Before the beginning of heredity as a huge science, many scientist were wondering about special features that were found in animals and how it transported from one generation to another like the tall neck of a giraffe.Some explain that the old giraffes used to stretch their necks to reach high trees, you know it seems funny if we try to apply this theory on humans. For example, we may have long arms if we stretch them in the gym, and the snake known as “cobra” has a flat neck that maybe because it squeezes it self under a tree! This is totally unbelievable. Then the well-known scientist “Charles Darwin” started to establish his theory which is known as “natural selection”.
Darwin and Natural Selection:
Charles Darwin has a well-known theory which is natural selection. He used three ideas to drive it.
First:
All living things are able to have many offspring than they actually do. Darwin chose to illustrate this with the slowest breeding animal he knew, the elephant, assuming that it start to breed in its 30s, stopped when it is 90 and has six young over 60 years. He suggested that after 740 years there would be 19 million elephants existing, descending from the first pair.
Second:
Elephants face environmental pressures such as limited supply of resources. The trees and grass they eat can not multiply as elephants. So in every generation some generation will starve to death.
Finally:
In each generation, the offspring slightly differs from one another. The members who are strong enough will survive and increase in number and will dominate over weak members.
Humanity owes a great deal to peas and drosophila:
Mendel...the father of heredity:
Gregor Mendel was a monk and like many monks at that time, a scientist. Born as Johann Mendel, he changed his name when he entered the monastery, north of Vienna in what is now the Czech republic.
Mendel was a gifted scientist who set out to discover how new species arose. He studied verity of peas that differs in single easily recognised traits. Some were tall, others short, some have white flowers, others purple. In all, Mendel worked with seven pairs of traits. Mendel made hybridisation between two kinds of peas, that has not happened in nature, and sat to study difference in the first and second offspring f1, f2. His notes stated that in the first generation one character is dominate over another but in the second generation he found that the weak character appears in low present. From these trials Mendel put three laws.
Mendel Laws of Inheritance:
1st the law of uniformity:
When plants that differ in a particular trait are crossed with each other, the offspring are uniform and resemble one parent.
2nd the law of segregation of alleles:
The alleles in the parents separate and recombine in the offspring. This explains why the expression of a particular trait in an organism.
3rd the law of independent assortment of alleles:
The alleles of different characters pass to the offspring independently so inheritance of, say, green peas says nothing about whether the peas will be wrinkled or smooth.
The researches of Mendel were ignored for a long time and no one could understand his theory. Until three men simultaneously and independently did a similar experiment and finally understood Mendel’s work.
They were the Dutch botanist Hugo De Vries, an Austrian, Erich Tschermak and German Carl Correns. Through Hugo De Vries’ trials he discovered a new phenomena in heredity, which is “mutation”. As he studied evaluation by making crosses between garden varieties of evening primrose plants. Among the offspring he grew, a few were noticeably different from there parents. De Vries coined the term “mutation” to describe there sudden changes.
Morgan’s Drosophila:
Morgan arrived at Columbia University in 1904 to determine development. He started to chose suitable animals for his study. He found that mice and rats bred too slow for his purpose. Morgan turn to the fruit fly, drosophila. It is small, a thousand or more flies can fit into a small bottle. It breeds quickly, producing 30 generations a year, it is easily observed as its eggs develop outside the female’s body. As consonance plays an important rule in heredity, as in case of Mendel and de vries. Also with Morgan as the study of development and heredity in fruit fly allowed him to make a new discovery which was that some characters are related to sex and transported to the generation attached to the sex chromosome.
But what is “chromosome” and what carries these features through generations?
In the Next issue we will know more about genes, chromosomes, and we will try to discover the mystery of the double helix “DNA”.