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Incomplete Dominance and Codominance – What is the Difference?

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.

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Homologous and Homozygous – What is the Difference?

Homologous and Homozygous – What is the Difference?

Homologous or homozygous? Let’s understand the difference in this post. We will start with homologous and move to homozygous because that would make it easy to understand.

  • Homologous –

    this term we use many times with reference to chromosome, isn’t it? Homologous chromosomes right! What this means is, in humans; as we are diploid organisms we have total of 46 chromosomes or 23 pairs. Half of these chromosomes come from mother (23 chromosomes) and half of these chromosomes come from father (23 chromosomes). That is how we have total of 46 chromosomes or 23 pairs of chromosomes.

Therefore, for every given pair, one chromosome is maternal and one chromosome is paternal (figure 1). Both these chromosomes in a pair would have the same genes which codes for the same traits. Even the location of all these genes would be same on both the chromosomes (because basically they are the same chromosome but coming from two parents).

Figure 1. Chromosome in pair having the same genes at same locations

Let’s say for example, we are talking about genes which codes for eye color and hair color. Figure 1 shows the position of both these genes on chromosome number 1. Whether it is a maternal chromosome 1 or paternal chromosome 1, these two genes are located on the same place for these chromosomes. This means for every given pair of chromosome, all the genes are same and their location is also same. That is why we call them homologous chromosomes.

Homologous Chromosome = Chromosomes in pair having the same genes (codes for same traits) at same locations (where ever it is present on maternal chromosome, it will be present in the same location on paternal chromosome).

  • Homozygous –

    as just discussed above, we are diploid organisms. This means we have two copies of every chromosome and thus we have two copies of every gene (figure 2). In one of our previous post we had seen that copies of the same gene are called alleles .

In short, alleles are different form of the same gene which might differ slightly in sequence giving us slightly different phenotype such as gene for eye pigmentation can give brown, green, blue etc. pigmentation.

Figure 2. Chromosome pair Homozygous for eye and hair color trait

Figure 2 shows gene for eye color. If both the chromosome 1, maternal as well as paternal codes for the same color eye pigment (blue) that means they have the same allele. In this case they are called homozygous for eye color trait. Let’s say for hair pigmentation both the chromosome 1 has allele for brown hair then they are called homozygous for hair color trait.

Homozygous = when on chromosome pair for a given trait both the alleles are same.

It is also possible that a given chromosome pair has two different alleles for the same gene, right! In that case they are called heterozygous for a given trait.

Remember, we use the term homozygous or heterozygous for individual gene. We cannot use it for the complete chromosome because a chromosome would have many different genes present and for each of them they might be either homozygous or heterozygous. Whereas homologous is the term we use with reference to chromosome in a pair.

I hope this post was helpful 🙂

To understand more details, watch a video on this topic here.