genetics essay intro

Introduction to Genetics

Natasha Ramroop Singh, Kamloops, British Columbia

Copyright Year: 2009

Publisher: Thompson Rivers University

Language: English

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Reviewed by James Langeland, Professor, Kalamazoo College on 1/30/23

This text does what it claims to do. It provides an introductory overview of a broad swath of genetics. read more

Comprehensiveness rating: 4 see less

This text does what it claims to do. It provides an introductory overview of a broad swath of genetics.

Content Accuracy rating: 4

No glaring errors. One could always nitpick any text book.

Relevance/Longevity rating: 3

The text is relevant, but not particularly unique in any sense. One could find virtually the same information in any number of genetics textbooks, presented in largely the same way. A major problem here is that the filed is presented more or less historically with many of the experiments and concepts being described having little to no relevance to genetics today. This is a problem with many texts so I do not single this one out.

Clarity rating: 4

As with many open source texts, this one suffers from substandard figures, which directly influences clarity. The words on the age are fine, but the adage is true-a picture can be worth a thousand words. The mainstream publishers spent a lot of money on figures and it shows--they can be really good.

Consistency rating: 4

No comments here.

Modularity rating: 4

There seem to be appropriate and logical chapter and section breaks.

Organization/Structure/Flow rating: 3

The flow is the same as nearly any other genetics textbook. It suffers from a rigid historical framework. Better than most at Muller's morphs however!

Interface rating: 5

No problems here. I do really like the integrated you tube links. I did not dive into the content of those videos (beyond the scope of my review), but the fact that they are there in abundance is a good use of the open source approach.

Grammatical Errors rating: 5

No problems here.

Cultural Relevance rating: 3

No comment.

A very timely section on SARS-Cov-2 at the end! Rich with study questions and answers. Genetics is and should be very problem based, so this is good. I appreciate what is being offered here and I understand the market. There is nothing "wrong" with this textbook. There is also no wow factor that would cause me to adopt it at this time.

Table of Contents

• Chapter 1- Mendel's First Law and Meiosis
• Chapter 2- Mendel's Second Law: Independent Assortment
• Chapter 3- The Cell Cycle and Mitosis
• Chapter 4- Pedigree Analysis
• Chapter 5- The Complementation Test
• Chapter 6- Alleles at a Single Locus
• Chapter 7- The Central Dogma- Mutations and Biochemical Pathways
• Chapter 8- Gene Interactions 
• Chapter 9- Linkage and Recombination Frequency
• Chapter 10- Sex Chromosomes & Sex Linkage
• Chapter 11- Recombination Mapping of Gene Loci
• Chapter 12- Physical Mapping of Chromosomes and Genomes
• Chapter 13- Genes and COVID-19 Susceptibility in Humans 

Ancillary Material

About the book.

Genetics, otherwise known as the Science of Heredity, is the study of biological information, and how this information is stored, replicated, transmitted and used by subsequent generations. The study of genetics can be sub-divided into three main areas: Transmission Genetics, Molecular Genetics, and Population Genetics. In this Introductory text, the focus is on Transmission or Classical Genetics, which deals with the basic principles of heredity and the mechanisms by which traits are passed from one generation to the next. The work of Gregor Mendel is central to Transmission Genetics; as such, there is a discussion about the pioneering work performed by him along with Mendel’s Laws, as they pertain to inheritance. Other aspects of Classical Genetics are covered, including the relationship between chromosomes and heredity, the arrangement of genes on chromosomes, and the physical mapping of genes.

About the Contributors

Natasha Ramroop Singh , Thompson Rivers University

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Genetics is the study of how genes bring about characteristics, or traits, in living things and how those characteristics are inherited. Genes are specific sequences of nucleotides that code for particular proteins. Through the processes of meiosis and sexual reproduction, genes are transmitted from one generation to the next.

Augustinian monk Gregor Mendel developed the science of genetics. Mendel performed his experiments in the 1860s and 1870s, but the scientific community did not accept his work until early in the twentieth century. Because the principles established by Mendel form the basis for genetics, the science is often referred to as Mendelian genetics. It is also called classical genetics to distinguish it from another branch of biology known as molecular genetics (see Chapter 10).

Mendel believed that factors pass from parents to their offspring, but he did not know of the existence of DNA. Modern scientists accept that genes are composed of segments of DNA molecules that control discrete hereditary characteristics.

