Incomplete Dominance

Hey All AIPMT Biology Aspirants, read out the next AIPMT Biology Study material/ Notes of Incomplete Dominance in Genetics, useful for AIPMT Biology.

Law of Dominance states that heterozygote (F1 offspring) having two alleles expresses dominant allele in its phenotype. But in some conditions, this law fails. One such condition is called Incomplete dominance.

  • Incomplete dominance is exhibited when the heterozygote has a phenotype intermediate between the phenotypes of the two homozygotes, i.e. phenotype of F1 offspring (heterozygote) is intermediate between the parental (homozygotes) phenotypes.
  • F1 offspring did not show the dominant phenotype, so law of dominance gets failed here. Incomplete dominance is exception to Mendel’s Law of dominance.
  • F1 phenotype did not resemble either of the two parents and was in between the two.
  • One such example is seen in Flower color of Snapdragon, Antirrhinum majus (Dog Flower). Also in flower color of Mirabilis jalapa(Four-o’clock plant).
  • When Red and White parental phenotypes (both homozygous for their respective alleles) are crossed, they produce heterozygotes that have Pink flowers.
AIPMT biology - Incomplete Dominance in Snapdragon flowers

Incomplete Dominance in Snapdragon flowers

  • The allele for red color (R) is therefore said to be incompletely, or partially dominant or semi dominant over the allele for white color (r).
  • Reason behind this incomplete dominance of Red color allele over white color allele lies in the strength of expression of alleles. Every allele has its own strength of expression. Some alleles are very dominant that their one copy is enough to mask the other allele.
  • In this case, red allele does not have full strength to show its phenotype and mask white allele in heterozygote condition, as it does in homozygote condition. So, the heterozygote showed intermediate phenotype of red and white, i.e. pink.
  • Incomplete dominance exhibits same genotypic ratio and phenotypic ratio (1:2:1) of the offspring because each genotype has its own phenotype.
  • Genotypic ratio was same as in any Mendelian monohybrid cross, but the phenotypic ratio had changed from the 3:1 to 1:2:1
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