LAWS OF INHERITANCE
Laws of Inheritance History
- • The assertion that life can instantaneously arise from non living matter is called spontaneous generation.
- • Here are the critical experiments that busted the myth.
- • Although today we understand that living things arise from other living things, the idea of spontaneous generation was entrenched in the minds of man throughout most of history.
- • Spontaneous generation is the belief that, on a daily basis, living things arise from non living material.
- • This debunked belief is not the same as abiogenesis, the study of how life on earth could have arisen from inanimate matter billions of years ago.
Aristotle and Spontaneous Generation (383-322)
- • Aristotle was one of the first to record his conclusions on the possible routes to life.
- • He saw beings as arising in one of three ways, from sexual reproduction, asexual reproduction or nonliving matter.
- • According to Aristotle, it was readily observable that aphids arise from the dew on plants, fleas from putrid matter, and mice from dirty hay; and this belief remained unchallenged for more than two thousand years.
Francesco Redi’s Experiments (late 1600s)
- • Redi was and Italian physician and one of the first to formally challenge the doctrine of spontaneous generation.
- • Redi’s question was simple, “Where do maggots come from?” According to spontaneous generation, one would conclude that maggots came from rotting food.
- • Redi hypothesized that maggots came from flies and designed an experiment, elegant in its simplicity, to challenge spontaneous generation.
Redi put meat into three separate jars:
Jar #1 he left open. He observed flies laying eggs on the meat and the eventual development of maggots.
Jar #2 he covered with netting. Flies laid their eggs on the netting and maggots soon appeared.
Jar #3 he sealed. Flies were not attracted to this jar and no maggots developed on the meat.
- • This seems to be a clear demonstration of life giving rise to life.
- • Yet it took another two hundred years for people to accept spontaneous generation as a fallacy.
Anthony van Leeuwenhoek’s “Animalcules” (1600-1700s)
- • Leeuwenhoek was a Dutch cloth merchant, and due to his trade, he frequently used lenses to examine cloth.
- • Rather than employing lenses made by others, he ground his own, and the expertise that he gained through lens crafting combined with a curious mind eventually led to an interest in microscopy.
- • During his life, Leeuwenhoek assembled more than 250 microscopes, some of which magnified objects 270 times.
- • Through magnification, he discovered presence of “micro” organisms – organisms so tiny that they were invisible to the naked eye.
- • He called these tiny living things “animalcules,” and was the first to describe many microbes and microscopic structures, including bacteria, protozoans and human cells.
John Needham & Lazzaro Spallanzani (1700s)
- • The debate over spontaneous generation was reignited with Leeuwenhoek’s discovery of animalcules and the observation that these tiny organisms would appear in collected rainwater within a matter of days.
- • John Needham and Lazzaro Spallazani both set out to examine Leeuwenhoek’s animalcules.
- • John Needham was a proponent of spontaneous generation, and his beliefs were confirmed when, after boiling beef broth to kill all microbes, within the span of a few days, cloudiness of the broth indicated the respawning of microscopic life.
- • Lazzaro Spallazani noted a flaw in Needham’s experiment.
- • The containers holding Needham’s beef broths had not been
sealed upon boiling.
- • So Spallazani modified Needham’s experiment, boiling infusions, but immediately upon boiling he melted the necks of his glass containers so that they were not open to the atmosphere.
- • The microbes were killed and did not reappear unless he broke the seal and again exposed the infusion to air.
Louis Pasteur (1800s)
- • Pasteur, a French scientist who made great contributions to our understanding of microbiology and for whom the process of “pasteurization” is named, repeated experiments similar to those of Spallazani’s and brought to light strong evidence that microbes arise from other microbes, not spontaneously.
Pasteur’s Swan-Necked Flasks
- • Pasteur created unique glass flasks with unusual long, thin necks that pointed downward.
- • These “swan-necked” flasks allowed air into the container but did not allow particles from the air to drift down into the body of the flask.
The End of Spontaneous Generation
- • After boiling his nutrient broths, Pasteur found that these swan- necked containers would remain free of microbes until he either broke the necks of the flasks, allowing particles from the air to drift in, or until he tilted the flask so that the liquid came in contact with dust that had accumulated at the opening of the flask.
- • It was these carefully controlled experiments of Pasteur that finally put to rest the debate over spontaneous generation.
Preformation theory (Swammerdam and Bonnet. 1720 1793)
- • A French biologist Maupertius in 1698-1759 discards the preformation theory and forwarded the concept of biparental through many tiny particles.
- • According to him both the parents produce the semen, which composed of many tiny particles.
- • The semen of both the parents unite and the embryo formed each organ of the embryo was supposed to be formed by two particles.
- • Each of which came from each parent.
- • In the year 1732-1806 J.C. Koelreuter was the first person to get fertile hybrids by artificial crossing two species of tobacco and concluded that the gametes were the physical basis of heredity.
- • Charles Darwin proposed this theory.
- • According to pangenesis that each organ of an individual produces very small almost invisible identical copies of itself called gemmules or pangenes.
- • These gemmules from various parts collected into the blood stream of animals.
- • The blood transports the gemmules into the reproductive organ, which produce gametes.
- • Male and female gametes unite to form zygotes.
- • When these gives rise to a new organism, the gemmules of different parts of the body give rise to the same kind of organs, tissues and cells, which produced them in the parents.
- • A French biologist Lamark (1774-1829) considered the inheritance of acquired characters to be the most important, if not the sole, mechanism of evolutionary changes.
- • According tourgent need, use and disuse of organs, the modification thus acquired will be transmitted to their off spring.
Germplasm theory August Weismanís (1834-1914)
- • Germplasm theory explains that body of individual consists of two distinct types of tissues, (1) somatoplasm (2) germplasm.
- • Somatoplasm consists of all body tissues, which do not contribute to the sexual reproduction.
- • The germplasm on the other hand produces gametes that are the basis of heredity.
- • It is only applied to animals and plants in which distinction between soma and germ can be made.
- • Weismannís famous experiment of cutting off the tail of mice for 22 generations and observing that the progeny still had tail of normal length, proved that the somatoplasm is not responsible for transmission of characters.
- • Schleiden and Schwann proposed cell theory 1838. They concluded that all plant and animal tissues were made of cells.
- • It was also postulated that cell is the functional unit of living organism.
- • In 1846 Negeli said that all cells originated from preexisting cells.
- • Virchow 1853 elaborated this and referred it as cell linkage theory.
Mendelian concept of hereditary
- • The laws of inheritance were derived by Gregor Mendel, a 19th century monk conducting hybridization experiments in garden peas (Pisum sativum).
- • Between 1856 and 1863, he cultivated and tested some 29,000 pea plants.
- • From these experiments he deduced two generalizations which later became known as Mendel’s Laws of Heredity or Mendelian inheritance.
- • He described these laws in a two part paper, “Experiments on Plant Hybridization” that he read to the Natural History Society of Bruno on February 8 and March 8, 1865, and which was published in 1866.
- • 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 measured only absolute (binary) characteristics, such as color, 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 careful attention to procedure and detail, Mendel’s work could not have had the impact it made on the world of genetics.
- • Mendel discovered that by crossing white flower and purple flower plants, the result was not a hybrid offspring.
- • 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 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.