What is Genetics? – Definition, Example, Importance, and History

 

What is Genetics?

Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. Though heredity had been observed for millennia, Gregor Mendel, Moravian scientist and Augustinian friar working in the 19th century in Brno, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance".

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, medicines and biotechnology.

This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

Trait inheritance and molecular inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the cell, the organism (e.g. dominance), and within the context of a population. Genetics has given rise to a number of subfields, including molecular genetics, epi-genetic and population genetics. Organisms studied within the broad field span the domains of life (Archaea, Bacteria, and Eukarya).

Genetic processes work in combination with an organism's environment and experiences to influence development and behavior, often referred to as nature versus nurture. The intracellular or extracellular environment of a living cell or organism may switch gene transcription on or off. A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate (lacking sufficient waterfall or rain). While the average height of the two corn stalks may be genetically determined to be equal, the one in the arid climate only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment.

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.

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.

 

Examples of Genetics

1. Classical or formal genetics: The study of the transmission of single genes within families and the analysis of more complex types of inheritance.

2. Clinical genetics: The diagnosis, prognosis and, in some cases, the treatment of genetic diseases.

3. Genetic counseling: An important area within clinical genetics involving the diagnosis, risk assessment, and interpersonal communication.

4. Cancer genetics: The study of genetic factors in inherited and sporadic cancer.

5. Cytogenetic: The study of chromosomes in health and disease.

6. Biochemical genetic: The biochemistry of nucleic acids and proteins including enzymes.

7. Pharmacogenetics: How genes govern the absorption, metabolism and disposal of drugs and untoward reactions to them.

8. Molecular genetics: The molecular study of genetics including particularly DNA and RNA.

9. Immunogenetics: The genetics of the immune system including blood groups, HLA, and the immunoglobulin.

10. Behavioral genetics: The study of genetic factors in behavior in health and disease including mental retardation and mental illness.

11. Population genetics: The study of genes within populations including gene frequencies, the gene pool, and evolution.

12. Reproductive genetics: The genetics of reproduction including genes and chromosomes in germ cells and the early embryo.

13. Developmental genetics: The genetics of normal and abnormal development including congenital malformations (birth defects). (Cancer genetics- The study of the genetic factors in inherited and sporadic cancer)

14. Eco-genetics: The interaction of genetics with the environment.

15. Forensic genetics: The application of genetic knowledge, including DNA, to legal matters.

 

Characteristics of Genetics

Parents pass on traits or characteristics, such as eye colour and blood type, to their children through their genes. Some health conditions and diseases can be passed on genetically too. 

Sometimes, one characteristic has many different forms. For example, blood type can be A, B, AB or O. Changes (or variations) in the gene for that characteristic causes these different forms. 

Each variation of a gene is called an allele (pronounced ‘AL-eel’). These two copies of the gene contained in your chromosomes influence the way your cells work.

The two alleles in a gene pair are inherited, one from each parent. Alleles interact with each other in different ways. These are called inheritance patterns.

Examples of inheritance patterns include:

a)  Autosomal dominant: Where the gene for a trait or condition is dominant, and is on a non-sex chromosome 

b) Autosomal recessive: Where the gene for a trait or condition is recessive, and is on a non-sex chromosome

c)  X-linked dominant: Where the gene for a trait or condition is dominant, and is on the X-chromosome

d) X-linked recessive: Where the gene for a trait or condition is recessive, and is on the X-chromosome

e) Y-linked: Where the gene for a trait or condition is on the Y-chromosome

f) Co-dominant: Where each allele in a gene pair carries equal weight and produces a combined physical characteristic

g) Mitochondrial: Where the gene for a trait or condition is in your mitochondrial DNA, which sits in the mitochondria (powerhouse) of your cells.

Dominant and recessive genes

The most common interaction between alleles is a dominant/recessive relationship. An allele of a gene is said to be dominant when it effectively overrules the other (recessive) allele. 

Eye colour and blood groups are both examples of dominant/recessive gene relationships.

