Principles Of Heredity

[Glauco Schlembach]

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Mendel's principles of heredity : a defence / by W. Bateson ; with a translation of Mendel's original papers on hybridisation. Attribution-NonCommercial 4.0 International (CC BY-NC 4.0). Source: Wellcome Collection.

In Biology, heredity is the term used for transmission of the traits from one generation to the next generation. It is due to heredity that the offsprings look similar to their parents. It also explains why dogs always give birth to puppies and never to kittens. The process of heredity is universal among all living organisms. Genetic variation refers to the variation in a population or species. Genetics is the study of heredity and variation in living organisms.

Transmission genetics and cytogenetics have helped scientists investigate the biological basis of heredity. In transmission genetics, organisms are crossed to study the inheritance pattern in offsprings. Cytological techniques help in understanding cellular reproduction. With the advancement of molecular biology and its tools and techniques, geneticists are able to understand the genetic basis of the inheritance of traits and variations present in various organisms.

A Code of Ethics is a document which attempts to clarify and guide the conduct of a professional so that the goals and values of the profession might best be served. Download a PDF copy of the NSGC Code of Ethics.

If you require additional assistance interpreting the Code of Ethics in the context of a specific clinical case, research case, or professional issue, please consider submitting an Ethics Consult Request to the NSGC Ethics Advisory Group.

Genetic counselors are health professionals with specialized education, training, and experience in medical genetics and counseling. The National Society of Genetic Counselors (NSGC) is the leading voice, authority and advocate for the genetic counseling profession. Through this code of ethics, the NSGC affirms the ethical responsibilities of its members. NSGC members are expected to be aware of the ethical implications of their professional actions and work to uphold and adhere to the guidelines and principles set forth in this code.

A code of ethics is a document that attempts to clarify and guide the conduct of a professional so that the goals and values of the profession are best served. The NSGC Code of Ethics is based upon the distinct relationships genetic counselors have with 1) themselves, 2) their clients, 3) their colleagues, and 4) society. Each section of this code begins with an explanation of the relevant relationship, along with the key values and characteristics of that relationship. These values are drawn from the ethical principles of autonomy, beneficence, nonmaleficence and justice, and they include the professional principles of fidelity, veracity, integrity, dignity and accountability.

No set of guidelines can provide all the assistance needed in every situation, especially when different values appear to conflict. In certain areas, some ambiguity remains, allowing for the judgment of the genetic counselor(s) involved to determine how best to respond to difficult situations.

Genetic counselors value professionalism, competence, integrity, objectivity, veracity, dignity, accountability and self-respect in themselves as well as in each other. Therefore, genetic counselors work to:

The relationships of genetic counselors with society include interest and participation in activities that have the purpose of promoting the well-being of society and access to genetic services and health care. These relationships are based on the principles of veracity, objectivity and integrity. Therefore, genetic counselors, individually or through their professional organizations, work to:

The principles of Mendelian inheritance were named for and first derived by Gregor Johann Mendel,[3] a nineteenth-century Moravian monk who formulated his ideas after conducting simple hybridization experiments with pea plants (Pisum sativum) he had planted in the garden of his monastery.[4] Between 1856 and 1863, Mendel cultivated and tested some 5,000 pea plants. From these experiments, he induced two generalizations which later became known as Mendel's Principles of Heredity or Mendelian inheritance. He described his experiments in a two-part paper, Versuche ber Pflanzen-Hybriden (Experiments on Plant Hybridization),[5] that he presented to the Natural History Society of Brno on 8 February and 8 March 1865, and which was published in 1866.[3][6][7][8]

Mendel's results were at first 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 roadblock to understanding their significance was the importance attached by 19th-century biologists to the apparent blending of many inherited traits in the overall appearance of the progeny,[citation needed] now known to be due to multi-gene interactions, in contrast to the organ-specific binary characters studied by Mendel.[4] 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 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' 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 and how much came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.[9][10]

Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was William Bateson, who coined the terms "genetics" and "allele" to describe many of its tenets.[11] The model of heredity was contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits.[12] Many biologists also dismissed the theory because they were not sure it would apply to all species. However, later work by biologists and statisticians such as Ronald Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed, thus demonstrating that Mendelian genetics is compatible with natural selection.[13][14] Thomas Hunt Morgan and his assistants later integrated Mendel's theoretical model with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and created what is now known as classical genetics, a highly successful foundation which eventually cemented Mendel's place in history.[3][11]

Mendel's findings allowed scientists such as Fisher and J.B.S. Haldane to predict the expression of traits on the basis of mathematical probabilities. An important aspect of Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding.[4][13] He only measured discrete (binary) characteristics, such as color, shape, and position of the seeds, rather than quantitatively variable 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 had the foresight to follow several successive generations (P, F1, F2, F3) of 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 the presence and proportions of recessive characters.[15]

Punnett Squares are a well known genetics tool that was created by an English geneticist, Reginald Punnett, which can visually demonstrate all the possible genotypes that an offspring can receive, given the genotypes of their parents.[16][17][18] Each parent carries two alleles, which can be shown on the top and the side of the chart, and each contribute one of them towards reproduction at a time. Each of the squares in the middle demonstrates the number of times each pairing of parental alleles could combine to make potential offspring. Using probabilities, one can then determine which genotypes the parents can create, and at what frequencies they can be created.[16][18]

For example, if two parents both have a heterozygous genotype, then there would be a 50% chance for their offspring to have the same genotype, and a 50% chance they would have a homozygous genotype. Since they could possibly contribute two identical alleles, the 50% would be halved to 25% to account for each type of homozygote, whether this was a homozygous dominant genotype, or a homozygous recessive genotype.[16][17][18]

Pedigrees are visual tree like representations that demonstrate exactly how alleles are being passed from past generations to future ones.[19] They also provide a diagram displaying each individual that carries a desired allele, and exactly which side of inheritance it was received from, whether it was from their mother's side or their father's side.[19] Pedigrees can also be used to aid researchers in determining the inheritance pattern for the desired allele, because they share information such as the gender of all individuals, the phenotype, a predicted genotype, the potential sources for the alleles, and also based its history, how it could continue to spread in the future generations to come. By using pedigrees, scientists have been able to find ways to control the flow of alleles over time, so that alleles that act problematic can be resolved upon discovery.[20]

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