Molecular basis of incomplete dominance.

INCOMPLETE DOMINANCE(THE BASIC IDEA):
Incomplete dominance is an interallelic or intragenic interaction which means interaction between 2 alleles of the same gene present on the same locus of a homologous chromosome. This results in the expression of a totally different phenotype(visual change) rather than the typical dominant-recessive phenotype.
so interallelic interaction are of 3 types:

1. Incomplete dominance
2. co-dominance
3. multiple alleles

WHAT EXACTLY HAPPENS IN INCOMPLETE DOMINANCE?
In this phenomenon, none of the 2 alleles of a gene is dominant over each other or none of the alleles expresses its phenotype, rather both the alleles interrelate or interact with each other to express a completely new and different phenotype which is intermediate between the individual expression of both alleles.

So let me make you understand this by some EXAMPLES:

INCOMPLETE DOMINANCE IN SNAPDRAGON AND FOUR O' CLOCK PLANT:

• in both snapdragon(Antirrhinum majus) and four o' clock(Mirabilis jalpa) there are only 2 types of phenotypes observed in the homozygous or pure state.
• when the 2 pure breeds have crossed the hybrids obtained in the F1 generation are all pink in color which is the intermediate phenotype of the two pure breeds.
• and later when the hybrids of the F1 generation are selfed or self crossed, the hybrids of the F2 generation are obtained in 1:2:1 ratio genotypically.
• that means 1 homozygous red color flower is obtained, 1 homozygous recessive white color flower is obtained and 2 heterozygous pink flowers are obtained.
• in the heterozygous condition when both the alleles are present, none expresses individually but an intermediate phenotype is observed.

MOLECULAR BASIS OF INCOMPLETE DOMINANCE:

when we discuss molecular basis we actually try to find out the reason behind incomplete dominance or why this particular phenomenon happens at the molecular level.

so there are some valid reasons behind this which are summarised below:

• one of the valid reasons for the pink color of flowers at heterozygous condition is due to something called haploinsufficiency. It can also be referred to as quantitative inheritance.
• In haploinsuficiency, the single dominant allele let 'R' makes enough pigment for pink color but the homozygous dominant condition is required for red color. And this case the recessive allele 'r' might make no protein or a defective protein which is not expressed.
• Another way is that the dominant allele 'R' makes a protein giving only red color and recessive allele 'r' makes a protein giving white color. and when both are present in the heterozygous condition they make a combined protein in which the white dilutes the red color and pink color is produced.
• actually, the wild type allele is fully functional and forms an RNA, protein or enzyme for expression.when mutation occurs a mutated allele is formed which produces a faulty or no product.
• the unmodified wild allele is the dominant allele and the mutated one is the recessive allele. if the mutated allele forms the same product as the dominant allele then it is called as equivalent allele and if it forms a modified product, it gives rise to incomplete dominance.
  
  
  
  
  
  
  
  
  .