Most complex organisms have cells that are diploid. Diploid cells have a double set of chromosomes, one from each parent. For example, human cells have a double set of chromosomes consisting of 23 pairs, or a total of 46 chromosomes. In a diploid cell, there are two genes for each characteristic. In preparation for sexual reproduction, the diploid number of chromosomes is reduced to a haploid number. That is, diploid cells are reduced to cells that have a single set of chromosomes. These haploid cells are gametes, or sex cells, and they are formed through meiosis (see Chapter 8). When gametes come together in sexual reproduction, the diploid condition is reestablished.

The offspring of sexual reproduction obtain one gene of each type from each parent. The different forms of a gene are called alleles. In humans, for instance, there are two alleles for earlobe construction. One allele is for earlobes that are attached, while the other allele is for earlobes that hang free. The type of earlobe a person has is determined by the alleles inherited from the parents.

The set of all genes that specify an organism’s traits is known as the organism’s genome. The genome for a human cell consists of about 20,000 genes. The gene composition of a living organism is its genotype. For a person’s earlobe shape, the genotype may consist of two alleles for attached earlobes, or two alleles for free earlobes, or one allele for attached earlobes and one allele for free earlobes.

The expression of the genes is referred to as the phenotype of a living thing. If a person has attached earlobes, the phenotype is “attached earlobes.” If the person has free earlobes, the phenotype is “free earlobes.” Even though three genotypes for earlobe shape are possible, only two phenotypes (attached earlobes and free earlobes) are possible.

The two paired alleles in an organism’s genotype may be identical, or they may be different. An organism’s condition is said to be homozygous when two identical alleles are present for a particular characteristic. In contrast, the condition is said to be heterozygous when two different alleles are present for a particular characteristic. In a homozygous individual, the alleles express themselves. In a heterozygous individual, the alleles may interact with one another, and in many cases, only one allele is expressed.

When one allele expresses itself and the other does not, the one expressing itself is the dominant allele. The “overshadowed” allele is the recessive allele. In humans, the allele for free earlobes is the dominant allele. If this allele is present with the allele for attached earlobes, the allele for free earlobes expresses itself, and the phenotype of the individual is “free earlobes.” Dominant alleles always express themselves, while recessive alleles express themselves only when two recessive alleles exist together in an individual. Thus, a person having free earlobes can have one dominant allele or two dominant alleles, while a person having attached earlobes must have two recessive alleles.

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"Half of your DNA is determined by your mother's side, and half is by your father. So, if you seem to look exactly like your mother, perhaps some DNA that codes for your body and how your organs run was copied from your father's genes ."

So close, yet so far. This quote, taken from a high school student's submission in a national essay contest, represents just one of countless misconceptions many people have about the basic nature of heredity and how our bodies read the instructions stored in our genetic material (Shaw et al . 2008). Although it is true that half of our genome is inherited from our mother and half from our father, it is certainly not the case that only some of our cells receive instructions from only some of our DNA. Rather, every diploid, nucleated cell in our body contains a full complement of chromosomes, and our specific cellular phenotypes are the result of complex patterns of gene expression and regulation . In fact, it is through this dynamic regulation of gene expression that organismal complexity is determined. For example, when the first draft of the human genome was published in 2003, scientists were surprised to find that sequence analysis revealed only around 25,000 genes, instead of the 50,000 to 100,000 genes originally hypothesized. Clues from studies examining the genomic structure of a variety of organisms suggest that much of human uniqueness lies not in our number of genes, but instead in our regulatory control over when and where certain genes are expressed. Additional examination of different organisms has revealed that all genomes are more complex and dynamic than previously thought. Thus, the central dogma proposed by Francis Crick as early as 1958 — that DNA encodes RNA, which is translated into protein — is now considered overly simplistic. Today, scientists know that beyond the three types of RNA that make the central dogma possible (mRNA, tRNA, and rRNA), there are many additional varieties of functional RNA within cells, many of which serve a number of known (and unknown) functions, including regulation of gene expression. Understanding how the structure of these and other nucleic acids belies their function at both the macroscopic and microscopic levels, and discovering how that understanding can be manipulated, is the essence of where genetics and molecular biology converge. Detailed comparative analysis of different organisms' genomes has also shed light on the genetics of evolutionary history . Using molecular approaches, information about mutation rates, and other tools, scientists continue to add more detail to phylogenetic trees, which tell us about the relationships between the marvelous variety of organisms that have existed throughout the planet's history. Examining how different processes shape populations through the culling or maintenance of deleterious or beneficial alleles lies at the heart of the field of population genetics . Within a population, beneficial alleles are typically maintained through positive natural selection, while alleles that compromise fitness are often removed via negative selection. Some detrimental alleles may remain, however, and a number of these alleles are associated with disease. Many common human diseases , such as asthma, cardiovascular disease, and various forms of cancer, are complex-in other words, they arise from the interaction between multiple alleles at different genetic loci with cues from the environment. Other diseases, which are significantly less prevalent, are inherited. For instance, phenylketonuria (PKU) was the first disease shown to have a recessive pattern of inheritance. Other conditions, like Huntington's disease, are associated with dominant alleles, while still other disorders are sex-linked-a concept that was first identified through studies involving mutations in the common fruit fly. Still other diseases, like Down syndrome, are linked to chromosomal aberrations that can be identified through cytogenetic techniques that examine chromosome structure and number . Our understanding in all these fields has blossomed in recent years. Thanks to the merger of molecular biology techniques with improved knowledge of genetics, scientists are now able to create transgenic organisms that have specific characters, test embryos for a variety of traits in vitro , and develop all manner of diagnostic tests capable of identifying individuals at risk for particular disorders. This interplay between genetics and society makes it crucial for all of us to grasp the science behind these techniques in order to better inform our decisions at the doctor, at the grocery store, and at home. As we seek to cultivate this understanding of modern genetics, it is critical to remember that the misconceptions expressed in the aforementioned essay are the same ones that many individuals carry with them. Thus, when working together, faculty and students need to explore not only what we know about genetics, but also what data and evidence support these claims. Only when we are equipped with the ability to reach our own conclusions will our misconceptions be altered.

