Klug, W., Spencer, C., Palladino, M., & Cummings, M. (2012). Concepts of genetics (10th ed., pp. 2 to 5).

1301 Sansome Street, San Francisco, California 94111: Pearson Benjamin Cummings.

(Klug, Spencer, Palladino & Cummings, 2012)

Winchester, A. (2020). genetics | History, Biology, Timeline, & Facts. Retrieved 5 August 2020, from
https://www.britannica.com/science/genetics

(Winchester, 2020)

Rosalind Franklin | Biography, Facts, & DNA. (2020). Retrieved 17 August 2020, from
https://www.britannica.com/biography/Rosalind-Franklin#:~:text=Rosalind%20Franklin%20discovered
%20the%20density,double-helix%20polymer%20in%201953.

("Rosalind Franklin | Biography, Facts, & DNA", 2020)

MacRae, C., & Vasan, R. (2016). The Future of Genetics and Genomics. Circulation, 133(25), 2634-2639.
doi: 10.1161/circulationaha.116.022547

(MacRae & Vasan, 2016)

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HomeHealth & MedicineGenetics & Evolution

WRITTEN BY

A.M. Winchester

Emeritus Professor of Biology, University of Northern Colorado, Greeley. Author of Genetics and others.

See Article History

ARTICLE CONTENTS

Genetics, study of heredity in general and of genes in particular. Genetics forms one of the central pillars
of biology and overlaps with many other areas, such as agriculture, medicine, and biotechnology.

Genetics

KEY PEOPLE

Jeffrey C. Hall

Michael Rosbash

Michael W. Young

Gregor Mendel

Sir Ian Wilmut

Thomas Hunt Morgan

Bruce Ames

Lancelot Thomas Hogben

August Weismann

Theodosius Dobzhansky

RELATED TOPICS

Heredity

Biology

Human genetics

Eugenics
Human genome

Genetic epidemiology

Genomics

Gene

Epigenomics

Behaviour genetics

Since the dawn of civilization, humankind has recognized the influence of heredity and applied its
principles to the improvement of cultivated crops and domestic animals. A Babylonian tablet more than
6,000 years old, for example, shows pedigrees of horses and indicates possible inherited characteristics.
Other old carvings show cross-pollination of date palm trees. Most of the mechanisms of heredity,
however, remained a mystery until the 19th century, when genetics as a systematic science began.

TOP QUESTIONS

What is genetics?

Is intelligence genetic?

How is genetic testing done?

Genetics arose out of the identification of genes, the fundamental units responsible for heredity.
Genetics may be defined as the study of genes at all levels, including the ways in which they act in the
cell and the ways in which they are transmitted from parents to offspring. Modern genetics focuses on
the chemical substance that genes are made of, called deoxyribonucleic acid, or DNA, and the ways in
which it affects the chemical reactions that constitute the living processes within the cell. Gene action
depends on interaction with the environment. Green plants, for example, have genes containing the
information necessary to synthesize the photosynthetic pigment chlorophyll that gives them their green
colour. Chlorophyll is synthesized in an environment containing light because the gene for chlorophyll is
expressed only when it interacts with light. If a plant is placed in a dark environment, chlorophyll
synthesis stops because the gene is no longer expressed.

Genetics as a scientific discipline stemmed from the work of Gregor Mendel in the middle of the 19th
century. Mendel suspected that traits were inherited as discrete units, and, although he knew nothing of
the physical or chemical nature of genes at the time, his units became the basis for the development of
the present understanding of heredity. All present research in genetics can be traced back to Mendel’s
discovery of the laws governing the inheritance of traits. The word genetics was introduced in 1905 by
English biologist William Bateson, who was one of the discoverers of Mendel’s work and who became a
champion of Mendel’s principles of inheritance.

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Historical Background

Ancient theories of pangenesis and blood in heredity

Although scientific evidence for patterns of genetic inheritance did not appear until Mendel’s work,
history shows that humankind must have been interested in heredity long before the dawn of
civilization. Curiosity must first have been based on human family resemblances, such as similarity in
body structure, voice, gait, and gestures. Such notions were instrumental in the establishment of family
and royal dynasties. Early nomadic tribes were interested in the qualities of the animals that they
herded and domesticated and, undoubtedly, bred selectively. The first human settlements that practiced
farming appear to have selected crop plants with favourable qualities. Ancient tomb paintings show
racehorse breeding pedigrees containing clear depictions of the inheritance of several distinct physical
traits in the horses. Despite this interest, the first recorded speculations on heredity did not exist until
the time of the ancient Greeks; some aspects of their ideas are still considered relevant today.

Hippocrates (c. 460–c. 375 BCE), known as the father of medicine, believed in the inheritance of
acquired characteristics, and, to account for this, he devised the hypothesis known as pangenesis. He
postulated that all organs of the body of a parent gave off invisible “seeds,” which were like miniaturized
building components and were transmitted during sexual intercourse, reassembling themselves in the
mother’s womb to form a baby.

