What you see on the screen are called 4’o clock plants: the plants that bloom with flowers usually in the evenings! We know they are used for decorative purposes, but can you think of any other uses for them? These plants have immense importance in the field of cytology and genetics! These plants were first used by the German botanist Carl Correns for his cytological and genetic studies. They were used to explain an interesting concept of Cytoplasmic inheritance. Also, these plants are used by many geneticists to study incomplete dominance! What does this mean? So far, we have studied only “Dominance” of characters in the pea plant. What does this concept of “Incomplete Dominance” mean? Let us begin with understanding the name! Incomplete here means partial. Does it indicate that the dominance is partial? That’s right! The dominant character expresses itself, but not completely. It means that the dominant alleles do not completely mask the effect of the recessive alleles. As a result, offsprings with intermediate characters are obtained. This is the reason why the phenomenon is also called “Partial Dominance” or “Semi-dominance”. Let us get back to our 4 o’clock plants to understand this! In this case, if a cross of homozygous red and white flowered plants is carried out, then the resultant offsprings are pink in colour. So the blend of red and white gives us a pink colour! This is because the dominant allele for the trait “red” does not completely mask the effect of the recessive allele for the trait “white”. So when the alleles are expressed in the offsprings, they give an effect which is like a blend of both! This is the reason why we get pink flowers! The same feature is seen in a snapdragon flower. A red variety crossed with a white variety gives us pink flowers! However, this is not a complete exception to Mendelism. If the pink flowers are crossed, then the F2 generation gives us the typical 1 to 2 to 1 genotypic ratio! How does this happen? Let us assume “RR” in capital to be the genes for Red flower colour. Let “ww” in lower case be the genes for representing white flowers. On crossing the two, we get all the pink flowers with the genotype “Rw”. Now till this step, we find the inheritance to be an exception to Mendelism. That’s because Mendelian genetics did not talk about blending of characters! However, on crossing any two from the F1 generation, we obtain the genotypic ratio of 1 to 2 to 1 in the F two generation. Even though the phenotypic ratio is not ‘3 to 1’, the genotypic ratio obtained is the same as we had obtained earlier! So this pattern of inheritance seen in snapdragon is an exception to Mendelism, but not entirely. Because the F2 generation still exhibits the genotypic ratio ‘1 to 2 to 1’! And if you noticed well, this is the phenotypic ratio as well! Now this was about incomplete dominance in plants. Is this seen in animals as well? Oh yes, it is seen in human beings too! Out of the many characters that show this kind of pattern, hair type is the one which exhibits the pattern of incomplete dominance! Curly and straight are the two types of hair traits which we know. Crossing these types gives us the third phenotype which is wavy hair. This is nothing but an example of incomplete dominance. Let us try out the theoretical cross now. Let us assume “CC” in capital to be the genotype for curly hair. Similarly, let “ss” in lower case be the genotype of straight hair. Here, the trait of curly hair type is dominant over the trait of straight hair which is recessive. Can you solve this cross further to find the genotypic ratio in the F two generation? This is what we get on solving this further! The F1 generation will all have Wavy hair; that is a blend of curly and straight hair. So all the offsprings here will have “C s” as the genotype and Wavy hair as the phenotype. And the F2 generation will be obtained in the ratio ‘1 to 2 to 1’, both phenotypic as well as genotypic! This is how incomplete dominance is found not only in plants but even in animals. But how does it help us? When it comes to plants, we obtain several beautiful phenotypes which can be used for many purposes! Animals exhibiting incomplete dominance are excellent models for genetic studies. Are there other types of exceptions to Mendelism? Yes! There is one more in the list! Let us have a look at it in the next part! Zeki visited his uncle’s poultry farm where he was amazed to see a variety of fowls around! However, what caught his attention was this rooster! Why? Notice that it has both: black and white feathers. Zeki was curious to know about this fowl. His uncle explained that such kind of chicken are commonly obtained when a black feathered and a white feathered chicken are bred! But this is a simple account. It does not explain the reason for this chicken to have checkered pattern or speckling feathers! Any idea how this pattern of feathers is obtained? Let us understand it with the help of Genetics! Yes! The logical and correct explanation of this pattern is given by the phenomenon called “CODOMINANCE” in genetics. This is another type of exception to Mendelism. The term “Codominance” can be divided into two parts, “CO” and “Dominance”. We know what “co” stands for. It means mutual or together. Just like “co-operation” which means operating or working together! The second part is dominance. And we know what dominance means! An allele that masks the effect of the other recessive allele is said to be the dominant one. This gives rise to a dominant phenotype! Now the question is: how do the two terms “CO” and “Dominance” come together to give us the effect? And what type of effect is exactly seen in the organisms? “Codominance” is the phenomenon where both the dominant alleles are equally strong and thus expressed in the offspring simultaneously. So the offspring will be heterozygous and will have both dominant genes in the genotype. As a result, the effect of both the alleles will be seen in the organism phenotypically. How do you think we got this chicken with checkered feathers? In such a case, we cross a black feathered chicken with a white feathered one. To understand this theoretically, let us say that we have the genes “BB” in capital for black coloured feathers and “WW” in capital for white coloured feathers. On crossing the two, we get a progeny with all heterozygous genotypes. Each offspring has one “B” and one “W” allele. That means each chicken has both the dominant alleles. And both the dominant alleles try to express themselves. As a result, we get both the characters expressed in all the offsprings phenotypically. Thus, all the chicken have checkered or in other words speckling feathers! Now this is like an exception to Mendelism, but not totally! Just like incomplete dominance, even Codominance follows a pattern. The F2 generation is obtained in the typical genotypic ratio 1 to 2 to 1. And the offsprings in the F2 generation exhibit the same ratio phenotypically as well! We get 1 black, 2 speckled and 1 white offspring in the F2 generation. This is the phenotype. And what about the genotype? We get one offspring with the genotype “BB”, two with “BW” and the last with genotype “WW”. This is how Codominance is seen in the chicken which gives us a completely different phenotype. Codominance is observed in humans as well when it comes to Blood grouping! Let us understand this in detail. How many blood group types do we have by the way? There are many types actually, but we mostly take into consideration the ABO type. And this type divides has how many groups? There are basically 4 groups according to this system. These are A, B, O and AB group. Let’s say this is a single red blood cell. There are various components attached to its surface which are called antigens. These can be long chains of proteins or sugars. Now in case of the blood grouping system, the different groups are classified on the basis of these antigens. So if the RBC has an “A” antigen, then the blood group is “A”. Similarly, presence of “B” antigen categorises the blood as “B”. In case of “O”, there are no antigens present at all! And now can you guess which antigen will be present on the RBCs if the group is “AB”? That’s right! The RBCs will have both the antigens “A” and “B” present on their surfaces. Getting back to our concept, how is Codominance seen in these blood groups? In the case of human blood groups, the types “A” and “B” are both dominant, while “O” is the recessive type. Now if the offspring has one allele for “A” blood group and the other for “B” blood group, then the phenotype obtained will be “AB” blood group. This is how Codominance is found in humans in the case of blood groups. Apart from humans, there are several other animals that exhibit Codominance. Cattle for example have roan coat colour. This is obtained on crossing cattle having white coat colour with cattle having a red coat colour. Many other animals have a skincoat with two colors, or have patches on their skin coat, which are examples of codominance. But note that not all animals with patches will always be examples of Codominance!