Heredity is the biological process responsible for passing traits from one generation to the next. You can think of it as a detailed set of instructions, like family recipes, that are handed down from parents to their children. These instructions determine a vast array of characteristics, from physical appearance to certain behavioral tendencies.
Heredity accounts for both the similarities that define a species and the differences among its individual members. An organism’s genetic information dictates its fundamental nature, ensuring a species remains consistent. However, subtle variations within this information create the unique differences between individuals, explaining why no two people, aside from identical twins, are exactly alike.
The Building Blocks of Heredity
The instructions for every inherited trait are encoded within a molecule called deoxyribonucleic acid, or DNA. This molecule is structured as a double helix, resembling a twisted ladder, and contains the complete genetic manual for an organism. The specific sequence of its components forms a code that directs the development and function of every cell. This entire set of DNA in an organism is known as its genome.
Within the vast DNA manual, individual instructions are known as genes. A gene is a specific segment of DNA that typically holds the information to build a particular protein, which in turn performs a specific job in the body. These genes are the fundamental units of heredity, controlling characteristics such as eye color or blood type.
To manage this immense amount of information, the long strands of DNA are tightly packaged into structures called chromosomes. Humans have 23 pairs of chromosomes in most of their cells, for a total of 46. You can visualize chromosomes as volumes of an encyclopedia, with each containing thousands of genes. During reproduction, one chromosome from each pair is passed down from each parent.
Patterns of Inheritance
For many traits, an individual inherits two copies of a gene, one from each parent. These different versions of the same gene are called alleles. The interaction between these two alleles determines which trait is physically expressed. Some alleles are dominant, meaning only one copy is needed for its trait to appear, while recessive alleles require two copies to be visible.
An organism’s genetic combination of alleles is its genotype, while the resulting observable physical trait is its phenotype. For example, in pea plants, the allele for purple flowers is dominant over the recessive allele for white flowers. A plant with at least one purple allele (genotype) will have purple flowers (phenotype), while a plant will only have white flowers if it inherits two recessive alleles.
The principles of how these traits are passed on were first described by Gregor Mendel through his experiments with pea plants. He demonstrated that hereditary factors do not blend but are passed on as distinct units. Parents transmit only half of their genes to each child, and the specific combination received is different, which is why siblings are not identical. This mechanism explains how a trait can skip a generation, reappearing if a descendant inherits the necessary pair of recessive alleles.
Heredity and Environmental Influence
An organism’s characteristics are rarely determined by genetics alone, but arise from an interaction between its genotype and environment. Genes provide the potential for a trait, but environmental factors influence how that potential is realized. The genotype remains constant, but the phenotype can change as the environment changes.
A clear example of this interplay is human height. An individual may inherit a set of genes that gives them the potential to be tall. However, poor nutrition during childhood and adolescence may prevent them from reaching their full potential height. The genetic blueprint sets the limits, but environmental inputs like diet are needed for that blueprint to be fully expressed.
Similarly, an individual might have a genetic predisposition for a certain health condition, but this does not guarantee they will develop it. Lifestyle factors such as diet and exercise can increase or decrease the likelihood of the condition manifesting. For instance, a person’s genotype may make them tan easily, but the suntan itself is a result of interacting with sunlight and is not passed down.