Introduction

Mendelian genetics – we have seen how the one gene inheritance occurs and how Mendel discovers the principle of inheritance. Today in this blog, we will continue the concept of Mendelian genetics, Punnett Square for Mendelian Genetics, Laws of Inheritance in Mendelian Genetics, The Law of dominance, What Is It Law of Segregation What Is Incomplete Dominance and Co-dominance. We know that the hereditary factors will pass from generation to generation, whether positive or negative. Positive in the sense of characteristics like good looks, color, eye lenses color, etc. Negative is any disease condition like cancer, diabetes, etc.

Genes are present in the allelic pair form. For example, in true-breeding tall or dwarf pea plants, the pair of alleles for tall and dwarf plants are identical or homozygous TT or tt. Hence the genotype is TT, and the tt while phenotype is the tallness and dwarfness of the plants. Then what do you think is the phenotype for the genotype Tt?

Initially, when Mendel got the heterozygous genotype after self-crossing F1 generation, he found that the heterozygous Tt plant shows a similar appearance to the TT tall parent plant. The complete experimental study concluded that in the pair of dissimilar alleles or factors, one is dominant over the other, like in the F1 generation. Henceforth the terms Dominant and Recessive are coined.

Due to the dominance of the T allele, over t shows identical characteristics with the parent TT plant, which is confirmed by the identical results with other crosses. Therefore, using capital and lower-case alphabets is quite convenient, and easy to remember the dominant and Recessive terms. However, we cannot use T for tall and d for a dwarf as it creates confusion for finding the alleles of the same or different gene or trait.

Punnett Square for Mendelian Genetics 

It is a graphical representation to calculate the probability for all the possible genotypes of offspring in a genetic cross. It was discovered by the British geneticist Reginald C. Punnett.

It clears that Tt’s case shows a tall phenotype that is ‘T’ is dominant over ‘t.’ That is, ‘T’ is a dominant character and ‘t’ is a recessive character. Recessive characters are mostly seen in the wild-type plant species. 

Crossing between the TT and tt is a monohybrid cross because it gives Tt as a result that controls only one character, height, which is termed monohybrid. When tall and dwarf plants produce gametes by meiosis, the separation of the parental pairs occurs. Therefore, among two alleles, only one is transmitted to a gamete. That means a 50% chance of a gamete containing any one allele is verified by crossing. 

One gamete TT gives two alleles, T and T, similarly tt givest and t. This whole calculation is get understood by the Punnett Square Method.

Here in the above monohybrid cross, when TT tall parent plant is cross with tt dwarf parent plant all the F1 generation is heterozygous Tt but tall due to presence of dominant allele T. When F1 is self- cross then it gives a mixture of tall and dwarf plants. So the ratio of tall and dwarf plants is 3 tall and 1 dwarf. And genotype is one TT, one tt, and two Tt.

When fertilization occurs, the pollen grains of genotype T have a 50% chance to pollinate the egg of the genotype T and t. Similarly, the genotype t has a 50% chance to pollinate the egg of genotype T and t.

Laws of Inheritance in Mendelian Genetics

The reproductive process gives rise to new individuals that are similar but subtly different. We know how the amount of variations is produced even in asexual and sexual characteristics. However, the most obvious result of the reproductive process still remains the generation of individuals of similar design is heredity. Mendel proposed the law of inheritance and proved the transfer of hereditary characteristics from parents to their children. 

To consolidate understanding of inheritance in the monohybrid cross, Mendel proposed some rules. And those laws or rules are now recognized as Laws of Inheritance. The first law is the Law of Dominance, and the second law is the Law of Segregation. 

The Law of dominance 

According to this law, all the characters are controlled by discrete units called alleles or factors. Among both the alleles, one is dominant over the other, which is recessive. And factors or alleles are always present in pairs. This law is used to explain only one parental character in the F1 generation and expression of both in the F2 generation for a monohybrid cross. In short, in heterozygous pairs, the allele that expresses is dominant. 

What Is It Law of Segregation

This law describes that the alleles do not show any blending and recovers both the parental characters. Though the F1 generation does not show all the parental characters, they get segregated or shown by the F2 generation. For example, when we cross true-breeding homozygous tall and true-breeding homozygous dwarf parents, that results in all heterozygous tall plants (F1). F1 generation does not show true-breeding homozygous tall or homozygous dwarf parental character. 

On the other hand, when we self-cross heterozygous F1 generation, it shows all the three types of genotype that is one homozygous tall (TT), two heterozygous tall (Tt), and one homozygous dwarf (tt) in the F2 generation. That is, all the characters segregate in the F2 generation. In addition, two more laws were included under this category are Incomplete Dominance and Co-dominance.

What Is Incomplete Dominance and Codominance

Incomplete dominance 

In simple language, when two parental traits are crossed and result in both parental traits and one additional trait different from parental traits, it is termed incomplete dominance. For example, we cross the true-breeding red flower (RR)plant with the true-breeding white flower (RR) plant. It results in the same genotype expected through mendelian monohybrid cross that is 1(RR):2(Rr):1(RR). But the phenotype is different that is a red flower, pink flowers (heterozygous), and white flower. 

The pink phenotype occurs due to red and white flowers crossing, which means the R dominant allele is not completely expressed. Instead, it is expressed partially and hence shows pink color instead of red color. This concept of partial expression is termed incomplete dominance.

Co-dominance 

Humans have different blood groups, namely A, B, AB, and O. These are the most common blood groups. The types of blood groups are the best example of co-dominance. Co-dominance occurs when two dominant alleles express themselves together. For example, in the human body blood group is controlled by the gene I. Gene I has three alleles, namely IA, IB, i. among all the three alleles IA and IB are dominant. 

The plasma membrane of RBC has sugar polymers that protrude from the surface and kind of sugar controlled by the gene. IA and IB produce slightly different sugars, while i do not produce any kind of sugar. This is because human beings are diploid organisms. Each person possesses any two of three I genes, and the dominant one will express. 

If the IA and i allele are present, the blood group is A. (because i does not produce sugar). The following table will explain more easily.

Parent 1 allele	Parent 2 allele	Genotype of child	Blood type of child
IA	IA	IA  IA	A
IA	IB	IA  IB	AB
IA	i.	IA i.	A
IB	IA	IA IB	AB
IB	IB	IB  IB	B
IB	i	IB i	B
i	i	i i	O

We can also study it under the examples of multiple alleles because here, three alleles govern the same function. So dominance is not an autonomous feature. It depends on the gene product and the phenotype produced by that product. 

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