Aristotle (384–322 BCE) emphasized the importance of blood in heredity. He thought that the blood
supplied generative material for building all parts of the adult body, and he reasoned that blood was the
basis for passing on this generative power to the next generation. In fact, he believed that the male’s
semen was purified blood and that a woman’s menstrual blood was her equivalent of semen. These
male and female contributions united in the womb to produce a baby. The blood contained some type
of hereditary essences, but he believed that the baby would develop under the influence of these
essences, rather than being built from the essences themselves.
Aristotle’s ideas about the role of blood in procreation were probably the origin of the still prevalent
notion that somehow the blood is involved in heredity. Today people still speak of certain traits as being
“in the blood” and of “blood lines” and “blood ties.” The Greek model of inheritance, in which a teeming
multitude of substances was invoked, differed from that of the Mendelian model. Mendel’s idea was
that distinct differences between individuals are determined by differences in single yet powerful
hereditary factors. These single hereditary factors were identified as genes. Copies of genes are
transmitted through sperm and egg and guide the development of the offspring. Genes are also
responsible for reproducing the distinct features of both parents that are visible in their children.

Preformation and natural selection

In the two millennia between the lives of Aristotle and Mendel, few new ideas were recorded on the
nature of heredity. In the 17th and 18th centuries the idea of preformation was introduced. Scientists
using the newly developed microscopes imagined that they could see miniature replicas of human
beings inside sperm heads. French biologist Jean-Baptiste Lamarck invoked the idea of “the inheritance
of acquired characters,” not as an explanation for heredity but as a model for evolution. He lived at a
time when the fixity of species was taken for granted, yet he maintained that this fixity was only found
in a constant environment. He enunciated the law of use and disuse, which states that when certain
organs become specially developed as a result of some environmental need, then that state of
development is hereditary and can be passed on to progeny. He believed that in this way, over many
generations, giraffes could arise from deerlike animals that had to keep stretching their necks to reach
high leaves on trees.

British naturalist Alfred Russel Wallace originally postulated the theory of evolution by natural selection.
However, Charles Darwin’s observations during his circumnavigation of the globe aboard the HMS
Beagle (1831–36) provided evidence for natural selection and his suggestion that humans and animals
shared a common ancestry. Many scientists at the time believed in a hereditary mechanism that was a
version of the ancient Greek idea of pangenesis, and Darwin’s ideas did not appear to fit with the theory
of heredity that sprang from the experiments of Mendel.

The work of Mendel

Before Gregor Mendel, theories for a hereditary mechanism were based largely on logic and
speculation, not on experimentation. In his monastery garden, Mendel carried out a large number of
cross-pollination experiments between variants of the garden pea, which he obtained as pure-breeding
lines. He crossed peas with yellow seeds to those with green seeds and observed that the progeny seeds
(the first generation, F1) were all yellow. When the F1 individuals were self-pollinated or crossed among
themselves, their progeny (F2) showed a ratio of 3:1 (3/4 yellow and 1/4 green). He deduced that, since
the F2 generation contained some green individuals, the determinants of greenness must have been
present in the F1 generation, although they were not expressed because yellow is dominant over green.
From the precise mathematical 3:1 ratio (of which he found several other examples), he deduced not
only the existence of discrete hereditary units (genes) but also that the units were present in pairs in the
pea plant and that the pairs separated during gamete formation. Hence, the two original lines of pea
plants were proposed to be YY (yellow) and yy (green). The gametes from these were Y and y, thereby
producing an F1 generation of Yy that were yellow in colour because of the dominance of Y. In the F1
generation, half the gametes were Y and the other half were y, making the F2 generation produced from
random mating 1/4 Yy, 1/2 YY, and 1/4 yy, thus explaining the 3:1 ratio. The forms of the pea colour
genes, Y and y, are called alleles.

Mendel also analyzed pure lines that differed in pairs of characters, such as seed colour (yellow versus
green) and seed shape (round versus wrinkled). The cross of yellow round seeds with green wrinkled
seeds resulted in an F1 generation that were all yellow and round, revealing the dominance of the
yellow and round traits. However, the F2 generation produced by self-pollination of F1 plants showed a
ratio of 9:3:3:1 (9/16 yellow round, 3/16 yellow wrinkled, 3/16 green round, and 1/16 green wrinkled;
note that a 9:3:3:1 ratio is simply two 3:1 ratios combined). From this result and others like it, he
deduced the independent assortment of separate gene pairs at gamete formation.

Mendel’s success can be attributed in part to his classic experimental approach. He chose his
experimental organism well and performed many controlled experiments to collect data. From his
results, he developed brilliant explanatory hypotheses and went on to test these hypotheses
experimentally. Mendel’s methodology established a prototype for genetics that is still used today for
gene discovery and understanding the genetic properties of inheritance.

