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It grows easily and abundantly.

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LIVINGS

In

BIOLOGY

FOR

GENERAL SECONDARY CERTIFICATE

Unit (II)

Chapter (1)

The Inheritance in Livings

Historical View

Historical View

Genetics:

Is the branch of Biology that studies the inheritance of characteristics in Livings from parents to offspring generation after generation.

Gregor Mendel:

He born in 1822 in Brunn, Austria. He carried out experiments on Pea plant in the garden of a monastery at Brunn. In Mendel’s age, there was a belief that both parents share in transmitting their characteristics to the offspring. Recently, it became known that the factors of heredity are transmitted through the reproductive cells which are called gametes (sperms in males, and ova in females). The results of Mendel’s experiments concluded that the inherited characteristics are transmitted through separate units and segregate in different ways each generation. These units were called later, genes.

Mendel’s experiments:

Mendel studied 32 varieties of Pea for many years before beginning his experiments. As a result of his observations, he selected only seven characteristics for his experiments, these that appeared to be clearly different in varieties of the plant. As examples:

- He obtained one variety of yellow seeded plant (pure breeding for successive generations) and another plant with green seeds.

- He obtained one variety of wrinkled seeds after drying and another with smooth seeds.

Each pair of contrasting (alternating) characteristics is called a pair of allelomorphic characters, such as the seed colour which may be yellow or green, the flower colour which may be pink or whit, and the height of the stem which is either tall or short,…etc.

Mendel performed his experiments by removing the stamens before maturation from a pea variety. Then he dusted the pollen grains taken from the anthers of another pea variety. Then he covered the flowers with paper sacs. The resulting seeds were collected, and cultivated which gave rise to the first filial generation (F1)

Mendel chose Pea plants for his experiments for the following reasons:



It grows easily and abundantly.

2. The petals of the Pea flowers (corolla) completely surround the reproductive organs (stamens and carpels) in the form of a keel, even after these organs become ripe. This gives the chance for self pollination, in addition to cross pollination to be carried out easily when required in the experiment (in case of self pollination, the flower can be protected from the reach of foreign pollen grains by surrounding it with a paper sac. While in case of cross pollination, the stamens are removed before maturation to prevent self pollination, then the flower is covered with paper sac, and the desired pollen grains are transferred artificially at the suitable time). Mendel expressed this adaptability in his papers stating that: The value and benefit of any experiment performed is determined by the degree of adaptation of the tools and materials to the experiment.

Reasons of Mendel’s success:

  1. He selected for his study hereditary characteristics of clear sharp differences and avoided these which were not.
  2. He continued studying the inheritance of characteristics not only in the first filial generation (F1), but also in the second filial generation (F2), and the following generations.
  3. He counted the number of individuals in the resulting generations, and applied Mathematics to his conclusions. So, the idea of applying quantitative studies was adopted in studying biological problems for the first time.
  4. He arranged his data in a manner so as to facilitate the evaluation of his results in an easy and objective way, and he described his experiments so that they could be easily repeated and tested by other scientists.

Experiments of the first law of Mendel (Law of Segregation of Factors):



Mendel crossed two pea plant varieties, one with pure red flowers, and the other variety with pure white flowers, the resulting seeds from this cross gave plants with red flowers after cultivation (F1 generation). The white flowered plants did not appear in this generation. The characteristic which appeared in the F1 (red) was called the dominant characteristic, while the characteristic which did not appeared (white) was called the recessive characteristic.

- What happened to this pair of allelomorphic characteristic??

To answer this question, Mendel let the F1 plants to pollinate themselves, and then he cultivated the resulting seeds to obtain the F2 plants. Mendel observed that the recessive characteristic (white) appeared in some of the resulting plants (F2 generation). Actually, the seeds produced plants with red flowers, and others with white flowers. This means that the parental red flowered plants were not really pure red, but were concealing the characteristic of the white flowers which appeared in the second generation (F2).

 

The table includes the seven allelomorphic characteristics which were studied by Mendel and the number of each character obtained in the F2 generation:

If you analyze the data in the table qualitatively, and quantitatively you will find that the ratio of dominant to recessive characteristic is 3:1 in the F2.

- Why did the recessive characteristic in each pair disappear completely in the F1 generation, then reappear in the F2 generation??

- Why did the two characteristics appear in the F2 generation, dominant: recessive in the ratio 3:1??

The answer these questions as follows:

The appearance or disappearance of characteristics (in F1) or their appearance together (in F2) is controlled by factors (now called genes). Mendel assumed that these factors exist in pairs. The factors of each pair separate (segregate) during the formation of gametes (pollen grains, and ovules). Mendel assumed that each individual carries pairs of factors for controlling the allelomorphic characteristics (a pair of factors for controlling each allelomorphic characteristic).

Law of Segregation of Factors:

When two pure members that differ in any pair of allelomorphic characteristic are crossed, only the dominant characteristic appears in the F1 generation, while the two characteristics appear in the F2 generation in the ratio of 3 dominant : 1 recessive.

N.B.:


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