The Composition of Matter

Chemistry is defined as an organized body of knowledge concerning the composition of matter and how matter interacts and changes . Matter can be defined as anything that possesses the properties of mass and volume. Mass is the measure of a chunk of matter's ability to resist a change in movement or direction. The chemist calls this "inertia". Volume is the three dimensional space occupied by matter. It is a universal axiom that two chunks of matter can not occupy the same space simultaneously. All matter is composed of basic particles of atoms, molecules or ions.

What are atoms?

The basic of these particles are the atom. Atoms are particles that are composed of three sub-atomic particles. The three sub-atomic particles are the proton, neutron, and the electron. The proton is a positively charged particle that has the mass approximately equal to a hydrogen atom. It is said to have the mass of 1 atomic mass unit (amu). The protons reside in the nucleus or center of the atom. The neutron is a neutrally charged particle that is approximately the mass of a proton. The neutron does not have an electrical charge, but it also resides in the nucleus. The particles of the nucleus are called nucleons. The third particle of an atom is the electron. The electron is a negatively charged particle of negligible but finite mass. It resides on the outer edges of the atom with an enormous amount of space between the nucleus and the electrons.

Because the protons and the neutrons are relatively well protected in the nucleus and the electron is on the periphery of the atom, it is the electrons that undergo change during a chemical reaction. It is the electrons that the chemist focuses upon. It takes millions of times more energy for the nucleus to be penetrated so the nucleus is not affected in normal chemical reactions. Atoms are neutral so that means that the positive protons and the negative electrons must be equal for the atom to be neutral. There are some 111 known different atoms that are differentiated by two numbers, atomic number and mass number. The atomic number is equal to the number of protons found in the nucleus. For a neutral atom that would also be the number of electrons in the atom. The mass number is defined as the sum of the protons and the neutrons in the nucleus of an atom. In other words, the mass number is the number of nucleons found in the atom. If I know the atomic number of an atom and the mass number then I can determine the number of electrons, protons, and neutrons that the atom has.

What are molecules?

Molecules are clusters of atoms that are held together by strong electrical forces called bonds. Molecules are neutral like atoms. Substances that form molecular units are called molecular substances. Just as elements can be symbolized by a symbol, molecules can be symbolized by the symbols of the elements that make up the compound. These are called formulas. Non-metallic elements react with other non-metallic elements to form molecular compounds

What are ions?

Ions are the third basic particle of matter. Ions are charged atoms or groups of atoms possessing an electrical charge. Atoms are neutral possessing equal numbers of negative electrons and positive protons. However some atoms have a tendency to lose one or more negative electrons during a Chemical change. If the atom loses electrons that would mean that the atom would have more positive protons than negative electrons and the atom would become positively charged. These ions are called "cations". Metallic elements tend to lose electrons and become cations.

Other atoms prefer to gain one or more negative electrons. This would give such an atom more negative electrons than protons and the atom would be negatively charged. These are referred to as "anions". Non-metallic elements have atoms that tend to gain electrons to become anions.

Why do atoms lose or gain electrons?

The whole motivation of why some atoms lose and some gain electrons is to make the atoms chemically more stable. They gladly sacrifice their electrical neutrality in favor of a more stable state. The more stable state is an ion that has the same number of electrons as one of the noble gases found in the last column of elements on the right of the periodic table. This condition is referred to as being "isoelectronic" to a Noble gas. These elements used to be called the "inert gases" because there were no known compounds of these elements. These elements are still relatively inert which is a testament of their great stability. All other elements have atoms that would like to emulate these noble gases. In other words, to be isoelectronic to a Noble Gas. The Noble gases were found to react with certain very chemically reactive elements like Fluorine and Oxygen and only Xenon has been known to form compounds of Fluorine and Oxygen. It is not surprising that such relatively stable elements would react with these two non-metals (Fluorine and Oxygen) If they were to react it would be the two most reactive non-metals in the universe.

Why do elements lose or gain different numbers of electrons?

