Heredity and evolution

Heredity- Mendelian Genetics

Do you know what is heredity? We have heard of the word hereditary. What does this word mean? The word hereditary is derived from the word heredity. The word hereditary refers to some trait or characteristic or even an item that is inherited from our parents.

In the article on reproduction, we had learned that both the parents contribute an equal amount of genetic material to the offspring. Thus, for each trait, we inherit one copy from our mother and one from our father. So, which trait will be expressed in the offspring? Are there any rules that determine which character is expressed and to what extent?

These were some of the questions on heredity that were answered by Gregor Mendel, an Austrian scientist. Mendel is often hailed as the father of modern genetics. He performed many experiments to elucidate the nature of heredity. He took pea plants with many different characteristics like round/wrinkled seeds, tall/short plants, white/violet flowers, etc. He cross-bred these plants and noted the features of the offspring.

In the first generation offspring, he noted that there were no midway characteristics. For e.g. when a tall plant was bred with a short plant, the offspring was all tall. When this offspring was allowed to produce by self-pollination, Mendel noted that a quarter of the offspring was short while ? of the offspring were tall in character. Therefore, he inferred that that the 1st generation offspring or F1 had inherited the tall and short trait from its parents, but only the tall character was manifest in the 1st generation of offspring. He called the tall trait as dominant trait while the short characteristic was the recessive trait.


Mendelian inheritance

A question naturally arises- why were a quarter of plants in the 2nd generation short?

Mendel postulated that the 1st generation offspring would have inherited both traits (tall and short) from its parents. We shall call these traits T & t. So, the tall parent is TT, and the short parent is tt.

The first generation offspring is Tt. When this plant reproduces by self-pollination, then there are four possible combinations that can be produced- TT, Tt, Tt and tt. Therefore, ? of the 2nd generation of plants are short. The characteristic T & t are now referred to as genes.

Based on his experiments with peas, Mendel proposed a set of rules of heredity. Organisms that reproduce by these rules of heredity are said to follow Mendelian genetics. However, not all traits are inherited in accordance with Mendel?s laws. We will learn about non-Mendelian inheritance in another article.

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Modes of reproduction: Asexual Vs Sexual reproduction

How many modes of reproduction are known to you? Do all living beings reproduce in the same way? Obviously not. Plants produce seeds; birds and reptiles reproduce by producing eggs and mammals like humans give birth to live offspring. Each organism?s mode of reproduction is evolved to suit its ecological niche.

There are two modes of reproduction ? asexual and sexual.

Asexual reproduction

Asexual reproduction is that mode of reproduction where an offspring inherits all its genetic material from a single organism. Asexual reproduction is the most fundamental means of reproduction and single-celled organisms like bacteria and protozoa reproduce by asexual reproduction. Organisms can reproduce asexually in many different ways- fission, fragmentation, vegetative propagation, spore formation, etc.

During asexual reproduction, there is no exchange of genetic material. In the previous article on reproduction, we learned that there is a transfer of genetic material during the process of reproduction. However, the transfer of genetic material does not occur in asexual reproduction. The offspring inherits all its genetic material from a single organism only. Therefore, genetic diversity is not high. We also learned that genetic diversity is essential for evolution. So why do organisms reproduce asexually?

There are many reasons why organisms reproduce asexually. Firstly, asexual reproduction is one of the simplest modes of reproduction. Therefore, you will note that most less evolved organisms reproduce asexually. Secondly, asexual reproduction may help with rapid growth in population, especially in a stable environment. And thirdly, some organisms, due to developmental constraints have entirely relinquished the sexual mode of reproduction.

Sexual Reproduction

Sexual reproduction is a mode of reproduction that requires two individuals to mate to produce an offspring. There is an interchange of genetic material during sexual reproduction. Therefore, sexual reproduction helps with genetic diversity.

Sexual reproduction is the most complex of the modes of reproduction. Sexual reproduction requires specialised organs. Therefore, sexual reproduction is mostly seen in multicellular organisms. Gametes are the reproductive cells of an organism. The sperm is the male reproductive cell and the ova or eggs are the female reproductive cells.

The first process in sexual reproduction is meiosis. During meiosis, the number of chromosomes in the gametes are halved. When the two gametes fuse, the number of chromosomes is again restored to the usual number. For example, the human cell contains 46 chromosomes while the sperm and ova contain only 23 chromosomes. However, the fertilised ova, which forms the embryo has 46 chromosomes (23 from the mother and 23 from the father).

Apart from being complex, sexual reproduction is also slow and requires a much higher expenditure of energy.

To summarise, there are two modes of reproduction- asexual and sexual. Asexual reproduction is a simple and rapid method of reproduction but does not maintain genetic diversity. On the other hand, sexual reproduction is a the more complex of the modes of reproduction, requires specialised organ systems and it is useful for maintaining genetic diversity.

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Reproduction: Why do organisms reproduce?

What is a reproduction and why do organisms reproduce? It would be very interesting to listen to the answers to these questions. Reproduction is not an essential life process. Unlike other life processes like respiration or nutrition, if an organism does not reproduce, it is not going to die. Also, reproduction requires the expenditure of energy. If reproduction is a wasteful process, why do organisms reproduce?

There are many different reasons why organisms reproduce. One reason is to ensure the survival of a species. The species is the most fundamental unit in ecology. A species is any large group of organisms that can interbreed to produce offspring. Cows belong to one species and horses belong to another. Cows and oxen can breed to produce calves. However, cows cannot mate with other animals. Therefore, they are two different species.

Imagine the following scenario- What would happen if cows do not reproduce? Then one by one all the cows would die at the end of their lifespan and then the entire species would become extinct. Therefore, organisms reproduce to ensure the survival of their species.

But is the survival of the species, the only reason for reproduction?

Reproduction also has additional functions. Reproduction also helps with maintenance of genetic diversity. Do all members of a species look alike? No. Take humans for example. Does each one of us look alike? Even brothers look different. So how does reproduction affect the way we look?

Previously, we have learned that how we look is a function of our DNA. DNA is the genetic code of life. During reproduction, there is an exchange of DNA and this exchange of DNA leads to an offspring that is different from the parents in subtle ways. Meiosis is the process by which there is an exchange of genetic material during the process of reproduction.

Why is genetic diversity or variation necessary?

Genetic diversity is important for evolution. The environment is always changing. The Dinosaurs used to rule the earth at some point in time but are now extinct. Why did they become extinct? They could not adapt to the changing environment, therefore, they became extinct. However, the cockroach as a species are still thriving. For a species to survive, it needs to evolve constantly to the changing environment. Genetic diversity is important for this adaptive process.

Do you know how bacteria become resistant? Amongst the many millions of bacteria, a few bacteria are resistant to particular antibiotics. When we take antibiotics, the bacterium that is resistant survives while the other bacteria die. The resistant bacteria then reproduce, producing offsprings that are resistant to the antibiotic. Thus, the resistant bacteria thrive and hence the species continues.

Therefore, reproduction is vital for ensuring the continuity of a species and it is also necessary for maintaining genetic variation in the nature.

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