B.A. / B.Sc. (Honours) 5 th Semester (THEORY) (2/13)

Transcription

1 Course Name: PHYSICAL ANTHROPOLOGY Paper No. & Title: B.A. / B.Sc. (Honours) 5 th Semester (THEORY) Topic No. & Title: (2/13) Theories of inheritance: Man as an object of Genetical study, Laws of Heredity- Mendel's principles Introduction: All the characters of a living being may be grouped under one or other of two headingseither they are inborn or else they are acquired. Inborn characters may be defined as those which take origin in the germ cell. Among the higher animals at least offspring invariably differ inherently from their parents that is, they are born different from what their parents were born with. They make a different start in life. These congenital differences are technically termed as "variation." Thus a sixth digit on its first appearance in a family is a variation. It follows, since a variation takes origin in the germ, that it is not an acquirement, but an inborn trait. Theories of Inheritance: Many theories of inheritance have been proposed, and some dates back as far as ancient Greece.

2 1. Aristotle proposed the theory of pangenesis which held that particles (called pangenes) from all parts of the body come together to form the eggs and sperm. i. Changes that occurred in the various body parts during an organism's life could be passed on to the next generation. ii. Pangenesis was accepted by Lamarck and Darwin and was the prevailing theory into the nineteenth century. iii. Pangenesis is incorrect because reproductive cells are not composed of contributions from body cells and changes in body cells do not influence egg and sperm cells. 2. In the 17 th century, Aton van Leeuwenhoek observed the homunculus, a miniature human being, in human sperm cells. He and his followers (spermists) believed that: i. The mother serves only as an incubator for the homunculus. ii. All characteristics are inherited from the father. 3. Also during the 17 th century, Regnier de Graaf and his followers (ovists) proposed that: i. The egg contains an entire human in miniature and that semen only stimulates its growth. ii. All characteristics are thus inherited from the mother. Regnier de Graff was the first person to describe the ovarian follicle in which human egg cells are produced. 4. Based upon their observations with ornamental plant breeding, scientists in the 19 th century realized that both parents contribute to the characteristics of offspring. The "blending" theory then became the favoured explanation of inheritance. According to this theory: i. Hereditary materials from male and female parents mix to form the offspring, and once blended, the hereditary material is inseparable. ii. Since the hereditary material is inseparable, the population should reach a uniform appearance after many generations. 5. This theory was inconsistent with the observations that: i. Populations do not reach a uniform appearance. ii. Some traits are absent in one generation and present in the next. Mendelian inheritance or Mendelian genetics or Mendelism: It is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics. Modern genetics began in the 1860's with the experiments of Gregor Mendel a 19 th century Austrian Priest who discovered the fundamental principles of inheritance. He is often called the "father of genetics." The laws of inheritance were derived by Gregor Mendel, conducting hybridization experiments in garden peas (Pisum sativum) (Marantz, 2009). Between 1856 and 1863, he cultivated and tested some 29,000 pea plants. From these experiments he deduced two generalizations which later became

3 known as Mendel's Principles of Heredity or Mendelian inheritance. He described these principles in a two part paper, Experiments on Plant Hybridization that he read to the Natural History Society of Brno on February 8 and March 8, 1865, and which was published in His studies showed that there was particulate inheritance of traits according to basic laws of inheritance. The significance of Mendel's work was not recognized until the turn of the twentieth century. Mendel's conclusions were largely ignored. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major block to understanding their significance was the importance attached by 19th century biologists to the apparent blending of inherited traits in the overall appearance of the progeny, now known to be due to multigene interactions, in contrast to the organ-specific binary characters studied by Mendel. In 1900, however, his work was "re-discovered" by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has been somewhat debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries's paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work, or came only after reading Mendel's paper. Later scholars have blamed Von Tschermak of not truly understanding the results at all. Mendel's findings allowed other scientists to predict the expression of traits on the basis of mathematical probabilities. A large contribution to Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding. He also only measured absolute (binary) characteristics, such as colour, shape, and position of the offspring, rather than quantitative characteristics. He expressed his results numerically and subjected them to statistical analysis. His method of data analysis and his large sample size gave credibility to his data. He also had the foresight to follow several successive generations (f2, f3) of his pea plants and record their variations. Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters. Without his hard work and careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics. Mendel's Laws of Inheritance: Mendel discovered that when crossing white flower and purple flower plants, the result is not a blend. Rather than being a mix of the two, the offspring was purple flowered. He then conceived the idea of heredity units, which he called "factors", one of which is a recessive characteristic and the other dominant. Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. Each member of the pair becomes part of the separate sex cell. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. After Mendel self-fertilized the F1 generation and obtained the 3:1 ratio, he correctly theorized that genes can be paired in three different ways for each trait: AA, aa, and Aa. The capital "A" represents the dominant factor and lowercase "a" represents the recessive. Mendel stated that each individual has two factors for each trait, one from each parent. The two factors may or may not contain the same information. If the two factors are identical,

4 the individual is called homozygous for the trait. If the two factors have different information, the individual is called heterozygous. The alternative forms of a factor are called alleles. The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. An individual possesses two alleles for each trait; one allele is given by the female parent and the other by the male parent. They are passed on when an individual matures and produces gametes: egg and sperm. When gametes form, the paired alleles separate randomly so that each gamete receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals the only allele that is expressed is the dominant. The recessive allele is present but its expression is hidden. Mendel summarized his findings in three laws; the Law of Segregation, the Law of Independent Assortment and the law of dominance. First Law: The Law of Segregation The Law of Segregation states that every individual possesses a pair of alleles for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to its offspring. The offspring then receives its own pair of alleles for that trait. Whichever of the two alleles in the offspring is dominant determines how the offspring expresses that trait (e.g. the colour of a person's eyes). Mendel's second law: Law of independent assortment The Law of Independent Assortment, also known as "Inheritance Law" states that separate genes for separate traits are passed independently of one another from parents to offspring. That is, the biological selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait. Mendel s third Law: Law of Dominance Inheritance of many features (e.g. eye colour, flower colour, seed shape etc.) is controlled by one pair of genes. When the two genes are of the same kind (e.g. brown colour of eyes, red colour of flower) the condition is termed homozygous. When a pair of chromosomes has the gene controlling the same feature (flower colour) in two different forms (red flower gene on one chromosome and white flower gene on its pair, the condition is termed heterozygous. Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. His most important principle was the "principle of dominance", that the trait from the dominant allele is always expressed over the recessive allele. Conclusion From the present discussion, it can be said that human genetics is a science that examines the gene as a unit of storage, transmission, and realization of hereditary information for development and function of human beings. Many theories of inheritance were reported before Mendel s law of inheritance. Mendel's findings allowed other scientists to predict the expression

5 of traits on the basis of mathematical probabilities and can be applied on human beings. Further, many scientists have been worked out on genetic mechanisms step by step to molecular level. The basic principles of human genetics are derived originally from the genetic principles of plants and animals, as many of these are also valid for man.