Eye colour

The allele for brown eyes (B) is dominant over the allele for blue eyes (b). So, if you have one allele for brown eyes and one allele for blue eyes (Bb), your eyes will be brown. (This is also the case if you have two alleles for brown eyes, BB.) However, if both alleles are for the recessive trait (in this case, blue eyes, bb) you will inherit blue eyes.

Blood groups

For blood groups, the alleles are A, B and O. The A allele is dominant over the O allele. So, a person with one A allele and one O allele (AO) has blood group A. Blood group A is said to have a dominant inheritance pattern over blood group O.

If a mother has the alleles A and O (AO), her blood group will be A because the A allele is dominant. If the father has two O alleles (OO), he has the blood group O. For each child that couple has, each parent will pass on one or the other of those two alleles. This is shown in figure 1. This means that each one of their children has a 50 per cent chance of having blood group A (AO) and a 50 per cent chance of having blood group O (OO), depending on which alleles they inherit. 

Recessive genetic conditions

If a person has one changed (q) and one unchanged (Q) copy of a gene, and they do not have the condition associated with that gene change, they are said to be a carrier of that condition. The condition is said to have a recessive inheritance pattern – it is not expressed if there is a functioning copy of the gene present. 

If two people are carriers (Qq) of the same recessive genetic condition, there is a 25 per cent (or one in four) chance that they may both pass the changed copy of the gene on to their child. As the child then does not have an unchanged, fully functioning copy of the gene, they will develop the condition. 

There is also a 25 per cent chance that each child of the same parents may be unaffected, and a 50 per cent chance that they may be carriers of the condition.

Co-dominant genes

Not all genes are either dominant or recessive. Sometimes, each allele in the gene pair carries equal weight and will show up as a combined physical characteristic. For example, with blood groups, the A allele is as ‘strong’ as the B allele. The A and B alleles are said to be co-dominant. Someone with one copy of A and one copy of B has the blood group AB.

The inheritance pattern of children from parents with blood groups B (BO) and A (AO). Each one of their children has a 25 per cent chance of having blood group AB (AB), A (AO), B (BO) or O (OO), depending on which alleles they inherit.

Gene changes in cells

A cell reproduces by copying its genetic information then splitting in half, forming two individual cells. Occasionally, an alteration occurs in this process, causing a genetic change.

When this happens, chemical messages sent to the cell may also change. This spontaneous genetic change can cause issues in the way the person’s body functions.

Sperm and egg cells are known as ‘germ’ cells. Every other cell in the body is called ‘somatic’ (meaning ‘relating to the body’). 

If a change in a gene happens spontaneously in a person’s somatic cells, they may develop the condition related to that gene change, but won’t pass it on to their children. For example, skin cancer can be caused by a build-up of spontaneous changes in genes in the skin cells caused by damage from UV radiation. Other causes of spontaneous gene changes in somatic cells include exposure to chemicals and cigarette smoke. However, if the gene change occurs in a person’s germ cells, that person’s children have a chance of inheriting the altered gene. 

Genetic conditions

About half of the Australian population will be affected at some point in their life by a condition that is at least partly genetic in origin. Scientists estimate that more than 10,000 conditions are caused by changes in single genes. 

The three ways in which genetic conditions can arise are:

a) A change in a gene occurs spontaneously in the formation of the egg or sperm, or at conception

b) A changed gene is passed from parent to child that causes health issues at birth or later in life

c) A changed gene is passed from parent to child that causes a ‘genetic susceptibility’ to a condition. 

Having a genetic susceptibility to a condition does not mean that you will develop the condition. It means that you are at increased risk of developing it if certain environmental factors, such as diet or exposure to chemicals, trigger its onset. If these triggering conditions do not occur, you may never develop the condition. 

Some types of cancer are triggered by environmental factors such as diet and lifestyle. For example, prolonged exposure to the sun is linked to melanoma. Avoiding such triggers means significantly reducing the risks.