How the gene idea became reality

Mendel’s genes were only hypothetical entities, factors that could be inferred to exist in order to explain
his results. The 20th century saw tremendous strides in the development of the understanding of the
nature of genes and how they function. Mendel’s publications lay unmentioned in the research
literature until 1900, when the same conclusions were reached by several other investigators. Then
there followed hundreds of papers showing Mendelian inheritance in a wide array of plants and animals,
including humans. It seemed that Mendel’s ideas were of general validity. Many biologists noted that
the inheritance of genes closely paralleled the inheritance of chromosomes during nuclear divisions,
called meiosis, that occur in the cell divisions just prior to gamete formation.

chromosome

chromosome

Strands of human chromosomes.

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The discovery of linked genes

chromosome

chromosome

Chromosomes carry hereditary information in the form of genes.

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international.com

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It seemed that genes were parts of chromosomes. In 1910 this idea was strengthened through the
demonstration of parallel inheritance of certain Drosophila (a type of fruit fly) genes on sex-determining
chromosomes by American zoologist and geneticist Thomas Hunt Morgan. Morgan and one of his
students, Alfred Henry Sturtevant, showed not only that certain genes seemed to be linked on the same
chromosome but that the distance between genes on the same chromosome could be calculated by
measuring the frequency at which new chromosomal combinations arose (these were proposed to be
caused by chromosomal breakage and reunion, also known as crossing over). In 1916 another student of
Morgan’s, Calvin Bridges, used fruit flies with an extra chromosome to prove beyond reasonable doubt
that the only way to explain the abnormal inheritance of certain genes was if they were part of the extra
chromosome. American geneticist Hermann Joseph Müller showed that new alleles (called mutations)
could be produced at high frequencies by treating cells with X-rays, the first demonstration of an
environmental mutagenic agent (mutations can also arise spontaneously). In 1931 American botanist
Harriet Creighton and American scientist Barbara McClintock demonstrated that new allelic
combinations of linked genes were correlated with physically exchanged chromosome parts.

sex-linked inheritance

sex-linked inheritance
Sex-linked inheritance of white eyes in Drosophila flies.

Encyclopædia Britannica, Inc.

Early molecular genetics

In 1908 British physician Archibald Garrod proposed the important idea that the human disease
alkaptonuria, and certain other hereditary diseases, were caused by inborn errors of metabolism,
suggesting for the first time that linked genes had molecular action at the cell level. Molecular genetics
did not begin in earnest until 1941 when American geneticist George Beadle and American biochemist
Edward Tatum showed that the genes they were studying in the fungus Neurospora crassa acted by
coding for catalytic proteins called enzymes. Subsequent studies in other organisms extended this idea
to show that genes generally code for proteins. Soon afterward, American bacteriologist Oswald Avery,
Canadian American geneticist Colin M. MacLeod, and American biologist Maclyn McCarty showed that
bacterial genes are made of DNA, a finding that was later extended to all organisms.

DNA and the genetic code

Study DNA's double helix structure to learn how the organic chemical determines an organism's traits

Study DNA's double helix structure to learn how the organic chemical determines an organism's traits

James Watson and Francis Crick revolutionized the study of genetics when they discovered the structure
of DNA.

Encyclopædia Britannica, Inc.

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A major landmark was attained in 1953 when American geneticist and biophysicist James D. Watson and
British biophysicists Francis Crick and Maurice Wilkins devised a double helix model for DNA structure.
Their breakthrough was made possible by the work of British scientist Rosalind Franklin, whose X-ray
diffraction studies of the DNA molecule shed light on its helical structure. The double helix model
showed that DNA was capable of self-replication by separating its complementary strands and using
them as templates for the synthesis of new DNA molecules. Each of the intertwined strands of DNA was
proposed to be a chain of chemical groups called nucleotides, of which there were known to be four
types. Because proteins are strings of amino acids, it was proposed that a specific nucleotide sequence
of DNA could contain a code for an amino acid sequence and hence protein structure. In 1955 American
molecular biologist Seymour Benzer, extending earlier studies in Drosophila, showed that the mutant
sites within a gene could be mapped in relation to each other. His linear map indicated that the gene
itself is a linear structure.
Illustrated strands of DNA. Deoxyribonucleic acid, biology.

BRITANNICA QUIZ

Genetics Quiz

Who laid the foundation for James Watson and Francis Crick to suggest the helical structure of DNA?

