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Gregor Mendel’s Laws of Heredity

Posted in Discoveries, Nature, Science on Monday, 28 February 2011

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This edited article about Gregor Mendel’s laws of heridity originally appeared in Look and Learn issue number 909 published on 23 June 1979.

In the middle of the last century there lived in Austria an abbot called Gregor Mendel. He is usually described in history books as an amiable amateur scientist, who called the plants he experimented with his “beloved children”.

Gregor Mendel's theories on inheritance. Illustration by L R Brightwell

Gregor Mendel’s theories on heredity. Illustration by L R Brightwell

To the world, this monk was to give knowledge of the laws of heredity.

In Mendel’s day, little was known about why children should resemble their parents. It was known that both the male cell and the female cell contributed to the child’s characteristics, but in what way it was not understood.

Mendel decided to investigate the problem, and he chose for his experiments two kinds of pea plants, one very tall and the other very short, or dwarf.

He fertilized the dwarf plants’ female cells by transferring to them with a brush some of the pollen (male cells) from the tall plants.

He realized that somehow the seeds that were formed must contain the characteristics of the new plant that would grow from them. But when he grew these seeds the following year, the plants were all tall. None of them was short.

Mendel was very surprised with this result; but it made him even more determined to continue with his experiments. He decided to allow these tall plants (first or F1 generation) to fertilize themselves – pollen from each plant fertilizing the female cells on the same plant.

When the seeds grew, some of the plants were again dwarfs. It was as though the factors that made a plant a dwarf had been hidden for one generation, and had come to light again in the next generation (F2).

Of course, at that time no one knew anything about the genes and chromosomes which really carry the information about your characteristics. So how was Mendel to explain his results?

Mendel suggested that tallness and shortness were carried in the plant by special “factors”. When a male cell and a female cell combined to produce a seed, these factors separated and then recombined. The seed received factors for height from both parents – one for tallness and one for shortness. But these “factors” did not blend together to make the plants medium height. All the plants were tall. So Mendel concluded that the factors for tallness and shortness remained independent, but the tall factors “dominated” over the short factors.

The factor for dwarfness remained hidden, but when the plants that appeared tall produced their own seeds, some of the seeds received factors for dwarfness from both parents and these seeds grew into dwarf plants.

A quick way of seeing what happened is to make a little diagram.

Suppose we call Mendel’s “factor” for tallness T. Since the tall plant has factors for tallness from both its parents, we can represent its factors for height by the letters TT. The short factor we can call t, and the short plants will be represented by tt.

When the seeds are formed, they receive a tall T factor from the tall plant, and a short t factor from the short plant. They are therefore called Tt, and they all look tall, because the factor T dominates over the factor t.

When these tall Tt plants produce their own seeds, some of the seeds grow into dwarf plants because the seeds have received factors for shortness from both parents.

You can see that one out of every four of the last generation plants should be short. This is exactly what Mendel found. You can also see that two of the tall plants had factors for both tallness and shortness just like their parents.

We know now that Mendel’s factors do exist – as little particles called genes which are banded together in the cell nucleus into dark stringy objects called chromosomes. What was so amazing was that Mendel should have stumbled upon the explanation years before chromosomes were even discovered.

When Mendel first read out his results to a small Austrian scientific society, his words fell on deaf ears.

Yet 40 years later, when chromosomes had been discovered, Mendel’s work was reappraised and published. Since that day, in 1901, Mendel’s laws have been the basis of our knowledge of heredity and Mendel has taken his rightful title as one of the most brilliant men of science.

When we come to look at human beings, and their genes, we realize that in some ways Mendel was lucky in choosing the pea to experiment with. In the pea, tallness is determined by a single factor T, and shortness by a single factor t.

But in man, there are perhaps a million genes in the chromosomes, and few of these genes determine by themselves a single characteristic like height, or intelligence.

But when we do find certain characteristics in man behaving exactly according to Mendel’s laws, we can infer that there are single genes controlling the characteristics.

One of the things that scientists have studied is the factors that make a person a boy rather than a girl. This is rather easy to study, because the factor involved is not just a single gene – it is a whole chromosome (group of genes).

In a human, the forty-six chromosomes that are in every living cell can be arranged in twenty-three pairs, and in each pair one chromosome comes from the father and one from the mother.

Usually the members of each pair are alike. But in every boy, there are two chromosomes (the twenty-third pair) that do not make a proper pair: one is short and the other long. In a girl, the twenty-third pair is quite normal – both chromosomes are long. The short chromosome in a boy is called the “Y chromosome”. The long ones, which are like the other chromosomes in the nucleus, are called the “X chromosomes”.

Suppose we imagine the children born to a father with his XY chromosomes, and a mother with her XX chromosomes.

Some of the children will receive a Y chromosome from the father and an X chromosome from the mother. These children will be boys. Others will receive an X chromosome from the father and an X chromosome from the mother. These children will be girls.

In the overall population we usually find that half the babies born are boys and half girls. Some families have only boys, and others only girls, but no-one really knows why this should be so.

Other things that behave according to Mendel’s laws are the blood types that you inherit from your parents. If you ever need a blood transfusion, your doctor will first make sure what kind of blood you have.

Before blood groups were discovered in the 1920s, giving blood transfusions was almost impossible. When doctors gave new blood to a patient the blood cells that carry oxygen around the body clotted together and died.

It is now known that every person has a special blood type that is inherited from his parents. When he is given blood that is different from his own, this clotting may occur.

One of the things that determine your blood type is whether you have in your blood special sub stances which are called A and B. These substances determine what kind of blood you can accept.

It is inferred, from seeing how these blood substances are inherited from parents, that single genes control their manufacture.

Suppose, for instance, two people with a blood containing A and B substances marry. Some of their children could have AB blood like them, others could have B blood and others A blood.

Unfortunately there are few human characteristics that are as simple as blood groups. Most of the things that make you unique, like your height, or the colour of your eyes and hair, are determined by many genes acting together.

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