Biological inheritance is the process by which parents pass down traits to their children. This transfer of information occurs in all organisms, from simple bacteria to complex animals. At the center of this process is a molecule called deoxyribonucleic acid, or DNA, which contains the instructions for building and operating a living being.
DNA The Molecule of Heredity
Deoxyribonucleic acid, or DNA, is the instruction manual for building and operating a living organism. Its structure resembles a twisted ladder, a shape known as a double helix. The sides of this ladder are made of alternating sugar and phosphate groups, while the rungs are pairs of nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases follow a pairing rule where A always pairs with T, and G always pairs with C, ensuring the two strands of the helix are complementary.
Specific segments of this DNA molecule are called genes, which contain the instructions for building proteins or performing particular functions. These proteins are responsible for creating most of the components of the body and directing its functions. In organisms like plants and animals, DNA is located inside the cell nucleus, where it is tightly coiled and packaged into structures called chromosomes.
From Parents to Offspring The Transmission Process
The transfer of DNA to the next generation occurs through sexual reproduction. This process begins with producing reproductive cells called gametes—sperm in males and eggs in females. These cells are unique because they contain only half the number of chromosomes as other body cells, holding one set of 23 chromosomes instead of 23 pairs.
This reduction is achieved through a type of cell division called meiosis. During meiosis, the parent cell’s DNA is replicated and then divided twice, resulting in four gametes, each with a single set of chromosomes. A process called crossing over occurs during meiosis, where chromosomes exchange genetic material. This shuffling creates new combinations of genes, contributing to genetic diversity.
Inheritance continues with fertilization, when a sperm cell fuses with an egg cell. This event combines the single set of chromosomes from the male parent with the single set from the female parent. The resulting cell, called a zygote, now has a complete set of 46 chromosomes, or 23 pairs. This zygote will then divide and develop into a new individual with a unique genetic makeup inherited from both parents.
How Genes Determine Traits Basic Principles
An organism’s observable characteristics, its phenotype, are determined by its genetic makeup, or genotype. For each gene, an individual inherits two versions, one from each parent. These different versions of a gene are called alleles. The interplay between these two inherited alleles determines which trait is expressed.
Some alleles are dominant, meaning only one copy is needed for the associated trait to be expressed. Other alleles are recessive, and the trait will only appear if an individual inherits two copies of that allele. For example, if an allele for brown eyes is dominant and an allele for blue eyes is recessive, a person with one of each will have brown eyes. The brown-eye allele masks the effect of the blue-eye allele.
An individual’s specific combination of alleles is their genotype. A person is homozygous for a gene if they have two identical alleles and heterozygous if they have two different alleles. While examples like eye color are used to explain these principles, most human traits are polygenic. This means they are influenced by multiple genes acting together, along with environmental factors.
Variations in the Code Mutations and Genetic Diversity
A mutation is a permanent change in an organism’s DNA sequence. These alterations can arise from errors during DNA copying or from exposure to environmental factors known as mutagens, such as ultraviolet radiation and certain chemicals. Mutations are the source of all new genetic variation within a population.
The effects of a mutation can vary widely. Many are neutral, having no discernible impact on an organism’s health or traits. Some mutations can be harmful. If a mutation alters a gene that provides instructions for a protein, it can disrupt normal function and lead to a genetic disorder.
Conversely, some mutations can be beneficial, providing a trait that gives an organism a survival advantage. Over generations, individuals with these traits may be more likely to survive and reproduce, passing the new allele to their offspring. This process is the basis for evolution by natural selection, allowing species to adapt over time. Genetic variation makes each individual unique and provides the raw material for this adaptation.