-Kenna Shaw, Ph.D

Image: Mehau Kulyk/Science Photo Library/Getty Images.

Shaw, K. (2008) Genetics. Nature Education 2(10):1

© 2014 Nature Education

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4 Introduction to Molecular Genetics

The three letters “DNA” have now become associated with crime solving, paternity testing, human identification, and genetic testing. DNA can be retrieved from hair, blood, or saliva. With the exception of identical twins, each person’s DNA is unique and it is possible to detect differences between human beings on the basis of their unique DNA sequence.

DNA analysis has many practical applications beyond forensics and paternity testing. DNA testing is used for tracing genealogy and identifying pathogens. In the medical field, DNA is used in diagnostics, new vaccine development, and cancer therapy. It is now possible to determine predisposition to many diseases by analyzing genes.

DNA is the genetic material passed from parent to offspring for all life on Earth. The technology of molecular genetics developed in the last half century has enabled us to see deep into the history of life to deduce the relationships between living things in ways never thought possible. It also allows us to understand the workings of evolution in populations of organisms. Over a thousand species have had their entire genome sequenced, and there have been thousands of individual human genome sequences completed. These sequences will allow us to understand human disease and the relationship of humans to the rest of the tree of life. Finally, molecular genetics techniques have revolutionized plant and animal breeding for human agricultural needs. All of these advances in biotechnology depended on basic research leading to the discovery of the structure of DNA in 1953, and the research since then has uncovered the details of DNA replication and the complex process leading to the expression of DNA in the form of proteins in the cell.

Introductory Biology: Evolutionary and Ecological Perspectives Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Origins of human genetics. A personal perspective

Eberhard passarge.

Institut für Humangenetik, Universitätsklinikum Essen, Essen, Germany

Associated Data

Genetics evolved as a field of science after 1900 with new theories being derived from experiments obtained in fruit flies, bacteria, and viruses. This personal account suggests that the origins of human genetics can best be traced to the years 1949 to 1959. Several genetic scientific advances in genetics in 1949 yielded results directly relating to humans for the first time, except for a few earlier observations. In 1949 the first textbook of human genetics was published, the American Journal of Human Genetics was founded, and in the previous year the American Society of Human Genetics. In 1940 in Britain a textbook entitled Introduction to Medical Genetics served as a foundation for introducing genetic aspects into medicine. The introduction of new methods for analyzing chromosomes and new biochemical assays using cultured cells in 1959 and subsequent years revealed that many human diseases, including cancer, have genetic causes. It became possible to arrive at a precise cause-related genetic diagnosis. As a result the risk of occurrence or re-occurrence of a disease within a family could be assessed correctly. Genetic counseling as a new concept became a basis for improved patient care. Taken together the advances in medically orientated genetic research and patient care since 1949 have resulted in human genetics being both, a basic medical and a basic biological science. Prior to 1949 genetics was not generally viewed in a medical context. Although monogenic human diseases were recognized in 1902, their occurrence and distribution were considered mainly at the population level.