All atoms would like to have the same number of electrons as a Noble gas because of their great stability. In order to accomplish that, some atoms must lose one and some more than one electron in order to have the same number of electrons as the nearest Noble Gas in the Periodic Table. For the first column of elements called Group 1 located in the first column on the left side of the Periodic Table each element in this Group have atoms that have only one electron that needs to be lost in order to reach this isoelectronic state with the nearest Noble Gas located on the last column on the extreme right of the Table called Group 18. The second column of elements in the Periodic Table called Group 2 has elements whose atoms have two electrons that must be lost to become isoelectronic to the nearest Noble Gas. On the other end of the Periodic Table, there are Groups of elements like Group 15 where atoms must gain three electrons in order to become isoelectronic to the nearest Noble Gas. Group 16 elements have atoms that must gain only two electrons to become isoelectronic to the nearest Noble Gas. Group 17 elements which are only one column to the left of the Noble Gas Group requires that its atoms need only to gain one electron.

The Classification of Matter

Matter is classified in one of three ways:

1. Elements
2. Compounds
3. Mixtures

Elements

Elements is the most basic of all matter. Elements are pure substances that have a set of unique properties and cannot be further broken down or decomposed into other elements by chemical methods. There are some 111 different elements or more depending upon the time that you read this. Each element differs in the kind of atom particle that makes up the elemental substance. Each element has a different atom from other elements differing in the number of sub-atomic particles (electrons, protons, and neutrons). Each atom is symbollically distinguished by a specific symbol to represent the element. These symbols were not always as standard and defined as they are now. At one time in midieval Europe the alchemist,the fore runner of the present day Chemist, used a set of symbols to represent the elements. The only problem is that there were no one set of symbols agrreable to all Alchemists. A man by the name of Berzelius and people like him realized that unless scientists had a common symbol table to use, communication would be garbled at best. Therefore a set of symbols were decided upon to be used by all concerned. At first, these symbols were based on the Latin word for the known elements in that day. We may think that the symbol for Hydrogen comes from the English word for Hydrogen, but it originated from the latin word which happens to have the first letter an "H". Latin was used because during the renaisance, Latin was the accepted language among the clergy and intellectuals of that time. With the passage of time, other sources were used to select a symbol. Greek and Roman Methology is used such as Plutonium and Helium named after the Greek God of the Sun, Helios. Neptunium named after the ancient God of the Sea, Neptune. Other sources were famous geographical settings such as Californium and Berkylium. Still others were named after distinguished scientiests such as Einsteinium, Fermium, and Mendeleevium. The names and consequent symbols for the elements require international collaboration to decide upon a name and symbol for newly discovered elements. Sometimes disagreements over what names will apply arise. For instance, just recently the American Chemical Society and the IUPAC have a disagreement over some of the Newer Elements.

The atoms of a specific element can differ in the number of neutrons that the nucleus of the atom possesses. These different "forms" of the element are called Isotopes. The atoms of all the isotopes of a single element will have the same number of electrons and protons, but their neutrons and therefore their masses will differ. It is the statistical distribution in nature along with the isotopic mass of each Isotope that will decide the atomic mass of the element itself.

Elements chemically combine their atoms to produce another form of matter called the compound.

Compounds

Compounds are pure substances having a unique set of properties that are produced when elemental substances chemically combine. Unlike elements whose numbers are finite, the number of compounds theoretically possible are limitless. Compounds are symbolized by a formula which is the use of the symbol for each element represented in the compound and subscripts that appear right after the symbol of the element. The subscripts have a duel purpose. They can be interpreted as the number of atom particles making up a single unit of the compound. However a more practical interpretation of the subscript is to represent the number of mole units of the element in each mole unit of the compound. Compounds interact with each other when brought into contact. Compounds can be ionic consisting of ion particles. Compounds can also be molecular composed of molecular units instead of ionic units. Ionic compounds are usually as a result of Metallic elemets chemically combining with Non-metallic elements. Molecular compounds are formed when Non-metallic or Metalloid elements chemically combine with each other. Compounds are much more varied to study. When we physically combine elements and/or compounds together we get a third kind of matter, a mixture.

