Unit – II C

Mendel’s Laws of Inheritance

Mendel’s First Law of Inheritance (Law of Dominance):

  • In a cross between two organisms pure for any pair (or pairs) of contrasting characters, the character that appears in F1 generation is called dominant and the one which is suppressed is called recessive.
  • This law can be explained using monohybrid cross experiment.
  • The law is significant and true but is not universally applicable.
  • There are some cases where dominance is not complete or absent. Thus there are cases of incomplete dominance or co-dominance. This cases can be explained by studying cases of deviations from Mendelian inheritance. Hence the law of dominance is not universally applicable.

Mendel’s Second Law of Inheritance (Law of Segregation):

  • Members of an allelic pair in hybrid remain together without mixing with each other and separate or segregate during gamete formation.
  • Thus gametes receive only one of the two factors and are pure for a given trait. Hence this law is also known as the Law of Purity of Gametes.
  • All the sexually reproducing organisms are diploid (2n) i.e. with two sets of chromosomes and gametes are haploid (n) i.e. with one set of chromosomes.
  • This law is universal.

Significance of Law of Segregation:

  • This law introduced the concept of heredity factors as discrete, physical entities that they do not become blended or altered when present together in the same individual.
  • He disproved the blending theory by showing that although traits caused by recessive alleles disappear in the F1 generation, they reappear unchanged in F2 generation.

Limitations of Law of Segregation (Mendel’s Laws of Inheritance):

  • This law is applicable to only diploid organisms which form haploid gametes during sexual reproduction.
  • This is law is applicable to organisms having single gene pair containing two alleles one dominant over the other.
  • This law is not applicable to alleles that are incompletely dominant or co-dominant.
  • This law is not applicable to genes those collaborate or vary in their expression or in penetrance.
  • This law is not applicable to genes which are pleiotropic or complementary.
  • This law is not applicable to traits caused by many gene pairs.

Mendel’s Third Law of Inheritance (Law of Independent Assortment):

  • When two homozygous parents differing in two pairs of contrasting traits are crossed, the inheritance of one pair is independent of other.
  • In other words, when a dihybrid (or polyhybrid) forms gametes, assortment (distribution) of alleles or different traits is independent of their original combinations in the parents. This law can be explained by help of dihybrid cross and dihybrid ratio.
  • It is immaterial whether both dominant characters enter the hybrid from the same or two different parents but the segregation and assortment remain the same.
  • The appearances of new combination prove the law.
  • The law is universally applicable.

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