Introduction

With the completion of the Human Genome Project in 2004 [ 1 ] human genetics moved into a new era of exploring the whole genome and its relation to the causes of genetic disorders. New approaches based on numerous new technological advances, such as different automated DNA sequencing methods [ 2 ], the elucidation of different types of individual genetic variation [ 3 ] and others, allow high resolution analysis of the human genome in various genetic etiologies of diseases [ 4 , 5 ] in a great number of individuals in different geographic populations [ 6 – 9 ] or analysis of single cells [ 10 ]. Earlier genetic studies in human genetics were aimed at individual genes or groups of linked genes. In contrast, during the first 4–5 decades of increasing knowledge of general genetics since 1900, aspects relating to humans could rarely be considered [ 11 – 17 ]. The term “human genetics” has only been in wide use since 1949 on. “Man is one of the most unsatisfactory of all organisms for genetics studies.” One sentence later: “Obviously no geneticist would study such a refractory object, were it not for the importance that a knowledge of the subject has in other fields.” Thus wrote Alfred H Sturtevant in 1954 [ 18 ], expressing an opinion widely held among geneticists before the advent of human genetics (Extended Text #1 in Supp. Mat.).

How did human genetics arise? Here I propose that the origins of human genetics as an independent scientific field can best be traced to the years between 1949 and 1959, when genetic advances could be applied to humans. Several scientific events took place in 1949 that support this idea. In addition, I will briefly review advances relating to human genetics as they apply to medicine and patient care before and after 1949, much of it as a personal witness since 1963.

The year 1949

Two new important insights in 1949 serve as hallmarks in the development of early human genetics. James V Neel described sickle cell anemia as an autosomal recessive trait [ 19 ] and four months later in the same volume of Science Linus Pauling identified this disorder as a “molecular” disease [ 20 ]. In 1949 JBS Haldane estimated the mutation rate in humans based on an analysis of seven human diseases to be about 4 × 10 −5 [ 21 ]. Also in 1949, in a publication entitled “Disease and Evolution” JBS Haldane viewed infectious diseases as having potential as an “agent for natural selection” in man [ 22 ].

Another landmark paper in 1949 described the serendipitous discovery of a cytologically visible structure in the nucleus of neurons of female cats, but not in males [ 23 ]. Subsequently named Barr body, later X-chromatin, this eventually led to the principle of X-chromosome inactivation [ 24 ]. The examples above constitute a shift in the paradigm in scientific progress as postulated by Kuhn [ 25 ]. According to this theory science not only progresses as continuous accumulation of knowledge, but also by periods of a new paradigm by asking completely new questions in a new context [ 26 ].

For additional reasons the year 1949 can be considered a watershed time point from which modern human genetics developed. In 1949 the American Journal of Human Genetics was established, a year after the founding of the American Society of Human Genetics (ASHG). Curt Stern (1902–1981), one of the leading geneticists between 1923 and 1970, published the first textbook in this field, Principles of Human Genetics [ 27 ].

The first two meetings of the ASHG took place in September 1948 in Washington, DC, and December 1949 in New York City, both under HJ Muller as president. The title of Muller´s presidential address presented at the second annual meeting of the ASHG in 1949 was “Our Load of Mutations” [ 28 ]. This was mainly concerned with the consequences of mutations in humans at the population level.

In 1940 in Britain, a textbook appeared entitled An Introduction to Medical Genetics by Fraser Roberts [ 29 ]. This was the first textbook on medical genetics, and the only one for many years.

The year 1949 is also noteworthy for human genetics in post-war Germany (Extended Text #2 in Supp. Mat.).

Early advances

The transition from general genetics to human genetics is characterized by recognizing the medical aspects. Newly discovered chromosome abnormalities, hereditary metabolic defects and molecular technology resulted in defining new human diseases due to different genetic causes. Human genetics includes medical genetics , devoted to all of its medical aspects and clinical genetics , the practice of diagnosis and management of genetic disorders. McKusick in 1993 stated that clinical genetics originated in 1959 when human cytogenetics and biochemical genetics developed into mainstream subjects of research and its medical applications [ 30 ]. The term genomics , derived from genome (coined by Winkler in 1920), was introduced in 1987 [ 31 ]. It relates not only to all genes, but also to the molecules regulating their functions and nuclear structures.

The European Society of Human Genetics (ESHG) was founded at the Third International Congress of Human Genetics in 1966 in Chicago, with the author of this review and Albert de la Chapelle present. Its first annual meeting was held 1968 in Paris.

