MENDEL’S LAWS OF INHERITANCE AND EXCEPTIONS TO THE LAWSHistoryThe assertion that life can instantaneously arise from non living matter is calledspontaneous generation. Here are the critical experiments that busted the myth. Although todaywe understand that living things arise from other living things, the idea of spontaneousgeneration was entrenched in the minds of man throughout most of history. Spontaneousgeneration is the belief that, on a daily basis, living things arise from non living material. Thisdebunked belief is not the same as abiogenesis, the study of how life on earth could have arisenfrom 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. Hesaw beings as arising in one of three ways, from sexual reproduction, asexual reproduction ornonliving matter. According to Aristotle, it was readily observable that aphids arise from the dewon plants, fleas from putrid matter, and mice from dirty hay; and this belief remainedunchallenged 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 ofspontaneous generation. Redi's question was simple, “Where do maggots come from?”According to spontaneous generation, one would conclude that maggots came from rottingfood. Redi hypothesized that maggots came from flies and designed an experiment, elegant inits 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 developmentof maggots.Jar #2 he covered with netting. Flies laid their eggs on the netting and maggots soonappeared.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 hundredyears 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 usedlenses to examine cloth. Rather than employing lenses made by others, he ground his own, andthe 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 250microscopes, some of which magnified objects 270 times. Through magnification, he discoveredpresence of “micro” organisms - organisms so tiny that they were invisible to the naked eye. Hecalled these tiny living things “animalcules,” and was the first to describe many microbes andmicroscopic structures, including bacteria, protozoans and human cells.John Needham & Lazzaro Spallanzani (1700s)The debate over spontaneous generation was reignited with Leeuwenhoek’s discoveryof animalcules and the observation that these tiny organisms would appear in collectedrainwater within a matter of days. John Needham and Lazzaro Spallazani both set out toexamine Leeuwenhoek's animalcules.Needham’s ExperimentJohn Needham was a proponent of spontaneous generation, and his beliefs wereconfirmed 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.Spallazani’s ExperimentLazzaro Spallazani noted a flaw in Needham’s experiment. The containers holdingNeedham’s beef broths had not been sealed upon boiling. So Spallazani modified Needham’sexperiment, boiling infusions, but immediately upon boiling he melted the necks of his glasscontainers so that they were not open to the atmosphere. The microbes were killed and did notreappear 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 ofmicrobiology and for whom the process of “pasteurization” is named, repeated experimentssimilar to those of Spallazani’s and brought to light strong evidence that microbes arise fromother microbes, not spontaneously.Pasteur’s Swan-Necked FlasksPasteur created unique glass flasks with unusual long, thin necks that pointeddownward. These “swan-necked” flasks allowed air into the container but did not allow particlesfrom the air to drift down into the body of the flask.The End of Spontaneous GenerationAfter boiling his nutrient broths, Pasteur found that these swan-necked containers wouldremain free of microbes until he either broke the necks of the flasks, allowing particles from theair to drift in, or until he tilted the flask so that the liquid came in contact with dust that hadaccumulated 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)Preformation theory proposes that the only male and female is responsible for heredity.The male gamete consists of a miniature figure of man’s body called as homunculus which isresponsible for heredity. Epigenesis (C.f.wolf (1733-1794) and K.E. Von Baer (1792-1876) saidthat the different organs and tissues of adult plant and animals developed from the uniformembryonic tissue and not from mere growth expansion of the miniature homunculi present ineggs / sperms. Von Baer proposed that they developed through a sequential modification of theembryonic tissue. This concept is universally accepted.Swammerdam (1637-1680), for example, thought that a tiny preformed frog occurred inthe animal hemisphere of the frog egg and that became simply larger by feeding on the foodstored in the vegetal hemisphere of the egg. Another biologist, Hartsoeker (1695) published afigure showing a miniature man known as mankin or homunculus in the head of the humanspermatazoa. Such preformation theories had been supported by Leeuwenhoek (1632-1723),Malpighi (1673), Reaumur, Bonnet (1720- 1793), Spallanzani (1729-1799) and other workers of17th and early 18th centuries. With the development of improved microscopy and othercytological techniques in 17th and 18th centuries, it became clear to biologists that neither theegg nor the sperm contained a preformed individual but that each was a relatively uniform,homogeneous mass of protoplasm.Particulate TheoryA French biologist Maupertius in 1698-1759 discards the preformation theory andforwarded the concept of biparental through many tiny particles. According to him both theparents produce the semen, which composed of many tiny particles. The semen of both theparents unite and the embryo formed each organ of the embryo was supposed to be formed bytwo particles. Each of which came from each parent. In the year 1732-1806 J.C. Koelreuter wasthe first person to get fertile hybrids by artificial crossing two species of tobacco and concludedthat the gametes were the physical basis of heredity.PangenesisCharles Darwin proposed this theory. According to pangenesis that each organ of anindividual produces very small almost invisible identical copies of itself called gemmules orpangenes. These gemmules from various parts collected into the blood stream of animals. Theblood transports the gemmules into the reproductive organ, which produce gametes. Male andfemale gametes unite to form zygotes. When these gives rise to a new organism, the gemmulesof different parts of the body give rise to the same kind of organs, tissues and cells, which