Mixtures

Mixtures are physical combinations of two or more elements or compounds. Being physical combinations, mixtures can be separated by using a difference in a physical property between the pure substances in the mixture. This is called Resolution of the Mixture and results in the physical separation of the componenets in the mixture.

There are three kinds of mixtures.

1. Heterogeneous
2. Homogeneous
3. Colloids

Heterogeneous mixtures are mixtures where the compoents are not uniform in their distribution throughout the mixture. The division between the components in such a mixture can be easily seen. Such mixtures can often but not always be separated by filtration or the use of a separatory funnel. If we take a sample of such a mixture the sampling will not always show the same distribution of each component in the mixture. For this reason, we say that the mixture is not uniform i its composition.

Homogeneous mixtures are uniform in their distribution. If we took a sampling anywhere in the mixture, and then analyzed it as to its composition for each component we would find that the distribution was the same throughout the mixture. All solutions are said to be Homogeneous mixtures.

Colloids are sometimes classified as a heterogeneous mixture. However, unlike a heterogeneous mixture whose components will separate, colloids are mixtures whose components will not easily separate out. This is because the size of the particles making up the mixture are not large enough for gravitational force to pull them apart. The particles can't be seen as in the case of a heterogeneous mixture, but the particles of a colloid will scatter a beam of light passed through the colloid. This scattering of light by the particles is called the Tyndall Effect and it is a property of colloids. Colloids can not be filtered since the particles are small enough to pass through the tiny openings of the filter paper. Colloids can be destabilized by removing charged particles that are responsible for keeping the particles from becoming larger. Electrostatic precipitators are devices that can destabilize a colloid and cause the components to separate. Heterogeneous mixtures have the largest particles, followed by colloidal particles. The smallest particles are found in homogeneous mixtures such as solutions. These particles are too small to be seen as in Heterogensous mixtures and too small to scatter a beam of light passed through the mixture as a colloid exhibits in the Tyndall Effect.

The Changes Matter Undergoes

Matter can undergo two major kinds of Changes.

Physical Changes

Physical Change is a change where there is no change in the composition of the substance and only the physical state of the substance takes place. Examples include the melting of a solid, The freezing of a liquid, sublimation of a solid to a gas, formation of a solution, evaporation, boiling process, crystallization, etc. In a physical change the molecular particles remain unchanged and no fragmentation of the molecules take place. Physical changes may be accompanied with energy being absorbed or given off during the physical change.

Chemical Changes

Chemical change involves a change in the composition as well as the physical state (perhaps). In a chemical change molecules fragment and recombine to form new molecules with a different composition. Evidence of Chemical Change include formation of evolution of gas at room temperature, the formation of a precipitate at the mixing of two solutions. Examples of Chemical Change include the souring of milk, fermentation, combustion (burning), cooking of foods, metal corrosion, electroplating of objects, etc.

Which clues can be used to decide if a chemical change has taken place?

Whenever we wish to understand a phenomenon that is occurring in real life, there are two basic viewpoints with which we can view the phenomenon whether it be a chemical change or physical change.

One view in which we can perceive such a change is the "sub-microscopic" view. This view is what I call the "molecular" view. It is a view that we can only hope to envision with our mind's eye. We can't actually view the particles (molecules or ions), but by envisioning molecules and atoms dynamically where the particles are in constant movement we can better understand what is actually happening.

The second view is called the "macroscopic view". This view is a view that we observe in the world around us. It is what we observe in a laboratory as we observe chemical and physical changes.

The relationship between the two viewpoints is important. Often we can't really understand what we are viewing macroscopically until we have developed a sub-microscopic or molecular view of that change.

There are a number of clues that will point to a Chemical change taking place. A chemical change will take place when the molecules or ions of matter undergo rearrangement or fragmentation where in many cases the fragments recombine to form new molecules or ions. The bottom line from a molecular view is that molecules undergo change in structure or shape. However from a "macro-scopic" view, the view that we can see in the laboratory what are some laboratory observations?

