Tag: codomi

Incomplete Dominance and Codominance

To understand the concept of incomplete and codominance it is important that we know the concept of dominant and recessive alleles in diploid organisms. In humans, there are two alleles for every given gene (since we are diploid) which can be dominant or recessive. Dominant alleles are those where only one copy of the allele is enough to show the phenotype so in heterozygous condition also this allele will be expressed. Recessive alleles are those which have to be in pair to show the phenotype so they have to be in homozygous condition.

Say for example, we are talking about two alleles R and r. R =dominant allele and r =recessive allele so for dominant condition it can be RR or Rr in either case dominant allele will be expressed. And for recessive allele it has to be rr for it to be expressed.

Alright now let’s move on to our topic.

• Incomplete Dominance

  – as the term says, dominance is incomplete. What does that mean? If we cross a plant with a red color flower (genotype R¹ R¹ ) and a plant which has white color flower genotype (R² R² ) then we would expected all the flowers in F1 generation to be either red or white based on whatever allele is dominant. But in incomplete dominance neither of the allele is completely dominant and that means both the alleles will be expressed but incompletely. As a result there will be interaction of products coded by both the alleles and we will see a “blending” of phenotype. So if red and white both products interact or blend what color will be produced? Pink, isn’t it! So here we will observe all pink color flowers in F1 generation.

It is giving us a new phenotype than what is expected right! R¹ allele for red flower and R² allele for white but we are observing pink color flowers. This shows that neither of the allele is dominant over the other (both alleles are expressed somewhat) so it is incomplete dominance. Easy to understand J

Example of Incomplete Dominance – Flower color in Snapdragons

When two true breeding lines of snapdragon flower; red (R¹ R¹) and white (R² R²) are crossed, in F1 generation all the flowers will be pink (R¹ R²). When F1 generation is self-pollinated, we get one red (R¹ R¹) flower, two pink (R¹ R²) flowers and one white (R² R²) flower. So the ratio for incomplete dominance is 1:2:1 for both; genotype as well phenotype in F2 generation.

Ratio = 1:2:1

                                           Figure 1. Incomplete Dominance

• Codominance

  – as the term says codominance and co means “together” which means both the alleles are dominant and both alleles will be completely expressed to give you both the phenotypes.

Don’t get confused with incomplete dominance; in case of incomplete dominance neither allele is dominant. Thus there will be interaction between the two phenotype to give a new phenotype but in codominance both the alleles are dominant which result in expression of both the phenotypes and we will observe two phenotypes together.

Example of Codominance – Red and white spotted flower

When two true breeding lines of red (R¹ R¹) and white (R² R²) color flower plants are crossed, in F1 generation all the flowers will have mixed petals (R¹ R²) – red and white petals together in the same flower. When F1 generation is self-pollinated, we get same ratio as we saw above 1:2:1. So we get 1 red flower, 2 red and white spotted flowers and 1 white flower in F2 generation.

Ratio = 1:2:1

                                                     Figure 2. Codominance

The ratio of F2 generation in both incomplete dominance and codominance is same i.e. 1:2:1. So if in exam we have such a case where ratio is given as 1:2:1, how can we differentiate which type it is? For that let me share a tip. Look at the F1 phenotype and compare it with the parental phenotype. If the F1 generation’s phenotype does not match with any of its parents then it is incomplete dominance. But if you see that F1 generation has phenotype of both of its parents then it is codominance.

Key to differentiate between incomplete dominance and codominance is;

Incomplete dominance – F1 generation shows different phenotype than both the parents.

Codominance – F1 generation shows both the parental traits together.

 Figure 3. Difference between incomplete dominance and codominance 

I hope this post helped J

Watch this video to understand this topic in more details.