Explained | Genetics and its Mendelian inheritance

Long before genes, genomes and genetics were known, Augustinian monk Gregor Mendel established three principles of inheritance from his experiments on pea plants.

July 22, 2022 04:06 pm | Updated July 23, 2022 05:42 pm IST

File photo of Gregor Mendel, widely called the father of genetics

File photo of Gregor Mendel, widely called the father of genetics

It’s a commonly observed habit in families to draw likenesses between members of the youngest generation and their ancestors. “This baby has a smile like her mother,” one would say, or, “He has exactly the temperament of his grandfather.” This project may be extended by some to even saying, “X writes so well, she has inherited this from her eldest aunt.” If someone should be outstandingly brilliant in math or history, quick follows the comment – “It’s all in the genes.”

Now, many of these traits may not be heritable, or little is known about whether they are inherited. So these statements often are mere speculations; however, they are speculations supported by a pervasive and instinctive knowledge of the laws of genetics and the mechanism of genes and heredity. Of course, it’s one thing to pull out statements like this based on purely anecdotal occurrences and quite another to demonstrate them through years of careful experimentation.

The latter is what Gregor Mendel did. In the 1860s, much before the gene was discovered or the term ‘genetics’ was coined, Mendel, a monk of the order of St. Augustine, discovered the cornerstone of genetics – the laws of Mendelian inheritance— using a methodical approach that is the envy of scientists even today. For this, he later came to be celebrated as the father of genetics.

For eight years, starting with 22 pea plants, he and his disciples allowed the plants to self-pollinate and cross-pollinate and came up with around 10,000 descendent plants. By observing them, Mendel came up with three laws. The first of these is the principle of uniformity – all the progeny of a cross between plants that differ in only one trait will look alike. This can be understood as follows – if the parent plants differ by only one trait – say, one has wrinkled seeds and one has round seeds – but are alike in all other aspects, the progeny will all have round seeds or all have wrinkled seeds depending on which trait is dominant. It was Gregor Mendel who first introduced the concepts of dominant and recessive traits.

But he believed, and showed experimentally, that the progeny contain the other traits within them and pass them on to subsequent generations. Continuing our previous example, if the four identical-looking plants from generation two, all with round seeds, were cross-fertilized, their offspring would have a mix of round and wrinkled seeds in a 3:1 ratio. This showed that even though they themselves had only round seeds, they carried in them something, let us say “particles”, that passed on the trait to their offspring.

Based on the above concept, Mendel put forth the second principle of inheritance, which is that the “particles” which determine these traits are separated during meiosis into gametes, and that meiosis produces an equal number of egg or sperm cells that contain particles carrying each trait.

The third principle he laid down was the principle of independent assortment, which statesthat alleles of one locus segregate into gametes independently of alleles of other loci. This means the choiceof one trait (say round or wrinkled seeds) does not affect the passing on of choices of a different trait (such as white or purple flowers).

With these laws, Mendel established the foundation of genetics even though he did not know anything about genes, genomes and genetics. No wonder, then, that he is known as the father of genetics. He submitted these findings and his observations of eight years in 1865 to the relatively lesser-known journal published by the Natural History Society of Brno (now in the Czech Republic).The paper was not noticed or celebrated much when he was alive. When Mendel died in 1884, all his papers were burnt, making it really difficult to trace out his contributions.

Decades later, in 1900, Mendel’s paper was independently discovered by three scientists working in the field. Further work has revealed much more complex details of genetics and inheritance, but Mendel has a place that cannot be denied.

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