Gene for plant height in Peas

Notes

Difference between dominance and Recessive:

Dominance  Recessive 
Dominance occurs when a factor (allele) expresses itself in the presence or absence of its dominant factor.  It can only manifest in the absence of or recessive factor allele. It can only manifest in the absence of or recessive factor allele.
Do not require the presence of a similar gene in the gene pair for the expression.  Require the presence of a similar gene in the gene pair for their effect. 
More likely to be inherited  Less likely to be inherited. 
E.g., Red colour flowers in pea.  E.g., White colour flowers in pea. 
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