1. A rapid evolution of bubbles at room temperature.

   This usually indicates that a new gaseous substance is being formed where the substance was not present to begin with.

2. A solid forms upon the mixture of two chemical solutions.

   This is called "precipitation". Do not confuse this with another process that takes place which would be described as a physical change, solidification or crystallization. When a pure liquid becomes a solid, a solid does appear (macroscopically), but this is the physical change of liquid molecules into more rigidly fixed solid molecules. Since the molecules are the same (whether in the liquid or solid state) no chemical change is taking place.

   A precipitation also results in a solid appearing (macroscopically). However the solid is caused as a result of two solutions (containing ion particles) being brought together. Two of the ions of opposite charge get together and a new substance is formed which is insoluble in the solvent present (usually water). This results in the new substance appearing as a solid. Since the new substance was not present before the change took place this would be described as a chemical change. The precipitate upon further examination of its properties would have a different property profile.

3. Energy is being exchanged (absorbed or liberated). However, we have to be very careful with this observation since physical changes can also involve energy exchanges. The important thing is that if we reverse the energy exchange that the pure substances before and after the energy exchange are the same. How can we know that the pure substances are the "same"? That brings us to our fourth observation.

4. The measurable properties of a pure substance before the change will be altered to a new value after the change if the change is truly a chemical change.

   Every pure substance (elements and compounds) have a set of measurable properties such as melting point, boiling point, density, etc. I call this its' "property profile" of the pure substance. If the property profile is altered after the change has taken place then we can suspect that a new pure substance is present and a chemical change has apparently taken place. For example, if we have a pure substance "A" and a change takes place where the boiling point of a pure substance "B" is measured and is shown to be different than the boiling point of substance "A", then we have to conclude that a new substance "B" has formed that was not there initially.

The Resolution of Mixtures

Mixtures are physical combinations of two or more pure substances (elements or compounds). As a physical combination one should be able to separate these substances from the mixture by physical methods so that no Chemical change can take place during the separation. Substances can be separated from the mixture by taking advantage of any differences the substances have in physical properties. For example, most pure substances have different boiling points (temperature at which a substance will boil). If we can heat a mixture so that the lowest boiling substance will boil off before any other substance begins to boil, we will effectively be able to separate that substance from the mixture. One such laboratory resolution method is known as distillation. There are several different types of distillation.

Types of Distillation

1. Simple Distillation- This form of distillation is best used when you have pure substances that you wish to separate that are 100 degrees or more apart in boiling point. Basically, it consists of a boiling flask in which the mixture is placed. The flask is connected to a distilling head with a thermometer inserted into its top. The distilling head is connected to a water cooled condenser which ,in turn, is connected to a recepticle where the pure substance that was boiled off would be found.

   The flask is usually heated by either a bunsen burner or a heating element called a Heating Mantle. Heating Mantles are used when there is a danger of vaporous substance igniting due to the presence of a burner flame.Other forms of heating are a water bath that is heated by a hot plate surface or a steam bathg where live steam is the source of heating. Milder and more controllable form of heating is the water bath, but it is only capable of heating up to 100 degrees Celsius.

   The flask is heated until the boiling point of the lowest boiling substance in the mixture is reached. Then the substance will vaporize and travel over to the condenser whose inner core is cooled by circulating water from the cold water tap. Water travels in at the bottom of the condenser and comes out at the top. As the hot vapors of the substance come into contact with the water cooled surface of the condenser core, the vapor will lose its energy it gained from being heated in the flask and will condense back into a liquid which will be pure in the lowest boiling substance in the mixture. The closer the boiling points of the substances are in the mixture, the more likely that the vapor coming over into the condenser will be partially of the two substances. If the substances within the mixture are within 60 degrees of one another, simple distillation techniques are not very efficient and a second type of distillation must be considered.

