Why do white rats have red eyes while black rats do not? - briefly
White rats lack melanin in the iris and retina, allowing the underlying blood vessels to appear as red eyes; black rats possess sufficient pigmentation that masks these vessels, so their eyes appear dark.
Why do white rats have red eyes while black rats do not? - in detail
White rats that appear completely white are usually albinos. Albinism results from a mutation that disables the enzyme tyrosinase, which is essential for converting the amino acid tyrosine into melanin. Without melanin, the iris, retina, and other ocular tissues lack pigment. The lack of pigment makes the thin layers of the eye transparent, allowing light to pass through and illuminate the underlying choroidal blood vessels. The reflected light gives the eye a reddish appearance, often called “red‑eye” or “ruby‑eye” in laboratory rodents.
Black rats possess functional melanin production. Their irises contain sufficient pigment to absorb light, preventing the underlying vasculature from being visible. Consequently, their eyes appear dark brown or black rather than red. The presence of melanin also protects retinal cells from phototoxic damage, contributing to normal visual function.
Key biological factors:
- Genetic mutation: Albino rats carry recessive alleles that block tyrosinase activity.
- Melanin deficiency: Absence of pigment in the iris, retinal pigment epithelium, and choroid.
- Transparency of ocular structures: Light penetrates the iris and is reflected by blood vessels in the choroid, producing the red glow.
- Pigmented counterparts: Black rats retain melanin, which masks the vascular reflection and yields dark eye coloration.
- Physiological consequences: Albino rats are more sensitive to bright light and may experience visual impairments due to the lack of protective pigment.
Thus, the red coloration of the eyes in white laboratory rats is a direct result of albinism‑induced melanin loss, while the dark‑eyed phenotype of black rats reflects intact pigment production that obscures the underlying vasculature.