Genes and genetics – Related Parents

Related parents are more likely than unrelated parents to have children with health problems or genetic conditions. This is because the two parents share one or more common ancestors and so carry some of the same genetic material. If both partners carry the same inherited gene change, their children are more likely to have a genetic condition.
Related couples are recommended to seek advice from a clinical genetics service if their family has a history of a genetic condition.

Genetic counseling and testing

If a family member has been diagnosed with a genetic condition, or if you know that a genetic condition runs in your family, it can be helpful to speak to a genetic counselor. 
Genetic counselors are health professionals qualified in both counseling and genetics. As well as providing emotional support, they can help you to understand a genetic condition and what causes it, how it is inherited (if it is), and what a diagnosis means for you and your family.

Genetic counselors are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs.

Genetic services in Victoria provide genetic consultation, counseling, testing and diagnostic services for children, adults, families, and prospective parents. They also provide referral to community resources, including support groups, if needed.

 

History of Genetics

Genetics is primarily and originally a science dealing with heredity i.e. the transmission of characteristics from parents to offspring. From such considerations, laws are derived concerning the relationships. In addition, genetics also involves a study of the factors, which show the relationship between parents and offspring and which also account for the many characteristics which organisms possess. You are familiar with the observations that “Like begets like”, that children tend to resemble their parents as well as their siblings (or sibs i.e. their brothers and sisters), but they also tend to vary or look different from one another in many ways.

Genetics is the science, which tries to account for similarities and variations between related individuals. The science studies the transmission of hereditary factors from parents to offspring. Put differently, it is a study of biological “communication” between generations using the hereditary factors. Another facet of the science is the study of the expression or effect of the factors during development.

If one were to put the above “descriptive definition” of Genetics in a capsule form, Bateson, who coined the term Genetics in 1906 aptly defines it as follows: Genetics is the science dealing with heredity and variation, seeking to discover laws governing similarities and differences in individuals related to descent. The factors which are transmitted were called “Genes” by Johannsen in 1909. As mentioned above, Genetics provide explanations to the phenomenon of heredity and variation. It is therefore, not surprising that the beginning of genetics dated back to the centuries before Christ. Around 400 BC Hippocrates theorized that small representative elements of all parts of the parental body are concentrated in the semen. It is these elements, which provide the building blocks for the corresponding parts of the embryo. According to this theory characteristics acquired by parents can be transmitted to offspring. Aristotle (384-322 BCE), one century later disproved the theory postulated by Hippocrates (about 470 BC-about 410 BC), pointing out the facts that crippled and mutilated parents do not always produce abnormal offspring.

Aristotle, in turn advanced the theory that the father’s semen provides the plans according to which the amorphous blood of the mother is to be shaped into the offspring. Put differently the semen supplied the FORM while the mother’s blood supplied the SUBSTANCES. It is important at this point to note that Aristotle recognized that biological inheritance consists of a transmission of information for embryonic development, and not simply a transmission of samples of body parts. The fact that the information in the seminal fluid could not be seen, it was regarded as a mystical influence. Early in the 17th century, Harvey called this influence the AURA SEMINALIS. In the 17th and 18th centuries, new theories of inheritance were propounded, following the discoveries of the egg and the sperm. One theory was the PREFORMATION THEORY, which depending on the school of thought, stated that either the egg or the sperm contains the entire organism in a miniaturized but perfect form. In the case of men, the theory postulated a miniature human being, called a homunculus, present in the sperm. This theory was postulated by Jan Swammerdam (1637-1680). Not too surprisingly there were scientists who claimed that they saw homunculi in spermatozoa. They even drew diagrams to illustrate what they saw. One person who made an elaborate drawing of homunculus was Nicolass Hartsoeker (1656-1725). The major drawback with the pre-formation theory is the fact that is implies that one homunculus contained another, which in turn contained yet another ad infinitum. Another theory of development was the THEORY OF EPIGENESIS. In the 18th century, Christian Wolff (1679-1754) discovers that adult structures in plants and animals arise from embryonic tissues, which do not resemble the corresponding adult structures. In other words, there is no pre-formation. But Wolff thought that mysterious vital forces were responsible for what he thought was a de novo origin of adult parts. Wolff’s view modified in the 19th century by Karl Ernst Von Baer (1792-1876) who stated that adult parts arise as a result of a gradual transformation or differentiation of embryonic tissues into increasingly specialized tissues. Although the modified epigenetic theory is correct. It did not account for the form in which the materials to be transformed existed in the original embryonic cell, zygote. Early in the 19th century, Pierre-Louis Maupertuis (1698-1759) postulated that minute particles from each part of the body of the parents are united in sexual reproduction such that during development particles from the male dominate in some cases; in other cases those form the female parent dominate. In one important aspect, this theory recognized the fact that an offspring receives two of each type of particle, one from each parent, but exhibits only one. However, by suggesting that the body parts contribute particles, this theory leads to the theory of evolution advanced by JeanBaptiste Lamarck (1744–1829). According to Lamarck’s interpretation characteristics such as well-developed muscles acquired by parents in the course of their life can be transmitted to their offspring. This idea was formalized by Charles Darwin (1809-1822) as the “Provisional Hypothesis of Pangenesis.” According to Darwin, exact miniature replicas, called gemmules, of the body parts and organs are carried in the blood stream, to be assembled in the gametes. In the zygote, the gemmules from both sexes come together and are parceled out to form the appropriate structures during development. Since a gemmule is an exact replica of a parental part it means that acquired characteristics should be inherited by the offspring. If that were so it would be easy to understand evolution. Recall that the theory of pangenesis is essentially the same theory advanced by Hippocrates in the 5th century B. C. and disproved by Aristotle.

