Variation and inheritance
Some characteristics of a living thing are caused by the environment, some by inherited factors, while others are caused by a combination of environment and inherited factors.
Male mammals carry XY sex chromosomes - female mammals carry XX sex chromosomes. Inherited disorders are caused by faulty genes on these chromosomes.
Different versions of a gene are called alleles, and these alleles can be dominant or recessive. Genetic diagrams can show the possible outcomes of a particular cross.
Some characteristics of an individual are caused by the environment. For example, the language we use or whether we have scars are environmental characteristics. Other characteristics are inherited.
Examples of inherited characteristics include:
Some characteristics - including intelligence, body mass and height - are the result of both environmental and inherited factors. But there is debate about the relative importance of these two types of factor in some human characteristics, such as intelligence, health and sporting ability.