Fingerprint

A fingerprint is the mark left by the friction ridges on a human finger. In forensic science, recovering even partial fingerprints from a crime scene is a key technique. These marks appear on surfaces like glass or metal because of moisture and grease on the skin. To get a full, deliberate print, ink or another substance is transferred from the ridge peaks to a smooth surface like paper. Standard fingerprint records usually include impressions from the pads of the last joint of each finger and thumb, but they also often capture parts of the lower finger joints.

Human fingerprints are detailed, unique, hard to change, and last throughout a person’s lifetime, which makes them useful for long-term identification. Authorities such as police use them to identify people trying to hide who they are, or to identify those who are unconscious or deceased and cannot identify themselves, for example after a natural disaster.

However, fingerprint evidence has been questioned by academics, judges, and the media. There are no uniform standards for counting points of comparison, and researchers argue that the error rate in matching fingerprints has not been properly studied, leaving the evidence without a solid statistical basis. Studies have also looked into whether experts can focus objectively on fingerprint features without being influenced by unrelated information, like the context of a case.

**Biology**

Fingerprints are impressions left on surfaces by the friction ridges on a human finger. Matching two fingerprints is one of the most widely used and reliable biometric techniques, though it only considers the obvious features of a print. The residue of a fingerprint is mostly water (95–99%), along with organic and inorganic substances. The organic part includes amino acids, proteins, glucose, lactase, urea, pyruvate, fatty acids, and sterols. Inorganic ions like chloride, sodium, potassium, and iron are also present. Other contaminants, such as oils from cosmetics, drugs and their metabolites, and food residues, may be found as well.

A friction ridge is a raised part of the epidermis on the fingers, toes, palms, or soles. It consists of one or more connected ridge units of friction ridge skin, also called epidermal ridges. These ridges form because of the interface between the dermal papillae of the dermis and the interpapillary pegs of the epidermis. They develop around the 15th week of fetal growth and remain until after death, when decomposition begins. During fetal development, around the 13th week, ledge-like formations appear at the bottom of the epidermis next to the dermis. Cells along these ledges start to multiply rapidly, creating primary and secondary ridges. These ridges then serve as a template for the outer skin layer, forming the friction ridges seen on the surface.

These epidermal ridges help amplify vibrations when fingertips brush across an uneven surface, better transmitting signals to sensory nerves involved in fine texture perception. While it seems unlikely that fingerprints generally improve gripping ability, the ridges may help with gripping rough surfaces and could improve contact in wet conditions.

**Genetics**

The scientific consensus is that the dermatoglyphic patterns on fingertips are hereditary. Fingerprint patterns between identical twins are very similar, though not identical, while fraternal twins show much less similarity. Significant heritability has been found for 12 dermatoglyphic traits. Current models suggest these traits are inherited through Mendelian transmission, with additional effects from either additive or dominant major genes.

Genes determine the general characteristics and type of pattern, but environmental factors cause slight differences in each fingerprint. However, the relative influence of genetics versus environment on fingerprint patterns is generally unclear. One study, using only total ridge count as a measure, suggested that about 5% of total variability comes from small environmental effects. Several models have been proposed for how fingerprint ridges form, leading to their vast diversity. One model suggests a buckling instability in the basal cell layer of the fetal epidermis creates the ridges. Blood vessels and nerves may also play a role. Another model indicates that changes in the amniotic fluid around each developing finger cause cells on each fingerprint to grow in different microenvironments. For a given individual, these factors affect each finger differently, preventing two fingerprints from being identical while still retaining similar patterns.

Determining how fingerprints are inherited is difficult because of the huge variety of patterns. Classifying a specific pattern is often subjective—there is no consensus on the best characteristic to measure quantitatively—which complicates analysis. Several modes of inheritance have been suggested and observed for various fingerprint patterns. Total fingerprint ridge count, a common measure of pattern size, has been suggested to have a polygenic mode of inheritance, influenced by multiple additive genes. However, this hypothesis has been challenged by other research.