Inbreeding describes the mating between individuals closely related by ancestry. Common in both natural and managed populations, it results in negative consequences that extend across generations. Its effects are rooted in genetic principles, influencing the health and viability of individuals and populations. This alters the genetic makeup of offspring, leading to various biological challenges.
The Genetic Basis of Inbreeding Effects
Inbreeding impacts offspring by increasing homozygosity, a state where an individual inherits two identical copies of a gene from their parents. Organisms carry two alleles for each gene, one from each parent, and these alleles can be either dominant or recessive. A dominant allele expresses its trait even if only one copy is present, effectively masking the trait of a recessive allele.
Individuals with shared ancestry are more likely to carry the same recessive alleles. If an offspring inherits two copies of a recessive allele, one from each parent, the trait associated with that allele will be expressed. Many recessive alleles are deleterious, meaning they carry harmful mutations that can cause genetic disorders or reduce an organism’s fitness.
In a diverse population, a harmful recessive allele often remains unexpressed, masked by a dominant allele. However, increased homozygosity due to inbreeding removes this protective masking. This elevated chance of inheriting two copies of a harmful recessive allele directly leads to the manifestation of these traits, which can range from subtle disadvantages to severe health conditions.
Inbreeding Depression
Inbreeding depression refers to reduced biological fitness resulting from inbreeding. It manifests in various ways, impacting an individual’s ability to survive and reproduce. Reduced fertility is a common effect, leading to lower reproductive rates, smaller litter sizes, or an increased incidence of stillbirths. Purebred dogs, for example, show poor puppy viability and survival due to inbreeding.
Survival rates decline in inbred individuals, including higher infant mortality and reduced lifespans. In addition, inbreeding can weaken the immune system, making organisms more susceptible to diseases. This increased vulnerability means inbred individuals may struggle to fight off common infections that outbred individuals overcome.
Physical vigor, growth, and overall health are diminished in inbred populations, including slower growth rates and a general lack of robustness. Genetic disorders are another consequence, often seen in purebred animal breeds due to selective breeding within closed gene pools. For instance, Shar Peis are prone to Familial Shar Pei Fever (FSF), a painful disease, while English Springer Spaniels can suffer from Fucosidosis, causing severe neurological problems. Cavalier King Charles Spaniels may develop syringomyelia due to small heads, and Dachshunds are predisposed to intervertebral disc disease due to short legs.
Broader Impacts on Population Health and Diversity
Beyond individual effects, prolonged inbreeding depletes genetic diversity within a population. Genetic diversity represents the total genetic characteristics in a species’ makeup, and its reduction limits the raw material for adaptation. Loss of genetic variation compromises a population’s capacity to adapt to changing environmental conditions, emerging diseases, or climate shifts.
This decreased adaptability increases a population’s vulnerability to extinction, especially for small or isolated groups like endangered species. For example, the Devils Hole Pupfish, confined to a single cave, is one of the most inbred animals known. The small size and isolation of the Southern Resident killer whale population have also led to inbreeding, contributing to their decline.
The “extinction vortex” illustrates this downward spiral: a small population size leads to inbreeding, which in turn reduces genetic diversity and individual fitness, further shrinking the population. This feedback loop accelerates extinction. These broader impacts are studied in conservation biology, where efforts focus on maintaining or restoring genetic diversity to enhance population resilience. Animal breeding programs also face these challenges, balancing desirable traits with risks of reduced genetic variability.