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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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Hematopoiesis – Overview [Blood Cell Formation]

Hematopoiesis

Hematopoiesis, also known as Hemopoiesis is formation and development of blood cells. Meaning of the term hematopoiesis is “Hemo = blood” and “poiesis = to form”. Hematopoiesis occurs in bone marrow and where is this bone marrow located?? To understand this, let us take a cross section of a bone (figure a).

Figure a. Bone Marrow

In figure a we can see the center cavity of bone has the red spongy tissue. This red spongy tissue in the center of the bone is called bone marrow. OK so we are clear with some terminology and we will see few more as we go on J

The starting point of Hematopoiesis is Hematopoietic Stem Cell (HSC) which is a multipotent stem cell. What is multipotent now?? The meaning of multipotent is “multi = many” and “potent = being able” which means the cell has ability to differentiate or give rise to several different types of cells. We will see how HSC which is a pluripotent stem cell gives rise to all different types of blood cells (figure b).

Figure b. Hematopoiesis

The first differentiation of HSC results in two lineages – Myeloid stem cell and Lymphoid stem cell. Now let me tell you couple of points which would make this chart (figure b) very easy to understand and remember.

First, the lymphoid stem cell lineage gives rise to all the lymphocytes and the myeloid stem cell will differentiate in rest of the blood cells (other than lymphocytes). Lymphoid stem cell = all lymphocytes, easy to understand and remember J Second, once the main two lineages are differentiated (myeloid and lymphoid), it will now differentiate into progenitor cells (green in color in fig b), followed by differentiation in precursor cells (blue in color in fig b) which are named by writing suffix “blast”, i.e. proerythroblast, megakaryoblast, monoblast etc. Now this precursor cells will undergo further development and becomes formed element (red in color in fig b).

So briefly it is,

 HSC – myeloid and lymphoid lineages – progenitor cells – precursor cells – formed elements  

Now that we are clear with these points, keeping them in mind let us see the differentiation process.

So first we will see what happens to myeloid stem cell. It will differentiate into Colony Forming Unit – Erythrocyte (CFU-E), Colony Forming Unit – Megakaryocyte (CFU-Meg) and Colony Forming Unit – Granulocyte and Macrophage (CFU-GM). If you look at these progenitor cell, you can understand what it is going get developed in. There are two more cell developing from myeloid cell but they don’t have progenitor cell phase, it will have direct precursor cell stage.

Alright so now once the progenitor cells are formed, the next stage is the formation of precursor cells. As I mentioned before they will have suffix “blast”. So from CFU-E, proerythroblast will form. From CFU-Meg, megakaryoblast will form. From CFU-GM two precursor cell will form as the term suggest, monoblast and myeloblast. And as I said there will be two more precursor cells directly differentiating from myeloid stem cell and they are eosionophilic myelolast and basophilic myeloblast.

Now proerythroblst is going to get developed in reticulocyte, at this point the nucleus of RBC will be ejected and eventually it will developed in erythrocyte (RBC). Megakaryoblast will differentiate in a huge megakaryocyte cell which gets splinter into thousands of small fragments and that is what we call thrombocyte (platelets). Monoblast differentiates into monocyte and circulating monocytes are called macrophages. The remaining three precursor cells are going to give rise to all granulocytes and if you notice that all the precursor cells of granulocyte are named myeloblast, i.e. myeloblast will developed into neutrophil, eosionophilic myeloblast will developed into eosinophil and basophilic myeloblast will developed into basophil.

Now let us see what happens to lymphoid stem cell. It will differentiate into two progenitor cells, i.e. prothymocyte and pre B-cell and they will develop into precursor cells T lymphoblast and B lymphoblast respectively. Here also there is one direct precursor cell formation without the progenitor cell stage and that is NK lymphoblast (again see suffix “blast” in all precursor cells). T lymphoblast will develop into T lymphocyte (T cell), B lymphoblast will develop into B lymphoblast (B cell) and NK lymphoblast will give rise to Natural killer cell (NK cell).

So that is it!! It is very easy to understand once we know all different stages and how they are named. Last but not least this is how you can remember it 🙂

First, you start with HSC which give rise to two lineages myeloid and lymphoid and here the term itself has the answer that lymphoid stem cell give rise to all lymphoid cells which include T cells, B cells and NK cells. All other remaining blood cells are derived from myeloid stem cell.

Second, progenitor cells are formed and in case of eosinophil, basophil and NK cell there is no progenitor cell.

Third, the progenitor cells differentiates into precursor cells (suffix “blast”) which then differentiates in to formed elements and they are nothing but our blood cells.

For better understanding watch this video on Hematopoiesis.