2. Fractional Distillation-Fractional Distillation is similar to simple distillation except for one very important extra item in the set-up. We have a vertical column that is inserted in between the distilling flask and the distilling head called the "fractionating column". This column is ,unlike the condenser, never heated. It is usually filled with glass beads or metal wire mesh inside its central core. Quite often, it is insulated using spun glass which is stuffed into the center of the column. This provides many surfaces for the vaporized substances to condense upon. As the mixture is heated the substances that are nearest to each other in boiling point will both vaporize. The vapor mixture will then rise into the fractionating column and will condense on the relatively cooler surface and the hot condensate will fall back into a lower portion of the column only to be met with hot vapors from below and the hot vapors will re-vaporize the liquid back into the vapor state pushing the mixture vapor even higher up the fractionating column where it will be recondensed and the cycle repeats itself over and over again. Each time the vapor mixture gets richer in the lower boiling substance. By the time it reaches the distilling head at the top of the fractionating column the vapor is pure in the lower boiling substance. Actually, each time there is a vaporization followed by a condensation, a simple distillation is being performed. Fractionating columns are rated in "theoretical plates". A theoretical plate equal to 1 is the same efficiency as one simple distillation. Usually theoretical Plates of 4 or 5 are considered good since that would yield the same efficiency of separation as if you had simple distilled each successive distillate (that which was distilled over) 4 or 5 times in succession.

   Often, the fractionating columns are wrapped in thermal insulation and even heated at the bottom of the column while gradually cooled at the top.

3. Vacuum (Reduced Pressure) Distillation- Often a mixture contains a pure substance which will have such a high boiling point that it will chemically decompose before it reaches its boiling point. This is obviously self defeating. However, boiling points vary according to the atmospheric pressure. The lower the pressure the lower the boiling point will be for the same substance. If the pressure inside the distilling flask could be reduced thereby reducing the boiling point below the decomposition point, then the substance could be effectively separated from the mixture. Vacuum distillation is also used when you have to separate a highly volatile and combustible solvent that has a low flash point. By reducing the pressure inside the flask you could effectively distill such a solvent without using even a heating mantle or hot plate. Just the warmth of the body temperature would be sufficient to separate it!!
4. Steam Distillation- An alternative way of distilling a liquid substance whose normal boiling point (boiling point at 760 mm Hg) is above its decomposition point is steam distillation. A substance must be insoluble (immiscible) in water. It must also have a polar group that could be attracted to the water molecule. Oils and tars are steam distillable. The advantage over vacuum distillation is that you don't have the excessive "bumping" that takes place in the vacuum distillation. The ancient Egyptians marketed a rose oil that was separated from rose petals by essentially a steam distillation process. The rose oil was at one time considered as valuable as its weight in Gold!! The distillate that comes over is actually two immiscible liquids, water and the organic substance. This is referred to as a "co-distillate". The organic by itself might boil hundreds of degrees Celsius but will come over with the steam at or below 100 degrees Celsius.

Psychology

What is psychology?

• Psychology: the science of the mind
• How do psychologists study the mind?
• Human behaviour: the raw data of psychology
• Psychology and other disciplines
• Branches of psychology

Psychology: the science of the mind

Psychology is the science of the mind. The human mind is the most complex machine on Earth. It is the source of all thought and behaviour.

How do psychologists study the mind?

But how can we study something as complex and mysterious as the mind? Even if we were to split open the skull of a willing volunteer and have a look inside, we would only see the gloopy grey matter of the brain. We cannot see someone thinking. Nor can we observe their emotions, or memories, or perceptions and dreams. So how do psychologists go about studying the mind?

In fact, psychologists adopt a similar approach to scientists in other fields. Nuclear physicists interested in the structure of atoms cannot observe protons, electrons and neutrons directly. Instead, they predict how these elements should behave and devise experiments to confirm or refute their expectations.

Human behaviour: the raw data of psychology

In a similar way, psychologists use human behaviour as a clue to the workings of the mind. Although we cannot observe the mind directly, everything we do, think, feel and say is determined by the functioning of the mind. So psychologists take human behaviour as the raw data for testing their theories about how the mind works.

Since the German psychologist Wilhelm Wundt (1832-1920) opened the first experimental psychology lab in Leipzig in 1879, we have learned an enormous amount about the relationship between brain, mind and behaviour.