produced them in the parents.LamarckismA French biologist Lamark (1774-1829) considered the inheritance of acquired charactersto 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 theiroff 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 notcontribute to the sexual reproduction. The germplasm on the other hand produces gametes thatare the basis of heredity. It is only applied to animals and plants in which distinction betweensoma and germ can be made. Weismannís famous experiment of cutting off the tail of mice for22 generations and observing that the progeny still had tail of normal length, proved that thesomatoplasm is not responsible for transmission of characters.Cell Theory(1838)Schleiden and Schwann proposed cell theory 1838. They concluded that all plant andanimal tissues were made of cells. It was also postulated that cell is the functional unit of livingorganism. In 1846 Negeli said that all cells originated from preexisting cells. Virchow 1853elaborated this and referred it as cell linkage theory.Mendelian concept of hereditaryThe laws of inheritance were derived by Gregor Mendel, a 19th century monkconducting 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 twogeneralizations which later became known as Mendel's Laws of Heredity or Mendelianinheritance. 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, andwhich was published in 1866.Mendel's findings allowed other scientists to predict the expression of traits on the basis ofmathematical probabilities. A large contribution to Mendel's success can be traced to hisdecision to start his crosses only with plants he demonstrated were true-breeding. He alsomeasured only absolute (binary) characteristics, such as color, shape, and position of theoffspring, rather than quantitative characteristics. He expressed his results numerically andsubjected them to statistical analysis. His method of data analysis and his large sample sizegave 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" (backcrossing descendants of the initial hybridization to the initial true-breeding lines) to reveal thepresence and proportion of recessive characters. Without his careful attention to procedure anddetail, Mendel's work could not have had the impact it made on the world of genetics.Mendel's LawsMendel discovered that by crossing white flower and purple flower plants, the result wasnot a hybrid offspring. Rather than being a mix of the two, the offspring was purple flowered. Hethen conceived the idea of heredity units, which he called "factors", one which is a recessivecharacteristic and the other dominant. Mendel said that factors, later called genes, normallyoccur in pairs in ordinary body cells, yet segregate during the formation of sex cells. Eachmember of the pair becomes part of the separate sex cell. The dominant gene, such as thepurple flower in Mendel's plants, will hide the recessive gene, the white flower. After Mendelself-fertilized the F1 generation and obtained the 3:1 ratio, he correctly theorized that genes canbe paired in three different ways for each trait; AA, aa, and Aa. The capital A represents thedominant 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,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. Thegenotype of an individual is made up of the many alleles it possesses. An individual's physicalappearance, or phenotype, is determined by its alleles as well as by its environment. Anindividual possesses two alleles for each trait; one allele is given by the female parent and theother by the male parent. They are passed on when an individual matures and producesgametes: egg and sperm. When gametes form, the paired alleles separate randomly so thateach gamete receives a copy of one of the two alleles. The presence of an allele doesn'tpromise that the trait will be expressed in the individual that possesses it. In heterozygousindividuals the only allele that is expressed is the dominant. The recessive allele is present butits expression is hidden. Mendel summarized his findings in two laws; the Law of Segregationand the Law of Independent Assortment.Law of Segregation (The "First Law")The Law of Segregation states that when any individual produces gametes, the copies ofa gene separate, so that each gamete receives only one copy. A gamete will receive one alleleor the other. The direct proof of this was later found when the process of meiosis came to beknown. In meiosis the paternal and maternal chromosomes get separated and the alleles withthe characters are segregated into two different gametes.Law of Independent Assortment (The "Second Law")The Law of Independent Assortment, also known as "Inheritance Law", states thatalleles of different genes assort independently of one another during gamete formation. WhileMendel's experiments with mixing one trait always resulted in a 3:1 ratio between dominant andrecessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1ratios. But the 9:3:3:1 table shows that each of the two genes are independently inherited with a3:1 ratio. Mendel concluded that different traits are inherited independently of each other, sothat there is no relation, for example, between a cat's color and tail length. This is actually onlytrue for genes that are not linked to each other.Independent assortment occurs during meiosis I in eukaryotic organisms, specificallymetaphase I of meiosis, to produce a gamete with a mixture of the organism's maternal andpaternal chromosomes. Along with chromosomal crossover, this process aids in increasinggenetic diversity by producing novel genetic combinations.
