Principle of Inheritance —We all are now well aware of the basic concepts of genetic material. In the previous blogs, we have briefly discussed the inheritance character in the basics of genetics. So today, we will discuss in detail the Principle of Inheritance, Flashback Before Discovery of Principle of Inheritance, Some Flashback Theories, What are Mendelian Genetics and Principle of Inheritance and Inheritance of One Gene.

At a Glance

What do you think is the reason behind the resemblance in some phenotypic characters of you and your parents?. Or what makes mango seed germinate and develop only into a mango tree and not into an apple tree? So, the reason is the principle of inheritance of the genes present in the genetic material. Gregor Johann Mendel is the person who resolves all these mysteries through his experiments.

Mendel was a mathematician, meteorologist, and biologist too. Born in a German family and now recognized as a founder of the modern science of genetics. As a biologist, he was well aware will all the basic researches in genetics. But still, the exact reasons and mechanism behind the passing of hereditary characters from parents to their offspring were unclear.

He devoted seven years from 1856 -1863 to study hereditary characteristics and established rules of heredity. In this blog, we will discuss in detail about

What is the principle of inheritance? And

what are the patterns of inheritance?

Flashback Before Discovery of Principle of Inheritance 

The study of genetics has been a vast field of work since the ancient era. Many great peoples like Hippocrates, Aristotle gave different influential theories in the world of genetics. Darwin also contributed to evolution which somehow bears the base of genetics. Hippocrates’ theory based on Anaxagoras shows similarity with the pangenesis theory of Darwin that we have discussed in the evolution blog.

Some Flashback Theories

Before discovering the principle of inheritance with Mendelian genetics, heredity was based on the various hypotheses nearly towards mendelian genetics but remain unsolved or unexplored. One of the theories given by Aristotle and according to that theory the form giving principle of any organism is transmitted through the semen and mothers menstrual blood interaction in the womb that leads to the early development of organisms. This theory had a good time until the actual detail theory came into the picture during the 19th century.

Not only this, but since the 300CE, the Indian medical writers had written some major facts in the Charaka Samhita. They have written the factors which can determine the characteristics of the child to be born. According to their believes and observation, those factors are from mothers’ reproductive material, father’s sperm factors that attendant with the soul which enters into the fetus, and some through the diet taken by the mother.

Summing up all these factors, at last, the parent’s and soul’s karma will decide the child’s attributes and characteristics. Though these theories were based on hypotheses still made an impact on various study streams related to genetics.

One more ancient theory that I found interesting was the theory of Preformation given by the Anaxagoras and was in trend from the 17th to 19th century. According to this theory, scientists believe that the entire organism is preformed in the sperm or egg just needs to unfold and grow. It means you don’t need to put much effort for the baby. It’s ready-made in folded form. You just need to give the command for growth, and it will fully develop or unfold like a folding chair.

What are Mendelian Genetics and Principle of Inheritance?

Coming to the real picture, let’s discuss mendelian genetics and inheritance. What is inheritance? It is the act of passing something. Now heredity is the transmission or inheritance of the heritable characters from parents to their offspring. Baldness can be the best example of heredity also some diseases like diabetes, cancer, etc.

 Mendel selected the plant species Pisum Sativum (Garden Pea) to prove the inheritance of hereditary characters. Then, he performs the hybridization on the garden pea. For that, he selects some characteristics that can show opposing traits in the same plants. Those characters based on the tall or dwarf height of the plant, yellow or green color seeds, etc., use of large sample size helps him to confirm observed results several times. Only true-breeding plants were used in the experiment. Because they can self-pollinate continuously, stable traits of inheritance and expression for several generations. Fourteen true-breeding plants were selected for the study.

As we know, every gene is the responsible factor for the inheritance of any trait. This gene is present in pair of alleles. That is when genes get to express both the alleles get expressed. For example, if AA or aa are genes, then A or a are alleles.

Mendel performed some crossing or hybridization experiments on the selected plants are as follows.

Inheritance of One Gene

To observe the inheritance pattern, Mendel crosses two plants selected based on one opposing character that is tall and dwarf. After finishing the cross, he collected the seeds from the resultant plant to germinate. He got the first filial progeny that is the F1 generation. The entire F1 generation found was tall, no dwarf, and the same result was shown by others too. In all the results, only F1 generation was expressed. From these results, it is observed that there is something that is passing the trait of tallness.

Hence the experiment was continued by self-cross of F1(tall) generation (F1(tall) × F1 (tall)). And surprisingly this time, the combination of dwarf and tall plants was seen. The proportion of dwarf plants was 1/4, and that of tall plants was 3/4. From all the F1 and F2 generation results, Mendel concluded that there is something that is transferring from generation to generation. And that something is a ‘Factor’ which we have known as ‘gene’ nowadays. Therefore, a gene is the unit of inheritance that is proved.

Remember that whenever we talk about descriptive characters or terms like tall and dwarf, it’s called Phenotype in the language of genetics. And when it’s about the ratio of presence of phenotypic characters, it is Genotype.  

For better clearance, alphabets were used to show the trait’s ‘T’ for tall ‘t’ for a dwarf. We know that a gene is a pair of alleles that represents different traits.

From all the previous experimental observations, Mendel proposed some rules to explain the behavior of the different alleles and their expression in living organisms. In upcoming blogs, we will discuss in detail all the rules till the time we stay connected.