Chromosomes

Human genetics is a theory-driven science, but it also greatly depends on advances in methods of investigation. Probably the most important single contribution to the development of modern human genetics was that of cytogenetics in 1959 [ 32 – 36 ]. At first, individual chromosomes in mitosis could not yet be individually identified distinguished except for a few chromosome pairs (Extended Text #3 in Supp. Mat.). New cell culture methods and improved mitotic chromosomal preparations for light microscopic analysis directly led to the recognition in 1959/60 that several human disorders result from defined aberrations in the number or structure of chromosomes (Trisomies 21, 18, 13; partial chromosomal deletions or duplications). Since each aberration was associated with a distinct phenotype, a relationship between a genotype and a phenotype could be defined. In 1959, individuals without a Y chromosome were shown to be female [ 37 ], whereas those with a Y chromosome were male no matter how many X chromosomes were present [ 38 ]. This was the first step towards defining the fundaments of mammalian sex determination. In the 1960s and 1970s it became apparent that fetal death is frequently caused by chromosomal aberrations that are not observed in newborns. Although chromosomes in metaphase were described as early as 1879, the correct number of human chromosomes was not established until 1956 (Extended Text #4 in Supp. Mat.).

Cell cultures and biochemical defects

From the 1960s on, cultured cells became widely used to investigate monogenic human diseases (somatic cell genetics). Cells homozygous for a genetic defect could be distinguished from heterozygous cells. Fused homozygous cells from different patients (cell hybrids) could result in a normal cellular phenotype, proving the disease in question to be genetically heterogeneous. Biochemical assays began to define human hereditary metabolic diseases such as amino acid disorders, lysosomal storage diseases, and others at the level of the phenotype and genotype. Prenatal genetic diagnosis was introduced in the late 1960s.

Molecular advances

Beginning in 1974 DNA could be analyzed by applying new recombinant DNA methods directly, or indirectly by using linked polymorphic DNA markers. New methods to sequence DNA nucleotides in 1977 and to amplify small amounts of DNA in 1985 (PCR) resulted in precise genetic diagnoses with correct assessment of the genetic risk within a given family. Molecular cytogenetics was introduced shortly after 1985. This allowed the analysis of mitotic chromosomes by in situ DNA hybridization. Submicroscopic chromosomal alterations (less than 4 million base pairs of DNA) became visible. New automated massive parallel DNA sequencing methods (“next generation”) introduced in 2005 have made it possible to sequence the DNA of large numbers of individuals and tumor cells at relatively low cost [ 2 , 4 ]. Other new approaches have become possible: genome-wide association studies (GWAS), exome sequencing, whole genome sequencing, and others.

Genetics in medicine

From about 1960 on genetics included its medical aspects. McKusick in 1992 reviewed the development of human genetics from the First International Congress of Human Genetics in 1956 at Copenhagen to 1991 [ 39 ]. He noted that by 1992 human genetics had become “medicalized, subspecialized, professionalized, molecularized, consumerized, commercialized”. Systematic genetic diagnostic services and genetic counseling became part of patient care [ 40 ]. The American Board of Medical Genetics was established in 1979, the American College of Medical Genetics in 1992.

Details of the early stages of developing human genetics are reviewed by McKusick [ 40 ], Polani [ 41 ], Harper [ 42 , 43 ], Harper et al. [ 44 ]; McKusick & Harper and Childs & Pyeritz [ 45 , 46 ], and more recently Clausnitzer et al. [ 4 ]. Childs in 1999 and 2013 [ 47 , 48 ] has drawn attention to two views of disease: the classification of diseases differs in medicine and medical human genetics. In medicine it is mainly based on the phenotype, i.e., clinical manifestation, whereas the genetic classification system is based on the genotype, i.e., different types of mutations or other structural rearrangements. Table  1 lists the main genetic features of genetic disorders first described by their phenotypes since 1949. It is remarkable that many of these recognizable phenotypes were not described earlier, such as, e.g., trisomy 18, whereas the phenotype of trisomy 13 was described in 1657 (Thomas Bartholin, “Monstrum sine oculis”). Most disorders listed in Table  1 can be classified according to their genotypes rather than their phenotypes. Their classification is based on different pathogenic causes, such as impaired functions in genome structure, chromatin regulation, cell receptors, transcription factors, signaling pathways, imprinting, and others (for other examples of genetic classification of diseases see Extended Text #5 in Supp. Mat.).

Examples of new genetic disorders described 1949–2009.

Table S 1 lists examples of major advances in human genetics between 1949 and 2020. The criteria for selection are based on how each entry has been perceived in the literature and personal observations since 1963. The left column contains advances directly relating to human genetics, and the right column entries indirectly contributing to human genetics.