In 1958 the strand-separation method for DNA replication (called the semiconservative method) was
demonstrated experimentally for the first time by American molecular biologist Matthew Meselson and
American geneticist Franklin W. Stahl. In 1961 Crick and South African biologist Sydney Brenner showed
that the genetic code must be read in triplets of nucleotides, called codons. American geneticist Charles
Yanofsky showed that the positions of mutant sites within a gene matched perfectly the positions of
altered amino acids in the amino acid sequence of the corresponding protein. In 1966 the complete
genetic code of all 64 possible triplet coding units (codons), and the specific amino acids they code for,
was deduced by American biochemists Marshall Nirenberg and Har Gobind Khorana. Subsequent studies
in many organisms showed that the double helical structure of DNA, the mode of its replication, and the
genetic code are the same in virtually all organisms, including plants, animals, fungi, bacteria, and
viruses. In 1961 French biologist François Jacob and French biochemist Jacques Monod established the
prototypical model for gene regulation by showing that bacterial genes can be turned on (initiating
transcription into RNA and protein synthesis) and off through the binding action of regulatory proteins
to a region just upstream of the coding region of the gene.

Recombinant DNA technology and the polymerase chain reaction

Technical advances have played an important role in the advance of genetic understanding. In 1970
American microbiologists Daniel Nathans and Hamilton Othanel Smith discovered a specialized class of
enzymes (called restriction enzymes) that cut DNA at specific nucleotide target sequences. That
discovery allowed American biochemist Paul Berg in the early 1970s to make the first artificial
recombinant DNA molecule by isolating DNA molecules from different sources, cutting them, and joining
them together in a test tube. Shortly thereafter, American biochemists Herbert W. Boyer and Stanley N.
Cohen came up with methods to produce recombinant plasmids (extragenomic circular DNA elements),
which replicated naturally when inserted into bacterial cells. These advances allowed individual genes to
be cloned (amplified to a high copy number) by splicing them into self-replicating DNA molecules, such
as plasmids or viruses, and inserting these into living bacterial cells. From these methodologies arose the
field of recombinant DNA technology that came to dominate molecular genetics. In 1977 two different
methods were invented for determining the nucleotide sequence of DNA: one by American molecular
biologists Allan Maxam and Walter Gilbert and the other by English biochemist Fred Sanger. Such
technologies made it possible to examine the structure of genes directly by nucleotide sequencing,
resulting in the confirmation of many of the inferences about genes originally made indirectly.

Learn how DNA thermal cycler employs polymerase chain reaction to copy DNA strands

Learn how DNA thermal cycler employs polymerase chain reaction to copy DNA strands

Specific segments of DNA are amplified (copied) in a laboratory using polymerase chain reaction (PCR)
techniques

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In the 1970s Canadian biochemist Michael Smith revolutionized the art of redesigning genes by devising
a method for inducing specifically tailored mutations at defined sites within a gene, creating a technique
known as site-directed mutagenesis. In 1983 American biochemist Kary B. Mullis invented the
polymerase chain reaction, a method for rapidly detecting and amplifying a specific DNA sequence
without cloning it. In the last decade of the 20th century, progress in recombinant DNA technology and
in the development of automated sequencing machines led to the elucidation of complete DNA
sequences of several viruses, bacteria, plants, and animals. In 2001 the complete sequence of human
DNA, approximately three billion nucleotide pairs, was made public.

Time Line Of Important Milestones In The History Of Genetics

A time line of important milestones in the history of genetics is provided in the table.

Timeline of important milestones in the history of genetics

year event

Gregor Mendel, c. 1865.

1866 Austrian botanist Gregor Mendel published the results of his experiments with pea plants. His
work later provided the mathematical foundation of the science of genetics.

Miescher, Friedrich

1869 Swiss biochemist Johann Friedrich Miescher became the first to isolate nuclein—now known as
DNA. Although he developed hypotheses explaining the role of nuclein in heredity, he ultimately
concluded that one molecule alone could not provide the level of variation observed in nature within
and between species.

Mendel's law of segregationCross of a purple-flowered and a white-flowered strain of peas. R stands for
the gene for purple flowers and r for the gene for white flowers.

1900 Mendel's experiments were rediscovered independently by Dutch botanist and geneticist Hugo
de Vries, German botanist and geneticist Carl Erich Correns, and Austrian botanist Erich Tschermak von
Seysenegg, giving rise to the modern science of genetics.

1928 English bacteriologist Frederick Griffith conducted experiments suggesting that bacteria are
capable of transferring genetic information and that such transformation is heritable.

McClintock, Barbara

1931 American scientists Harriet B. Creighton and Barbara McClintock published a paper
demonstrating that new allelic combinations of linked genes are correlated with physically exchanged
chromosome parts. Their findings suggested that chromosomes form the basis of genetics.

The initial proposal of the structure of DNA by James Watson and Francis Crick, which was accompanied
by a suggestion on the means of replication.

1944 Canadian-born American bacteriologist Oswald Avery and American biologists Maclyn McCarty
and Colin MacLeod reported that the transforming substance—the genetic material of the cell—was
DNA.

polynucleotide chain of deoxyribonucleic acid (DNA)

1950 Austrian-born American biochemist Erwin Chargaff discovered that the components of DNA are
paired in a 1:1 ratio. Thus, the amount of adenine (A) is always equal to the amount of thymine (T), and
the amount of guanine (G) is always equal to the amount of cytosine (C).

Rosalind Franklin.

