Exceptions to Mendel’s laws​; Allelic variation and gene function​ ​

Exceptions to Mendel’s laws​; Allelic variation and gene function​ ​

February 17, 2022

Exceptions to Mendel’s laws; Allelic variation and gene function

  • According to Mendel, a single gene controls one character and the alleles of a pair are related as dominant and recessive.
  • Various exceptions are existed of the above notion. In some cases more than one pair of genes influence the same character
  • These may interact in different ways by assign, subtracting, or modifying the characters in question in, or may inhibit or reverse the effects of another pairs of genes
  • This concept was introduced by Bateson known as Bateson factor hypothesis

Incomplete dominance (1:2:1ratio )

  • In heterozygous, when dominant allele partially dominant i.e. semi dominant and is not fully expressed such condition is known as incomplete dominance. 
  • In another word in the heterozygote the dominant allele does not fully mask the phenotypic expression of the recessive allele and that result in the intermediate phenotype in the heterozygote. 
  • e.g. when a red flower plant (RR) is crossed with white flowered plant (rr) then the F1 hybrid pea plants are found to be pink flowers. It shows that gene for red color could not completely dominate the gene for white color as shown in the figure. The phenotypic ratio of F2 hybrid is 1:2:1 (red 1: pink 2: white 1).

Codominance (1:2:1 ratio)

  • In codominance, both the genes of an allele pair express themselves equally in F1 heterozygous.  These follow the law of segregation and F2 progeny exhibit 1:2:1 ratio both genotypically as well as phenotypically.
  • At the MN locus, there are two alleles: the LM allele, which encodes the M antigen; and the LN allele, which encodes the N antigen. 
  • Heterozygotes with genotype LMLN exhibit codominance and express both the M and the N antigens; they have blood-type MN.
  • ther examples are expression of blood antigen alleles (ABO system): Gene A and B responsible for blood group A and B are codominant. The hybrid has AB blood group. 
  • Codominance in Andalusian fowl: A cross between pure black and pure white varieties results in blue hybrids in F1. These blue hybrids of F1 generation when crossed among them yield black, blue and white fowls in 1:2:1 ratio. 


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