Nowhere is the enormous progress in the medical aspects of human genetics ( medical genetics ), in particular for monogenic disorders, more visible than in Mendelian Inheritance in Man. A Catalog of Human Genes and Genetic Disorders (Fig. S 1 ). This was first established in 1966 by Victor A McKusick (1921–2008) at Johns Hopkins University in Baltimore and went through 12 printed editions (1966–1998). Since then it is maintained online as Online Mendelian Inheritance in Man (Ref. [ 49 ], online freely available at OMIM: www.ncbi.nlm.nih.gov/omim ). CF Fraser and H Harris in 1956 independently established genetic heterogeneity as a basic principle in medical genetics [ 50 – 53 ]. Scriver in 1999 [ 54 ] first demonstrated that modifying genes influence the phenotype, severity and course of illness in monogenic disorders [ 55 – 57 ]. An important shift of paradigm in genetics occurred when the concept of genetic counseling was introduced (Extended Text #6 in Supp. Mat.).

Advances in general genetics applied to humans prior to 1949

Prior to 1949 none of the many discoveries in genetics could be derived from direct observations in humans. Advances in genetics generally were not seen in a medical context with patient care. Knowledge of human genetic disorders was aimed at the population level rather than individually to patients and their families. Monogenic Mendelian disorders were viewed as being too rare to be relevant for medical applications and patient care. Complex disorders with multifactorial etiologies had not yet revealed their genetic components. Several of the early genetic investigations in humans were directed at the genetics of normal traits such as stature, color of the eye, skin, hair, mental abilities and the like. They came to erroneous conclusions because the underlying genetic properties are not as simple as assumed at the time. Several presidents of the American Society of Human Genetics and others have reflected on the status of human genetics before 1949 (Extended Text #7 in Supp. Mat.).

A few earlier attempts related genetic knowledge to humans. Neel in 1939 initiated a seminar on human genetics together with Curt Stern (Extended Text #8 in Supp. Mat.). In 1940 in Britain, a textbook appeared entitled An Introduction to Medical Genetics by Fraser Roberts [ 29 ]. This was the first textbook on medical genetics, and the only one for many years (Extended Text #9 in Supp. Mat.). In Germany in 1923 a 500-page textbook entitled “Human Heredity Science and Racial Hygiene” went through five editions until 1940 (Extended Text #10 in Supp. Mat.).

In 1934 A Følling described phenylketonuria (OMIM 261600) as a cause of mental retardation. After GA Jervis recognized the enzyme defect in 1947, and H Bickel in 1953 delineated an approach to dietary therapy, R Guthrie in 1962 set the stage for population-wide screening of newborns for early diagnosis and effective therapy. Today a great number of hereditary metabolic disorders can be identified in newborns prior to clinical manifestation.

In general however, advances in genetics were not considered in relation to medicine. This would have required a shift in paradigm, which did not occur at that time. A gross misconception in applying genetic considerations to humans in the 1920s and 1930s was Eugenics (Extended Text #11 in Supp. Mat.).

Prescient insights

Three remarkable exceptions with early genetic insights relating to humans can be cited here: William Bateson, Archibald E Garrod, and Theodor Boveri. They can be considered forerunners of human genetics. William Bateson (1861–1926) at Cambridge in his Principles of Heredity in 1913 [ 12 ] described several human pedigrees with autosomal dominant, recessive, and X-linked inheritance (pp. 203–234). Bateson states on page 233: “Similarly when we find that a condition such as retinitis pigmentosa sometimes descends in one way and sometimes in another, we may perhaps expect that a fuller knowledge of facts would show that more than one pathological state may be included under the same name” [ 12 ]. Thus, Bateson recognized genetic heterogeneity more than 40 years before CF Fraser and H Harris in 1956 independently established it as a basic principle in human genetics (see above). Other examples of early descriptions of Mendelian inheritance of human diseases are heritable biochemical defects, described by Archibald Garrod as “inborn errors of metabolism” [ 58 – 60 ] or brachydactyly type A1 (OMIM 112500) by WC Farabee in a PhD thesis published in 1905, reviewed by Haws & McKusick in 1963 [ 61 ] and Bateson, 1913, page 210–216 [ 12 ].

Archibald E Garrod (1857–1936) at Great Ormand Street Hospital London recognized the genetic individuality of man. In a letter to Bateson on 11 January 1902, Garrod wrote: “I believe that no two individuals are exactly alike chemically any more than structurally (Ref. [ 60 ], Bearn, 1993, page 61). In his prescient monograph Inborn Factors of Disease of 1931 Garrod considered predisposition to disease to be important [ 47 , 48 , 60 ]. A remarkable insight pointing to the importance of genetics in human diseases is contained in Thomas H Morgan’s Nobel lecture in 1934, The relation of genetics to physiology and medicine : “… considering the present attitude of medicine and the dominating place of the constitutional researches, the role of the inner, hereditary factors to health and disease appears in a still clearer light. For the general understanding of maladies, for prophylactic medicine, and for the treatment of diseases, hereditary research thus gains still greater importance” (cited by Bearn, 1993, ref. [ 60 ], page 193).

