Variation is the difference between individuals of the same species. In any randomly-selected group of humans, there are differences in features such as skin colour, eye colour, height, foot size, hair curliness and hair colour. These differences are divided into two groups: acquired variation and inherited variation.
An acquired characteristic is a non-heritable change in a function or structure of a living organism, caused after birth. Acquired characteristics can be caused by disease, accident or injury, deliberate modification, or other environmental influences. Two examples are sun tanning and hair curling.
An inherited characteristic is genetically determined – i.e. it is decided by what we receive from our parents at conception. Two examples are eye colour, and whether your earlobes are attached or free.
A trait is a specific characteristic of an organism (e.g. hair colour or blood type). Traits can be determined by genes or the environment, or (more commonly) by interactions between them.
The actual object that is inherited is called a gene. (Genes determine the appearance of inherited traits.) It is a unit of heredity, and is transferred from parent to offspring.
The genotype is the individual organism's unique set of all their genes. The genotype (along with environmental influences) determines the phenotype, which is the outward expression of the individual's genes – i.e. the observable physical properties of the individual (including their appearance, development and behaviour).
(“Genotype” can also refer to the two alleles inherited for a particular gene.)
A genome is all of the genetic material in an organism. It is made up of DNA (or RNA in some viruses), and includes genes and other elements that control the activity of these genes.
The human genome is mostly the same for all humans, though there are of course variations across the genome. This genetic variation accounts for about 0.001% of a person's DNA.
Visualization of sections of the human genome sequence at the Wellcome Sanger Institute (England).
A characteristic that changes gradually over a range of values shows continuous variation.
One example is human height. It ranges from the shortest to the tallest person, with any height between those values being possible. On a bar graph, this would likely show a bell curve.
A characteristic with a limited number of possible values shows discontinuous variation.
One example is human blood group. There are only 4 possible blood types (A, B, AB and O). There are no values in between.
Other examples of discontinuous variation include eye colour and sex.
Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. It studies how genes are expressed in plants, animals and human beings.
A basic assumption of genetics is that acquired variation cannot be passed on. Only inherited variation can be passed on through reproduction.
Siblings often have similar features, but there are also many differences. Offspring can resemble one parent, or both parents.
Identical twins in utero.
Identical twins display a remarkable degree of similarity in their appearance, but non-identical twins are no more alike than regular siblings.
The principles of genetics can be used to explain how similarities and differences occur in a family.
Asexual and Sexual Reproduction
In some species (such as aphids & dandelions), a single parent can produce a single offspring. This process is called asexual reproduction. It produces offspring that are genetically identical to each other, and to the parent (because they have inherited the same set of genes).
Asexual reproduction can rapidly produce a large number of offspring, but there is no genetic variation.
Most plant & animal species reproduce via sexual reproduction. This involves two organisms, who combine their genes in some way to produce offspring.
This results in offspring that differ, i.e. it generates variation.
Source: Science: NCEA Level 1 (New Zealand Pathfinder series).