In independent assortment the chromosomes that end up in a newly-formed gamete arerandomly sorted from all possible combinations of maternal and paternal chromosomes.Because gametes end up with a random mix instead of a pre-defined "set" from either parent,gametes are therefore considered assorted independently. As such, the gamete can end upwith any combination of paternal or maternal chromosomes. Any of the possible combinations ofgametes formed from maternal and paternal chromosomes will occur with equal frequency. Forhuman gametes, with 23 pairs of chromosomes, the number of possibilities is 2 23 or 8,388,608possible combinations. The gametes will normally end up with 23 chromosomes, but the originof any particular one will be randomly selected from paternal or maternal chromosomes. Thiscontributes to the genetic variability of progeny.Rediscovery of Mendelís workMendel's conclusions were largely ignored. Although they were not completely unknownto 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 tounderstanding their significance was the importance attached by 19th century biologists to theapparent blending of inherited traits in the overall appearance of the progeny, now known to bedue to multigene interactions, in contrast to the organ-specific binary characters studied byMendel. In 1900, however, his work was "re-discovered" by three European scientists, Hugo deVries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has beensomewhat 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 thathe himself did not have priority. De Vries may not have acknowledged truthfully how much of hisknowledge of the laws came from his own work, or came only after reading Mendel's paper.Later scholars have accused Von Tschermak of not truly understanding the results at all.Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its mostvigorous promoter in Europe was William Bateson, who coined the term "genetics", "gene", and"allele" to describe many of its tenets.The model of heredity was highly contested by other biologists because it implied thatheredity was discontinuous, in opposition to the apparently continuous variation observable formany traits. Many biologists also dismissed the theory because they were not sure it wouldapply to all species, and there seemed to be very few true Mendelian characters in nature.However, later work by biologists and statisticians such as R.A. Fisher showed that if multipleMendelian factors were involved in the expression of an individual trait, they could produce thediverse results observed. Thomas Hunt Morgan and his assistants later integrated the
theoretical model of Mendel with the chromosome theory of inheritance, in which thechromosomes of cells were thought to hold the actual hereditary material, and create what isnow known as classical genetics, which was extremely successful and cemented Mendel'splace in history.Mendel's Laws of InheritanceMendel postulated three laws, which are now called after his name as Mendel’s laws ofheredity. These are:1. Law of dominance and recessive2. Law of segregation3. Law of independent assortment1. Law of DominanceDefinition: When two homozygous individuals with one or more sets of contrasting charactersare crossed, the characters that appear in the F1 hybrids are dominant characters and those do notappear in F1 are recessive characters.Law of dominance- If there are two alleles coding for the same trait and one is dominant it willshow up in the organism while the other won'tExplanation : The dominance and recessive of genes can be explained on the basis ofenzymatic functions of genes. The dominant genes - are capable of synthesizing activepolypeptides or proteins that form functional enzymes, whereas the recessive genes (mutant
genes) code for incomplete or non-functional polypeptides. Therefore, the dominant genesproduce a specific phenotype while the recessive genes fail to do so. In the heterozygouscondition also the dominant gene is able to express itself, so that the heterozygous andhomozygous individuals have similar phenotype.Critical appreciation of Law of DominanceScientists conducted cross-breeding experiments to find out the applicability of law ofdominance. The experiments were conducted by Correns on peas and maize, Tschermak on peas,by De Vries on maize etc., by Bateson and his collaborators on a variety of organisms, byDavenport on poultry, by Furst on rabbits, by Toyama on silk moth and by many others. Thesescientists observed that a large number of characters in various organisms are related as dominantand recessive.Importance of law of dominanceThe phenomenon of dominance is of practical importance as the harmful recessivecharacters are masked by the normal dominant characters in the hybrids. In Human beings aform of idiocy, diabetes, haemophilia etc. are recessive characters. A person hybrid for all thesecharacteristics appears perfectly normal. Thus harmful recessive genes can exist for severalgenerations without expressing themselves.Exceptions to Law of Dominance is the Incomplete Dominance. After Mendel several caseswere recorded by scientists, where F1 hybrids exhibited a blending of characters of two parents. Thesehybrids were found to be midway between the two parents. This is known as incomplete dominance orblending inheritance. It means that two genes of the allelomorphic pair are not related as dominantand recessive, but each of them expresses itself partially. As for example, in four-o'clock plant,Mirabilis jalapa, when plants with red