The third example is Theodor Boveri (1862–1915) at Würzburg. By 1902 he had recognized the individuality of chromosomes [ 62 ]. Subsequently Boveri related changes in chromosomes to the causes of cancer [ 63 , 64 ]. However, more than four decades went by until 1960 when the Philadelphia chromosome was described in chronic myelogenous leukemia [ 65 , 66 ]. The “One Gene – One Enzyme” hypothesis proposed by George W Beadle in 1941 could have become a corner stone of human biochemical genetics. Beadle referred to Garrod in his Nobel lecture in 1958 (cited by Bearn, 1993, ref. [ 60 ], page 150).

Diversity of modern human genetics

Modern human genetics has evolved in different directions mainly based on different methods of investigation, although in research it is by no means limited to Homo sapiens . Today it comprises genomics with several subsections (e.g., proteomics, epigenomics and others), molecular genetics, tumor genetics and -genomics, pharmacogenetics and -genomics, immunogenetics, epigenetics, cytogenetics, somatic cell genetics, biochemical genetics, population genetics, evolutionary bases of causes of disorders, bioinformatics and others. This is extensively reviewed in two current multivolume online textbooks [ 67 , 68 ]. No vertebrate genetics or genomics is better understood than that of man. Yet, human genetics is not an established curriculum of study within the faculties of either medicine or biology. Rather, to become a human geneticist one must study medicine or a basic science and complete approximately five years of formal postgraduate training. Thus, human geneticists represent either a medical or a non-medical basic science. This dual structure of being both a medical and a biological discipline makes human genetics unique among the medical subspecialities, as outlined in detail by Childs [ 47 , 48 ].

In summary, modern human genetics began when new advances in genetics were systematically applied in medicine from 1949 on. A close relationship between genetics and medicine evolved into human genetics. This contributes greatly to an understanding of the causes of human diseases. In addition, genetic counseling based on empathy and free decision-making of individuals has become part of patient care. Human genetics had become “medicalized” [ 40 ].

Supplementary information

Acknowledgements.

Frank Kaiser, Bernhard Horsthemke, Christel Depienne, Jasmin Beygo and Deniz Kanber provided valuable comments. Mary F Passarge made useful suggestions about the style of the text. I thank three anonymous reviewers for constructive criticisms and helpful suggestions.

Open Access funding enabled and organized by Projekt DEAL.

Compliance with ethical standards

The author declares no conflict of interest.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

The online version of this article (10.1038/s41431-020-00785-7) contains supplementary material, which is available to authorized users.

Genetics 101

What is a genome, and how are traits passed from generation to generation? Learn how pea plants helped launch the study of genetics and how the field of genetics research has evolved over time.

Biology, Genetics

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 6.

• Introduction to genetic engineering

Intro to biotechnology

• DNA cloning and recombinant DNA
• Overview: DNA cloning
• Polymerase chain reaction (PCR)
• Gel electrophoresis
• DNA sequencing
• Applications of DNA technologies
• Biotechnology

Key points:

• Biotechnology is the use of an organism, or a component of an organism or other biological system, to make a product or process.
• Many forms of modern biotechnology rely on DNA technology.
• DNA technology is the sequencing, analysis, and cutting-and-pasting of DNA.
• Common forms of DNA technology include DNA sequencing , polymerase chain reaction , DNA cloning , and gel electrophoresis .
• Biotechnology inventions can raise new practical concerns and ethical questions that must be addressed with informed input from all of society.

Introduction

What is biotechnology.

• Beer brewing . In beer brewing, tiny fungi (yeasts) are introduced into a solution of malted barley sugar, which they busily metabolize through a process called fermentation. The by-product of the fermentation is the alcohol that’s found in beer. Here, we see an organism – the yeast – being used to make a product for human consumption.
• Penicillin. The antibiotic penicillin is generated by certain molds. To make small amounts of penicillin for use in early clinical trials, researchers had to grow up to 500 ‍   liters of “mold juice” a week 1 ‍   . The process has since been improved for industrial production, with use of higher-producing mold strains and better culture conditions to increase yield 2 ‍   . Here, we see an organism (mold) being used to make a product for human use – in this case, an antibiotic to treat bacterial infections.
• Gene therapy. Gene therapy is an emerging technique used to treat genetic disorders that are caused by a nonfunctional gene. It works by delivering the “missing” gene’s DNA to the cells of the body. For instance, in the genetic disorder cystic fibrosis, people lack function of a gene for a chloride channel produced in the lungs. In a recent gene therapy clinical trial, a copy of the functional gene was inserted into a circular DNA molecule called a plasmid and delivered to patients’ lung cells in spheres of membrane (in the form of a spray) 3 ‍   . In this example, biological components from different sources (a gene from humans, a plasmid originally from bacteria) were combined to make a new product that helped preserve lung function in cystic fibrosis patients.