In many ways Genetics is a precise and somewhat mathematical science dealing with specific offspring ratios which are predictable on the basis of the known genetic constitutions of the parents. In the reverse process, the genetic constitution of the different types of offspring they produced. Gregor Johann Mendel (1822-1884), an Austrian monk, is regarded as the father of Genetics. It is generally agreed that Mendel’s success can be attributed to the fact that he was lucky in choosing the garden pea, Pisum sativum, for his studies. This plant, although, normally self-pollinating can be easily cross-pollinated. Mendel was also successful because he studied the inheritance of single contrasting characters (i.e. smooth versus wrinked), unlike his predecessors who studied several characters simultaneously. Equally important was the fact that Mendel counted and carefully recorded the numbers of each type of offspring from each of his crosses.

Three investigators unaware of Mendel’s work and results independently carried out similar plant breeding experiments. During the process of writing their findings for publication, they each came across Mendel’s paper and they referred to it in their rediscovery of the Mendelian laws of inheritance. Although the three people, Correns, Hugo de Vries and Tschermak are generally regarded as the rediscoverers, some scientists (Stern & Sherwood, 1966) do not think that Tschermak’s work on its own could have yielded the laws of inheritance. Hence, there should be only two rediscoverers. Although the laws of inheritance were first demonstrated with plants, Bateson in 1902 showed that the laws apply equally to animals. From this brief history of Genetics one would hope that you would derive and appreciate the tortuous steps leading to the establishment of various laws in science.

The history of genetics has been discussed in the Unit so as to let you appreciate the toils and labour of those who had worked in the field before. You could also develop your own ideas in Genetics and be reckoned with as one of the greats.

Genetics is a vital aspect of everyday life and of biology and biologists, and even non-biologists, should be fully exposed to it. Every father wants to be sure that the baby brought from the hospital is his own, and farmers want improved farm products – both animals and plants. These aspects are being further improved by genetic engineering which results in better agricultural products. Increased knowledge of heredity through genetics means increased power of control over living things. Genetics is the science dealing with heredity and variation and is governed by laws. The history of genetics dated even before Christ. Hippocrates, Aristotle Maupertuis, Lamark, Mendel and Charles Darwin are some of the eminent scientists who have contributed to the knowledge of Genetics. Genetics is a precise and somewhat mathematical science dealing with specific offspring ratios which are predictable on the basis of the known genetic constitutions of the parents. It should not be studied like literature but examples should be worked out so as to be acquainted with the mathematical rules guiding heredity.