Individuals in a population are usually similar to each other, but not identical. Some of this variation within a species is genetic, some is environmental - the conditions in which they have developed and some is a combination of both.
The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype. Most phenotypic features are the result of multiple genes rather than single gene inheritance.
Children generally look a little like their mother and their father, but are not identical to either. They inherit their features from each parent's DNA.
Every sperm and egg cell contains half of the genetic information needed for an individual. Each sex cell is described as haploid, and has half the normal number of chromosomes. When the chromosomes fuse during fertilisation, a new cell is formed, which is known as a zygote. It has all the genetic information needed for an individual, and is described as diploid. It has the full number of chromosomes.
Examples of genetic variation in humans include:
Whether you have lobed or lobeless ears is due to genetic causes.
Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will grow taller to reach more light.
Other examples of features that show environmental variation include:
Some features vary because of a combination of genetic and environmental causes. For example, tall parents will pass genes to their children for height. Their children have the genetic potential to also be tall. However, if their diet is poor then they will not grow very well - their environment also has an effect on their height.