What is DNA technology?

Examples of dna technologies.

• DNA cloning. In DNA cloning , researchers “clone” – make many copies of – a DNA fragment of interest, such as a gene. In many cases, DNA cloning involves inserting a target gene into a circular DNA molecule called a plasmid. The plasmid can be replicated in bacteria, making many copies of the gene of interest. In some cases, the gene is also expressed in the bacteria, making a protein (such as the insulin used by diabetics). Insertion of a gene into a plasmid.
• Polymerase chain reaction (PCR). Polymerase chain reaction is another widely used DNA manipulation technique, one with applications in almost every area of modern biology. PCR reactions produce many copies of a target DNA sequence starting from a piece of template DNA. This technique can be used to make many copies of DNA that is present in trace amounts (e.g., in a droplet of blood at a crime scene).
• Gel electrophoresis. Gel electrophoresis is a technique used to visualize (directly see) DNA fragments. For instance, researchers can analyze the results of a PCR reaction by examining the DNA fragments it produces on a gel. Gel electrophoresis separates DNA fragments based on their size, and the fragments are stained with a dye so the researcher can see them. DNA fragments migrate through the gel from the negative to the positive electrode. After the gel has run, the fragments are separated by size, with the smallest ones near the bottom (positive electrode) and the largest ones near the top (negative electrode). Based on similar diagram in Reece et al. 5 ‍  
• DNA sequencing. DNA sequencing involves determining the sequence of nucleotide bases (As, Ts, Cs, and Gs) in a DNA molecule. In some cases, just one piece of DNA is sequenced at a time, while in other cases, a large collection of DNA fragments (such as those from an entire genome) may be sequenced as a group. What is a genome? A genome refers to all of an organism's DNA. In eukaryotes, which have a nucleus in their cells to hold their DNA, the word genome is usually used for the nuclear genome (DNA found in the nucleus), excluding the DNA found in organelles such as chloroplasts or mitochondria.

Biotechnology raises new ethical questions

• Some of these relate to privacy and non-discrimination. For instance should your health insurance company be able to charge you more if you have a gene variant that makes you likely to develop a disease? How would you feel if your school or employer had access to your genome?
• Other questions relate to the safety, health effects, or ecological impacts of biotechnologies. For example, crops genetically engineered to make their own insecticide reduce the need for chemical spraying, but also raise concerns about plants escaping into the wild or interbreeding with local populations (potentially causing unintended ecological consequences).
• Biotechnology may provide knowledge that creates hard dilemmas for individuals. For example, a couple may learn via prenatal testing that their fetus has a genetic disorder. Similarly, a person who has her genome sequenced for the sake of curiosity may learn that she is going to develop an incurable, late-onset genetic disease, such as Huntington's.

Educate yourself and share your perspective

Works cited:.

• American Chemical Society. (2016). Discovery and development of penicillin. In Chemical landmarks . Retrieved from http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/flemingpenicillin.html .
• Meštrović, T. and Chow, S. (2015, April 29). Penicillin production. In News medical . Retrieved from http://www.news-medical.net/health/Penicillin-Production.aspx .
• Alton, E. W. F. W., Armstrong, D. K., Ashby, D., Bayfield, K. J., Bilton, Diana, Bloomfield, E. V., ... Wolstenholme-Hogg, P. (2015). Repeated nebulisation of non-viral CFTR gene therapy in patients with cystic fibrosis: A randomised, double-blind, placebo-controlled, phase 2b trial. Lancet Respiratory Medicine , 3 (9), 684-691. http://dx.doi.org/10.1016/S2213-2600(15)00245-3 .
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The DNA toolbox. In Campbell biology (10th ed., pp. 408-409). San Francisco, CA: Pearson.
• Reece, J. B., Taylor, M. R., Simon, E. J., and Dickey, J. L. (2012). Figure 12.13. Gel electrophoresis of DNA. In Campbell biology: Concepts & connections (7th ed., p. 243).

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