1951 British scientists Rosalind Franklin, Maurice Wilkins, and Raymond Gosling conducted X-ray
diffraction studies that provided images of the helical structure of DNA fibres.

1953 Using Chargaff's data and the X-ray images recorded by Franklin, Wilkins, and Gosling, British
biophysicists James Watson and Francis Crick determined the molecular structure of DNA. Watson,
Crick, and Wilkins shared the 1962 Nobel Prize for Physiology or Medicine for their discovery.

recombinant DNA

1960s Swiss microbiologist Werner Arber and American microbiologists Hamilton Othanel Smith and
Daniel Nathans discovered restriction enzymes, which cleave DNA into fragments. The discovery, for
which the three men shared the 1978 Nobel Prize for Physiology or Medicine, enabled scientists to
manipulate genes by removing and inserting DNA sequences.
polymerase chain reaction

1970s American molecular biologists Allan M. Maxam and Walter Gilbert and English biochemist
Frederick Sanger developed some of the first techniques for DNA sequencing. Gilbert and Sanger shared
the 1980 Nobel Prize for Chemistry for their work.

DNA sequencing

1983 American biochemist Kary B. Mullis invented the polymerase chain reaction (PCR), a simple
technique that allows a specific stretch of DNA to be copied billions of times in a few hours. Mullis
received the 1993 Nobel Prize for Chemistry for his invention.

human genetics

1990 The Human Genome Project (HGP) began. By the time of its completion in 2003, HGP
researchers had successfully determined, stored, and rendered publicly available the sequences of
almost all the genetic content of the human genome.

International HapMap Project

2002 The International HapMap Project, which was designed to identify genetic variations
contributing to human disease through the development of a haplotype (haploid genotype map of the
human genome), began. By completion of Phase II of the project in 2007, scientists had data on some
3.1 million variations in the human genome.

human chromosomes

2008 The 1000 Genomes Project, an international collaboration in which researchers aimed to
sequence the genomes of a large number of people from different ethnic groups worldwide with the
intent of creating a catalog of genetic variations, began. The project was completed in 2015.

Areas Of Study

Classical genetics

Use the Punnett square to track dominant and recessive allele pairings that make up a trait's genotype

Use the Punnett square to track dominant and recessive allele pairings that make up a trait's genotype

This video uses a Punnett square to illustrate how Gregor Mendel determined the way traits are
inherited.

Encyclopædia Britannica, Inc.

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Learn how dominant and recessive genes determine which traits and offspring will possess
Learn how dominant and recessive genes determine which traits and offspring will possess

Each offspring is a combination of its two parents, receiving some dominant traits from its mother and
others from its father.

Encyclopædia Britannica, Inc.

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Classical genetics, which remains the foundation for all other areas in genetics, is concerned primarily
with the method by which genetic traits—classified as dominant (always expressed), recessive
(subordinate to a dominant trait), intermediate (partially expressed), or polygenic (due to multiple
genes)—are transmitted in plants and animals. These traits may be sex-linked (resulting from the action
of a gene on the sex, or X, chromosome) or autosomal (resulting from the action of a gene on a
chromosome other than a sex chromosome). Classical genetics began with Mendel’s study of
inheritance in garden peas and continues with studies of inheritance in many different plants and
animals. Today a prime reason for performing classical genetics is for gene discovery—the finding and
assembling of a set of genes that affects a biological property of interest.

Cytogenetics

Cytogenetics, the microscopic study of chromosomes, blends the skills of cytologists, who study the
structure and activities of cells, with those of geneticists, who study genes. Cytologists discovered
chromosomes and the way in which they duplicate and separate during cell division at about the same
time that geneticists began to understand the behaviour of genes at the cellular level. The close
correlation between the two disciplines led to their combination.

Plant cytogenetics early became an important subdivision of cytogenetics because, as a general rule,
plant chromosomes are larger than those of animals. Animal cytogenetics became important after the
development of the so-called squash technique, in which entire cells are pressed flat on a piece of glass
and observed through a microscope; the human chromosomes were numbered using this technique.

Today there are multiple ways to attach molecular labels to specific genes and chromosomes, as well as
to specific RNAs and proteins, that make these molecules easily discernible from other components of
cells, thereby greatly facilitating cytogenetics research.

Microbial genetics

bacterial genetics: use of robots

A “robotic pipeline” used in bacterial genetics at University College Cork, Cork, Ireland.

University College Cork, Ireland (A Britannica Publishing Partner)

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Microorganisms were generally ignored by the early geneticists because they are small in size and were
thought to lack variable traits and the sexual reproduction necessary for a mixing of genes from different
organisms. After it was discovered that microorganisms have many different physical and physiological
characteristics that are amenable to study, they became objects of great interest to geneticists because
of their small size and the fact that they reproduce much more rapidly than larger organisms. Bacteria
became important model organisms in genetic analysis, and many discoveries of general interest in
genetics arose from their study. Bacterial genetics is the centre of cloning technology.

human chromosomes

READ MORE ON THIS TOPIC

heredity

…clear in the study of genetics. Both aspects of heredity can be explained by genes, the functional
units...

