Traits, observable features like eye color or height, are systematically passed down across generations. Heredity explains how offspring acquire these characteristics from their parents, creating continuity with variations. This transmission is why children often resemble their family.
The Genetic Blueprint
Inherited traits are based in deoxyribonucleic acid, or DNA. DNA serves as the instruction manual for building and operating an organism, containing its complete genetic information. Its double helix structure allows it to store data stably.
Specific segments of DNA are called genes. Each gene carries instructions for a particular characteristic, like eye color or growth. Genes are organized into larger structures called chromosomes, located within the nucleus of cells. Humans possess 23 pairs of chromosomes (46 total), with one from each pair inherited from each parent.
The Process of Inheritance
Genetic information transmits through specialized reproductive cells, called gametes. In humans, these are sperm and egg cells. Gametes contain only half the usual number of chromosomes (23 individual chromosomes).
This chromosome reduction occurs through meiosis, a cell division process where a parent cell produces four daughter cells, each with a single set of chromosomes. During fertilization, a sperm and egg fuse, combining their genetic material. This fusion restores the full set of 46 chromosomes in the offspring, with half from each parent.
How Traits Are Expressed
Inherited genetic information dictates how traits are expressed. Different versions of a gene are called alleles. For instance, an eye color gene might have alleles for brown or blue eyes. The specific combination of alleles an individual inherits for a trait is their genotype.
The observable characteristic that results from this genetic makeup is called the phenotype. One allele can mask the presence of another, a pattern known as dominant and recessive inheritance. A dominant allele expresses its trait even if only one copy is present, while a recessive allele only expresses its trait if two copies are inherited. For example, if an individual inherits one brown eye allele (dominant) and one blue eye allele (recessive), their phenotype will be brown eyes.
Other Factors Influencing Traits
While many traits follow straightforward inheritance patterns, others are more complex. Some characteristics, like height or skin color, are influenced by multiple genes working together, a phenomenon known as polygenic inheritance. This interaction of several genes contributes to a continuous range of variations for these traits.
Beyond genetics, environmental factors also shape traits. Nutrition, lifestyle, and sunlight exposure can interact with genetic predispositions to influence trait expression. For example, while genes contribute to potential height, adequate nutrition is necessary to reach that potential. Occasional DNA changes, called mutations, can introduce new variations or alter existing traits, serving as a source of genetic diversity.