Inheritance is the process by which characteristics are passed from parents to their offspring, ensuring the continuity of life across generations. This biological transfer ensures that animals generally resemble their parents, whether in the pattern of a cat’s fur or the migratory instincts of a wild bird. Inheritance generates both the recognizable consistency within a species and the subtle variations observed between individuals. The study of this transmission explains how traits like size, color, and behavior are encoded and expressed in every living animal.
The Building Blocks of Animal Inheritance
The fundamental instruction manual for every animal is deoxyribonucleic acid (DNA), a long, coiled molecule present in nearly every cell. DNA contains the complete set of specifications for building and operating an organism. Segments of DNA that code for a specific functional product, such as a protein, are called genes.
Every animal inherits two copies of each gene, one from each parent. These gene versions are known as alleles, and they determine the precise expression of a trait. For instance, a gene for coat color might have one allele for black pigment and another for brown pigment. The combination of these alleles dictates the final characteristic observed in the animal.
Simple and Complex Trait Patterns
Genetic traits are broadly categorized based on the number of genes that influence them. The simplest form is Mendelian inheritance, where a trait is determined by the alleles of a single gene pair. This results in clearly defined, discrete outcomes, such as the coat color of laboratory mice being entirely black or entirely brown based on a single dominant/recessive gene.
In contrast, many characteristics follow a polygenic inheritance pattern, governed by the combined action of multiple genes. These traits, such as an animal’s height or milk production yield, tend to show a continuous range of expression rather than simple “either/or” categories. Environmental factors, like nutrition, also play a role in shaping the final appearance of these complex traits.
Five Examples of Inherited Traits
Eye Color (in Primates)
Eye color in primates, including humans, is a complex trait governed by a polygenic inheritance pattern involving multiple genes. The OCA2 and HERC2 genes are the most significant, explaining the majority of pigmentation variation. The OCA2 gene codes for the P protein, which is involved in the maturation of melanosomes—the structures that produce and store melanin pigment in the iris.
The HERC2 gene contains a regulatory region that controls the activity of the OCA2 gene. A specific variation in HERC2 reduces OCA2 expression, leading to less melanin production in the iris stroma. This lowered pigment concentration causes the lighter appearance of blue or green eyes due to the way light is scattered within the iris tissue.
Horn Presence (in Livestock)
The absence of horns, known as polledness, in livestock species like cattle and goats is inherited as a simple Mendelian trait. The allele for polledness (\(P\)) is dominant over the allele for horns (\(p\)). This means an animal only needs to inherit one copy of the polled allele to be hornless. A horned animal must inherit two copies of the recessive allele, one from each parent. This predictable trait is extensively used in breeding programs to select for naturally hornless animals, simplifying management and reducing injury risk in herds.
Twinning Rate (in Sheep)
The ability of sheep to consistently produce twins or triplets is a complex trait influenced by several genes and environmental factors. Key genetic controllers are mutations in the BMPR1B, BMP15, and GDF9 genes, which regulate the development of ovarian follicles and the rate of ovulation in the ewe. For instance, a single copy of the FecB mutation in the BMPR1B gene can increase the average number of lambs born by about one. However, inheriting two copies of some of these mutations can lead to sterility in female sheep, illustrating a genetic balance between increased litter size and reproductive viability.
Sickle Cell Trait (in Humans)
Sickle cell trait in humans is a single-gene inheritance pattern that exhibits co-dominance at the molecular level. This condition is caused by a mutation in the HBB gene, which provides instructions for making the beta-globin protein in hemoglobin. Individuals who inherit one normal allele and one sickle cell allele have the sickle cell trait. These carriers produce both normal and abnormal hemoglobin, and their red blood cells are typically healthy. The full-blown sickle cell disease only manifests in individuals who inherit two copies of the mutated gene, demonstrating a classic recessive pattern for the disease itself.
Webbed Feet (in Ducks)
The development of webbed feet in ducks is a distinct morphological trait resulting from specific genetic changes during embryonic development. In most land animals, the skin between the developing digits is removed through programmed cell death, creating separate fingers or toes. In ducks and other aquatic birds, this process is genetically suppressed between the toes. The genes responsible for this suppression allow the interdigital tissue to remain, forming the web, which provides a large surface area for efficient propulsion in water. This trait demonstrates how a small genetic alteration can lead to a significant functional adaptation.