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Look at yourself. Are you the product of nature or nurture? Sometimes it may be both, especially for things like food. The stuff we eat are as alien to nature as we are to our ancestors. We had purposely bred crops and animals to carry on traits that are beneficial to us: fatter chickens, sweeter fruits, greater yielding grains, for instance. All of these are impossible without an understanding of heredity...
What about yourself? Are you a product of nature or nurture? |
"It is willingly granted that by cultivation the origination of new varieties is favored, and that by man's labor many varieties are acquired which, under natural conditions, would be lost; but nothing justifies the assumption that the tendency to formation of varieties is so extraordinarily increased that the species speedily lose all stability, and their offspring diverge into an endless series of extremely variable forms."
Gregor Mendel, 'Experiments in Plant Hybridisation', 1865
The basic rules of genetics were first discovered by a monk named Gregor Mendel in the 1850s, and published in 1866. For thousands of years, people had noticed how traits are inherited from parents to their children. However, Mendel's work was different because he did experiments on plants, and designed those experiments very carefully.
In his experiments, Mendel studied how traits were passed on in pea plants. He started his crosses with plants that bred true, and counted characters that were either/or in nature (either tall or short). He bred large numbers of plants, and expressed his results numerically. He used test crosses to reveal the presence and proportion of recessive characters.
In his experiments, Mendel studied how traits were passed on in pea plants. He started his crosses with plants that bred true, and counted characters that were either/or in nature (either tall or short). He bred large numbers of plants, and expressed his results numerically. He used test crosses to reveal the presence and proportion of recessive characters.
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The Man and The Discovery
Gregor Johann Mendel, born in Heinzendorf, Austria on in 1822, was an Austrian monk and botanist who founded genetics by his work cross-breeding pea plants. He discovered dominant and recessive characters (genes) from the crosses he performed on the plants in his greenhouse. What he learnt is known today as Mendelian inheritance. Mendel used the edible pea (Pisum sativum) for his crosses. He selected seven characters which were distinctive, and never blended; they occurred as either-or alternatives. Examples: plant height (short or tall); colour of peas (green or yellow); position of flowers (restricted to the top or distributed along the stem). When he crossed varieties which differed in a trait (e.g. tall crossed with short), the first generation of hybrids (F1) showed only one of the two alternatives. One character was dominant, and the other recessive. But when he crossed these hybrids with each other, the recessive character reappeared in the second (F2) generation. The proportion of plants showing the dominant as opposed to the recessive character was close to 3 to 1. Further analysis of the descendants (F3) of the dominant group showed that one-third of them were true-breeding and two-thirds were of hybrid constitution. The 3:1 ratio could therefore be rewritten as 1:2:1, meaning that 50 percent of the F2 generation were true-breeding and 50 percent were still hybrid. This was Mendel’s major discovery. |
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It could all be summed up by saying that inheritance was not blending, as Darwin had thought, it was particulate. The factors (genes) were not merged or mixed, they stayed separate and were passed on to the next generation unchanged.
He published his work in 1866, but at the time no-one saw how significant it was. 35 years later, the papers were rediscovered and, immediately, modern genetics began.
Mendelian Inheritance
Mendel explained the results of his experiment using two scientific laws:
1. Factors, later called genes, normally occur in pairs in ordinary body cells, yet separate during the formation of sex cells. This happens in meiosis, the production of gametes. Of each pair of chromosomes, a gamete only gets one.
The factors (genes) determine the organism's traits, and are inherited from its parents. As the pair of chromosomes separate, each gamete only receives one of each factor. This Mendel called the Law of segregation.
Mendel also noted that versions of a gene could be either dominant or recessive. We call those different versions alleles.
2. Alleles of different genes separate independently of one another when gametes are formed. This he called the Law of independent assortment. So Mendel thought that different traits are inherited independently of one another.
The second law is only true if the genes are not on the same chromosome. If they are, then they are linked to each other. This was the next great discovery after Mendel: that genes were carried on chromosomes. The closer they were on the chromosomes, the less likely was crossing over between them.
He published his work in 1866, but at the time no-one saw how significant it was. 35 years later, the papers were rediscovered and, immediately, modern genetics began.
Mendelian Inheritance
Mendel explained the results of his experiment using two scientific laws:
1. Factors, later called genes, normally occur in pairs in ordinary body cells, yet separate during the formation of sex cells. This happens in meiosis, the production of gametes. Of each pair of chromosomes, a gamete only gets one.
The factors (genes) determine the organism's traits, and are inherited from its parents. As the pair of chromosomes separate, each gamete only receives one of each factor. This Mendel called the Law of segregation.
Mendel also noted that versions of a gene could be either dominant or recessive. We call those different versions alleles.
2. Alleles of different genes separate independently of one another when gametes are formed. This he called the Law of independent assortment. So Mendel thought that different traits are inherited independently of one another.
The second law is only true if the genes are not on the same chromosome. If they are, then they are linked to each other. This was the next great discovery after Mendel: that genes were carried on chromosomes. The closer they were on the chromosomes, the less likely was crossing over between them.
Mendel's laws explained the results he got with his pea plants. Later, geneticists discovered that his laws were also true for other living things, even humans. Mendel's findings from his work on the garden pea plants helped to establish the field of genetics. His contributions were not limited to the basic rules that he discovered. Mendel's care towards controlling experiment conditions along with his attention to his numerical results set a standard for future experiments.
Mendel never enjoyed recognition in his lifetime. In fact, it was more than three decades later, in 1900, that three scientists doing agricultural research discovered his 1866 paper. Erich Tschermak, Hugo de Vries, and Carl Correns independently verified several of Mendel's experimental findings, and the "age of genetics" was born. In the next several decades, scientists would learn more about genes and the special substance called DNA that carried each living thing's specific traits.
Mendel never enjoyed recognition in his lifetime. In fact, it was more than three decades later, in 1900, that three scientists doing agricultural research discovered his 1866 paper. Erich Tschermak, Hugo de Vries, and Carl Correns independently verified several of Mendel's experimental findings, and the "age of genetics" was born. In the next several decades, scientists would learn more about genes and the special substance called DNA that carried each living thing's specific traits.
Ponder this
Why did Mendel chose peas as the subject of his studies? What sort of factors affect his choice? Are there better alternatives?
Does heredity only attributed to physical traits? What about behavioral or cognitive traits such as violence or intelligence?
Discuss
Lets assume if we are to apply the same principles of selective breeding on humans. What sort of traits are we looking for to maintain? What traits do we want to eliminate? How do we go about doing this?
Further readings
Gregor Mendel, at Biography.com
Mendelian genetics, a rather comprehensive coverage of the topic at the north Dakota State University.
StarGenetics Mandelian Genetics Simulator, a teaching software developed at MIT that allows student to simulate and analyse breeding experiments.