Psychology and other disciplines

Psychology lies at the intersection of many other different disciplines, including biology, medicine, linguistics, philosophy, anthropology, sociology, and artificial intelligence (AI).

For example, neuropsychology is allied with biology, since the aim is to map different areas of the brain and explain how each underpins different brain functions like memory or language. Other branches of psychology are more closely connected with medicine. Health psychologists help people manage disease and pain. Similarly, clinical psychologists help alleviate the suffering caused by mental disorders.

Branches of psychology

Any attempt to explain why humans think and behave in the way that they do will inevitably be linked to one or another branch of psychology. The different disciplines of psychology are extremely wide-ranging. They include:

• Clinical psychology
• Cognitive psychology: memory
• Cognitive psychology: intelligence
• Developmental psychology
• Evolutionary psychology
• Forensic psychology
• Health psychology
• Neuropsychology
• Occupational psychology
• Social psychology

You can learn more about these disciplines by selecting from the list of links on the right hand side of the page.

What all these different approaches to psychology have in common is a desire to explain the behaviour of individuals based on the workings of the mind. And in every area, psychologists apply scientific methodology. They formulate theories, test hypotheses through observation and experiment, and analyse the findings with statistical techniques that help them identify important findings.

Hereditary and Genetics

		Heredity is the passing on of characteristics from one generation to the next. It is the reason why offspring look like their parents. It also explains why cats always give birth to kittens and never puppies. The process of heredity occurs among all living things including animals, plants, bacteria, protists and fungi. The study of heredity is called genetics and scientists that study heredity are called geneticists. Through heredity, living things inherit traits from their parents. Traits are physical characteristics. You resemble your parents because you inherited your hair and skin color, nose shape, height, and other traits from them. Cells are the basic unit of structure and function of all living things. Tiny biochemical structures inside each cell called genes carry traits from one generation to the next. Genes are made of a chemical called DNA (deoxyribonucleic acid). Genes are strung together to form long chains of DNA in structures known as chromosomes. Genes are like blueprints for building a house, except that they carry the plans for building cells, tissues, organs, and bodies. They have the instructions for making the thousands of chemical building blocks in the body. These building blocks are called proteins. Proteins are made of smaller units called amino acids. Differences in genes cause the building of different amino acids and proteins. These differences cause individuals to have different traits such as hair color or blood types. A gene gives only the potential for the development of a trait. How this potential is achieved depends partly on the interaction of the gene with other genes. But it also depends partly on the environment. For example, a person may have a genetic tendency toward being overweight. But the person's actual weight will depend on such environmental factors as how what kinds of food the person eats and how much exercise that person does. Check your comprehension of the vocabulary terms answering the following questions: 1. The basic unit of structure and function in living things is called a. genes b. chromosomes c. cells d. proteins 2. The passing of characteristics from parent to offspring is called a. genetics b. heredity c. chromosome d. proteins 3. Genes are coded information telling the cell how to build these chemicals a. chromosomes b. DNA c. RNA d. proteins 4. Visible characteristics are called a. chromosomes b. DNA c. traits d. proteins 5. Genes are made of a chemical called a. chromosomes b. DNA c. RNA d. proteins 6. Genes are located on larger structures called a. chromosomes b. DNA c. RNA d. protein 7. Biochemical structures inside cells that carry traits from one generation to the next a. RNA b. chromosomes c. genes d. proteins

Genetics

What Is Genetics?

Genetics is the science of genes, heredity, and the variation of organisms. Humans began applying knowledge of genetics in prehistory with the domestication and breeding of plants and animals. In modern research, genetics provides important tools in the investigation of the function of a particular gene, e.g. analysis of genetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.

Genes encode the information necessary for synthesizing proteins, which, in turn play a large role in influencing, although, in many instances, do not completely determine, the final phenotype of the organism. The phrase to code for is often used to mean a gene contains the instructions on how to build a particular protein, as in the gene codes for the protein. Note that the "one gene, one protein" concept is now known to be simplistic. For example, a single gene may produce multiple products, depending on how its transcription is regulated.