Viral genetics is another key part of microbial genetics. The genetics of viruses that attack bacteria were
the first to be elucidated. Since then, studies and findings of viral genetics have been applied to viruses
pathogenic on plants and animals, including humans. Viruses are also used as vectors (agents that carry
and introduce modified genetic material into an organism) in DNA technology.

Molecular genetics

Molecular genetics is the study of the molecular structure of DNA, its cellular activities (including its
replication), and its influence in determining the overall makeup of an organism. Molecular genetics
relies heavily on genetic engineering (recombinant DNA technology), which can be used to modify
organisms by adding foreign DNA, thereby forming transgenic organisms. Since the early 1980s, these
techniques have been used extensively in basic biological research and are also fundamental to the
biotechnology industry, which is devoted to the manufacture of agricultural and medical products.
Transgenesis forms the basis of gene therapy, the attempt to cure genetic disease by addition of
normally functioning genes from exogenous sources.
Genomics

The development of the technology to sequence the DNA of whole genomes on a routine basis has given
rise to the discipline of genomics, which dominates genetics research today. Genomics is the study of
the structure, function, and evolutionary comparison of whole genomes. Genomics has made it possible
to study gene function at a broader level, revealing sets of genes that interact to impinge on some
biological property of interest to the researcher. Bioinformatics is the computer-based discipline that
deals with the analysis of such large sets of biological information, especially as it applies to genomic
information.

Population genetics

The study of genes in populations of animals, plants, and microbes provides information on past
migrations, evolutionary relationships and extents of mixing among different varieties and species, and
methods of adaptation to the environment. Statistical methods are used to analyze gene distributions
and chromosomal variations in populations.

Population genetics is based on the mathematics of the frequencies of alleles and of genetic types in
populations. For example, the Hardy-Weinberg formula, p2 + 2pq + q2 = 1, predicts the frequency of
individuals with the respective homozygous dominant (AA), heterozygous (Aa), and homozygous
recessive (aa) genotypes in a randomly mating population. Selection, mutation, and random changes can
be incorporated into such mathematical models to explain and predict the course of evolutionary
change at the population level. These methods can be used on alleles of known phenotypic effect, such
as the recessive allele for albinism, or on DNA segments of any type of known or unknown function.

Human population geneticists have traced the origins and migration and invasion routes of modern
humans, Homo sapiens. DNA comparisons between the present peoples on the planet have pointed to
an African origin of Homo sapiens. Tracing specific forms of genes has allowed geneticists to deduce
probable migration routes out of Africa to the areas colonized today. Similar studies show to what
degree present populations have been mixed by recent patterns of travel.

Behaviour genetics

Another aspect of genetics is the study of the influence of heredity on behaviour. Many aspects of
animal behaviour are genetically determined and can therefore be treated as similar to other biological
properties. This is the subject material of behaviour genetics, whose goal is to determine which genes
control various aspects of behaviour in animals. Human behaviour is difficult to analyze because of the
powerful effects of environmental factors, such as culture. Few cases of genetic determination of
complex human behaviour are known. Genomics studies provide a useful way to explore the genetic
factors involved in complex human traits such as behaviour.

Human genetics

Some geneticists specialize in the hereditary processes of human genetics. Most of the emphasis is on
understanding and treating genetic disease and genetically influenced ill health, areas collectively known
as medical genetics. One broad area of activity is laboratory research dealing with the mechanisms of
human gene function and malfunction and investigating pharmaceutical and other types of treatments.
Since there is a high degree of evolutionary conservation between organisms, research on model
organisms—such as bacteria, fungi, and fruit flies (Drosophila)—which are easier to study, often
provides important insights into human gene function.

Many single-gene diseases, caused by mutant alleles of a single gene, have been discovered. Two well-
characterized single-gene diseases include phenylketonuria (PKU) and Tay-Sachs disease. Other diseases,
such as heart disease, schizophrenia, and depression, are thought to have more complex heredity
components that involve a number of different genes. These diseases are the focus of a great deal of
research that is being carried out today.

Another broad area of activity is clinical genetics, which centres on advising parents of the likelihood of
their children being affected by genetic disease caused by mutant genes and abnormal chromosome
structure and number. Such genetic counseling is based on examining individual and family medical
records and on diagnostic procedures that can detect unexpressed, abnormal forms of genes.
Counseling is carried out by physicians with a particular interest in this area or by specially trained
nonphysicians.

