An inherited trait is any element of an organism's makeup passed down to it by one or both of its parents. For instance, eye color is an inherited trait, with the eye color of the parents affecting the eye color of the child. Inherited traits are passed from parent to offspring by genetic transmission, in which the child receives some of its genes from each parent.
Physical traits are the expression of genes, which determine how the body forms. A specific instance of a particular gene is called an allele. For example, in the case of eye color, the gene responsible has multiple alleles, such as an allele for brown eyes and an allele for blue eyes. Not all alleles have unique expressions. It is possible for several alleles to produce the same result in the organism's physical form, or phenotype.
In simple heredity, alleles are either dominant or recessive. In the case of eye color, the allele for brown eyes is dominant while the allele for blue eyes is recessive. A recessive allele is contained within an organism's genetic code but is not expressed in its phenotype. If either parent passes on the dominant allele, such as the allele for brown eyes, the child will show the inherited trait associated with the dominant allele. Only if both parents pass on the recessive allele will the child have the recessive trait of blue eyes, possibly resembling neither parent.
Some common human inherited traits are controlled by a single gene, including cleft chins, detached earlobes, widow's peaks and the ability to roll the tongue. Most traits are much more complicated than this, however, being controlled by a combination of alleles. An inherited trait usually derives from the interaction of several genes. Traits of this type are called polygenic traits.
Not all physical characteristics are inherited. Many arise from the interaction between an organism's genotype and its environment. For instance, the presence of a pigment called melanin determines the color of an individual's skin. Genetics influences the level of melanin the body produces, but other factors also play a role, including exposure to sunlight. A trait that results from interaction with the environment is called an acquired trait as opposed to an inherited trait.
18th-century French scientist Jean-Baptiste Lamarck hypothesized that acquired traits could be inherited by subsequent generations. According to this theory, changes acquired over the life of an organism could be passed on to its young. For example, an individual who built muscle through repeated exercise would be more likely to have strong children. This theory is now known to be incorrect; changes acquired during life, other than through mutation, affect the phenotype but not the genotype and are not passed on to the young.