Areas of genetics Classical genetics Main articles: Classical genetics, Mendelian inheritance

Classical genetics consists of the techniques and methodologies of genetics that predate the advent of molecular biology. After the discovery of the genetic code and such tools of cloning as restriction enzymes, the avenues of investigation open to geneticists were greatly broadened. Some classical genetic ideas have been supplanted with the mechanistic understanding brought by molecular discoveries, but many remain intact and in use, such as the Mendel's laws.

Molecular genetics Main article: Molecular genetics

Molecular genetics builds upon the foundation of classical genetics but focuses on the structure and function of genes at a molecular level. Molecular genetics employs the methods of both classical genetics (such as hybridization) and molecular biology. It is so-called to differentiate it from other sub fields of genetics such as ecological genetics and population genetics. An important area within molecular genetics is the use of molecular information to determine the patterns of descent, and therefore the correct scientific classification of organisms: this is called molecular systematics. The study of inherited features not strictly associated with changes in the DNA sequence is called epigenetics.

Some take the view that life can be defined, in molecular terms, as the set of strategies which RNA polynucleotides have used and continue to use to perpetuate themselves. This definition grows out of work on the origin of life, specifically the RNA world hypothesis.

Population, quantitative and ecological genetics Main articles: Population genetics, Quantitative genetics, Ecological genetics

Population, quantitative and ecological genetics are all very closely related subfields and also build upon classical genetics (supplemented with modern molecular genetics). They are chiefly distinguished by a common theme of studying populations of organisms drawn from nature but differ somewhat in the choice of which aspect of the organism on which they focus. The foundational discipline is population genetics which studies the distribution of and change in allele frequencies of genes under the influence of the four evolutionary forces: natural selection, genetic drift, mutation and migration. It is the theory that attempts to explain such phenomena as adaptation and speciation.

The related subfield of quantitative genetics, which builds on population genetics, aims to predict the response to selection given data on the phenotype and relationships of individuals. A more recent development of quantitative genetics is the analysis of quantitative trait loci. Traits that are under the influence of a large number of genes are known as quantitative traits, and their mapping to a location on the chromosome requires accurate phenotypic, pedigree and marker data from a large number of related individuals.

Ecological genetics again builds upon the basic principles of population genetics but is more explicitly focused on ecological issues. While molecular genetics studies the structure and function of genes at a molecular level, ecological genetics focuses on wild populations of organisms, and attempts to collect data on the ecological aspects of individuals as well as molecular markers from those individuals.

Genomics Main article: Genomics

A more recent development is the rise of genomics, which attempts the study of large-scale genetic patterns across the genome for (and in principle, all the DNA in) a given species.

Closely-related fields The science which grew out of the union of biochemistry and genetics is widely known as molecular biology. The term "genetics" is often widely conflated with the notion of genetic engineering, where the DNA of an organism is modified for some kind of practical end, but most research in genetics is aimed at understanding and explaining the effect of genes on phenotypes and in the role of genes in populations (see population genetics and ecological genetics), rather than genetic engineering.

History It was not until 1865 that Gregor Mendel first traced inheritance patterns of certain traits in pea plants and showed that they obeyed simple statistical rules. Although not all features show these patterns of Mendelian inheritance, his work acted as a proof that application of statistics to inheritance could be highly useful. Since that time many more complex forms of inheritance have been demonstrated.

From his statistical analysis Mendel defined a concept that he described as an allele, which was the fundamental unit of heredity. The term allele as Mendel used it is nearly synonymous with the term gene, whilst the term allele now means a specific variant of a particular gene.

The significance of Mendel's work was not understood until early in the twentieth century, after his death, when his research was re-discovered by other scientists working on similar problems.

Mendel was unaware of the physical nature of the gene. We now know that genetic information is normally carried on DNA. (Certain viruses store their genetic information in RNA). Manipulation of DNA can in turn alter the inheritance and features of various organisms.