Methods In Genetics

Experimental breeding

Genetically diverse lines of organisms can be crossed in such a way to produce different combinations of
alleles in one line. For example, parental lines are crossed, producing an F1 generation, which is then
allowed to undergo random mating to produce offspring that have purebreeding genotypes (i.e., AA, bb,
cc, or DD). This type of experimental breeding is the origin of new plant and animal lines, which are an
important part of making laboratory stocks for basic research. When applied to commerce, transgenic
commercial lines produced experimentally are called genetically modified organisms (GMOs). Many of
the plants and animals used by humans today (e.g., cows, pigs, chickens, sheep, wheat, corn (maize),
potatoes, and rice) have been bred in this way.

Cytogenetic techniques

Cytogenetics focuses on the microscopic examination of genetic components of the cell, including
chromosomes, genes, and gene products. Older cytogenetic techniques involve placing cells in paraffin
wax, slicing thin sections, and preparing them for microscopic study. The newer and faster squash
technique involves squashing entire cells and studying their contents. Dyes that selectively stain various
parts of the cell are used; the genes, for example, may be located by selectively staining the DNA of
which they are composed. Radioactive and fluorescent tags are valuable in determining the location of
various genes and gene products in the cell. Tissue-culture techniques may be used to grow cells before
squashing; white blood cells can be grown from samples of human blood and studied with the squash
technique. One major application of cytogenetics in humans is in diagnosing abnormal chromosomal
complements such as Down syndrome (caused by an extra copy of chromosome 21) and Klinefelter
syndrome (occurring in males with an extra X chromosome). Some diagnosis is prenatal, performed on
cell samples from amniotic fluid or the placenta.

Biochemical techniques

Biochemistry is carried out at the cellular or subcellular level, generally on cell extracts. Biochemical
methods are applied to the main chemical compounds of genetics—notably DNA, RNA, and protein.
Biochemical techniques are used to determine the activities of genes within cells and to analyze
substrates and products of gene-controlled reactions. In one approach, cells are ground up and the
substituent chemicals are fractionated for further analysis. Special techniques (e.g., chromatography
and electrophoresis) are used to separate the components of proteins so that inherited differences in
their structures can be revealed. For example, more than 100 different kinds of human hemoglobin
molecules have been identified. Radioactively tagged compounds are valuable in studying the
biochemistry of whole cells. For example, thymine is a compound found only in DNA; if radioactive
thymine is placed in a tissue-culture medium in which cells are growing, genes use it to duplicate
themselves. When cells containing radioactive thymine are analyzed, the results show that, during
duplication, the DNA molecule splits in half, and each half synthesizes its missing components.

Chemical tests are used to distinguish certain inherited conditions of humans; e.g., urinalysis and blood
analysis reveal the presence of certain inherited abnormalities—phenylketonuria (PKU), cystinuria,
alkaptonuria, gout, and galactosemia. Genomics has provided a battery of diagnostic tests that can be
carried out on an individual’s DNA. Some of these tests can be applied to fetuses in utero.
Physiological techniques

Physiological techniques, directed at exploring functional properties or organisms, are also used in
genetic investigations. In microorganisms, most genetic variations involve some important cell function.
Some strains of one bacterium (Escherichia coli), for example, are able to synthesize the vitamin thiamin
from simple compounds; others, which lack an enzyme necessary for this synthesis, cannot survive
unless thiamin is already present. The two strains can be distinguished by placing them on a thiamin-
free mixture: those that grow have the gene for the enzyme, those that fail to grow do not. The
technique also is applied to human cells, since many inherited human abnormalities are caused by a
faulty gene that fails to produce a vital enzyme; albinism, which results from an inability to produce the
pigment melanin in the skin, hair, or iris of the eyes, is an example of an enzyme deficiency in man.

Molecular techniques

Although overlapping with biochemical techniques, molecular genetics techniques are deeply involved
with the direct study of DNA. This field has been revolutionized by the invention of recombinant DNA
technology. The DNA of any gene of interest from a donor organism (such as a human) can be cut out of
a chromosome and inserted into a vector to make recombinant DNA, which can then be amplified and
manipulated, studied, or used to modify the genomes of other organisms by transgenesis. A
fundamental step in recombinant DNA technology is amplification. This is carried out by inserting the
recombinant DNA molecule into a bacterial cell, which replicates and produces many copies of the
bacterial genome and the recombinant DNA molecule (constituting a DNA clone). A collection of large
numbers of clones of recombinant donor DNA molecules is called a genomic library. Such libraries are
the starting point for sequencing entire genomes such as the human genome. Today genomes can be
scanned for small molecular variants called single nucleotide polymorphisms, or SNPs (“snips”), which
act as chromosomal tags to associated specific regions of DNA that have a property of interest and may
be involved in a human disease or disorder.

recombinant DNA

recombinant DNA

Steps involved in the engineering of a recombinant DNA molecule.

Encyclopædia Britannica, Inc.

Immunological techniques

immune system

immune system
The immune system and the field of immunogenetics.

HudsonAlpha Institute for Biotechnology (A Britannica Publishing Partner)

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Many substances (e.g., proteins) are antigenic; i.e., when introduced into a vertebrate body, they
stimulate the production of specific proteins called antibodies. Various antigens exist in red blood cells,
including those that make up the major blood groups of man (A, B, AB, O). These and other antigens are
genetically determined; their study constitutes immunogenetics. Blood antigens of man include
inherited variations, and the particular combination of antigens in an individual is almost as unique as
fingerprints and has been used in such areas as paternity testing (although this approach has been
largely supplanted by DNA-based techniques).

Immunological techniques are used in blood group determinations in blood transfusions, in organ
transplants, and in determining Rhesus incompatibility in childbirth. Specific antigens of the human
leukocyte antigen (HLA) genes are correlated with human diseases and disease predispositions.
Antibodies also have a genetic basis, and their seemingly endless ability to match any antigen presented
is based on special types of DNA shuffling processes between antibody genes. Immunology is also useful
in identifying specific recombinant DNA clones that synthesize a specific protein of interest.

Mathematical techniques

Because much of genetics is based on quantitative data, mathematical techniques are used extensively
in genetics. The laws of probability are applicable to crossbreeding and are used to predict frequencies
of specific genetic constitutions in offspring. Geneticists also use statistical methods to determine the
significance of deviations from expected results in experimental analyses. In addition, population
genetics is based largely on mathematical logic—for example, the Hardy-Weinberg equilibrium and its
derivatives (see above).

Bioinformatics uses computer-centred statistical techniques to handle and analyze the vast amounts of
information accumulating from genome sequencing projects. The computer program scans the DNA
looking for genes, determining their probable function based on other similar genes, and comparing
different DNA molecules for evolutionary analysis. Bioinformatics has made possible the discipline of
systems biology, treating and analyzing the genes and gene products of cells as a complete and
integrated system.

Applied Genetics
Medicine

Genetic techniques are used in medicine to diagnose and treat inherited human disorders. Knowledge of
a family history of conditions such as cancer or various disorders may indicate a hereditary tendency to
develop these afflictions. Cells from embryonic tissues reveal certain genetic abnormalities, including
enzyme deficiencies, that may be present in newborn babies, thus permitting early treatment. Many
countries require a blood test of newborn babies to determine the presence of an enzyme necessary to
convert an amino acid, phenylalanine, into simpler products. Phenylketonuria (PKU), which results from
lack of the enzyme, causes permanent brain damage if not treated soon after birth. Many different types
of human genetic diseases can be detected in embryos as young as 12 weeks; the procedure involves
removal and testing of a small amount of fluid from around the embryo (called amniocentesis) or of
tissue from the placenta (called chorionic villus sampling).

Gene therapy is based on modification of defective genotypes by adding functional genes made through
recombinant DNA technology. Bioinformatics is being used to “mine” the human genome for gene
products that might be candidates for designer pharmaceutical drugs.

Agriculture and animal husbandry

Agriculture and animal husbandry apply genetic techniques to improve plants and animals. Breeding
analysis and transgenic modification using recombinant DNA techniques are routinely used. Animal
breeders use artificial insemination to propagate the genes of prize bulls. Prize cows can transmit their
genes to hundreds of offspring by hormone treatment, which stimulates the release of many eggs that
are collected, fertilized, and transplanted to foster mothers. Several types of mammals can be cloned,
meaning that multiple identical copies can be produced of certain desirable types.

Dolly the sheep; cloning

Dolly the sheep; cloning

Dolly the sheep was successfully cloned in 1996 by fusing the nucleus from a mammary-gland cell of a
Finn Dorset ewe into an enucleated egg cell taken from a Scottish Blackface ewe. Carried to term in the
womb of another Scottish Blackface ewe, Dolly was a genetic copy of the Finn Dorset ewe.

Encyclopædia Britannica, Inc.

Plant geneticists use special techniques to produce new species, such as hybrid grains (i.e., produced by
crossing wheat and rye), and plants resistant to destruction by insect and fungal pests.
Plant breeders use the techniques of budding and grafting to maintain desirable gene combinations
originally obtained from crossbreeding. Transgenic plant cells can be made into plants by growing the
cells on special hormones. The use of the chemical compound colchicine, which causes chromosomes to
double in number, has resulted in many new varieties of fruits, vegetables, and flowers. Many
transgenic lines of crop plants are commercially advantageous and are being introduced into the
market.

Industry

Various industries employ geneticists; the brewing industry, for example, may use geneticists to improve
the strains of yeast that produce alcohol. The pharmaceutical industry has developed strains of molds,
bacteria, and other microorganisms high in antibiotic yield. Penicillin and cyclosporin from fungi, and
streptomycin and ampicillin from bacteria, are some examples.

Biotechnology, based on recombinant DNA technology, is now extensively used in industry. “Designer”
lines of transgenic bacteria, animals, or plants capable of manufacturing some commercial product are
made and used routinely. Such products include pharmaceutical drugs and industrial chemicals such as
citric acid.

A.M. Winchester