Summary
Highlights
Color blindness is common, affecting about 1 in 8 men and 1 in 50 women, many of whom are unaware they have it. John Dalton, known for atomic theory, was a famous example of color blindness. His description of seeing mainly yellow, blue, and purple in the spectrum helped define the condition. Post-mortem examination of his eyes, and later gene testing in 1995, revealed he was missing the green (M) cone pigment, classifying him with red-green color blindness or deuteranopia.
Normal human color vision is trichromatic, based on three color cones. Color deficiency usually occurs at two levels: either an entire cone class is missing (dichromacy) or one cone is a hybrid/chimeric cone. The genes for blue pigment are on chromosome 7, while red (L) and green (M) pigments are close together on the X chromosome (Xq28). This proximity leads to X-linked inheritance and frequent crossing over or recombination during cell division.
Chromosomes can exchange segments during cell division in a process called crossing over. Unequal crossover can lead to gains or losses of whole genes, creating variations in color vision. For example, a male receiving a chromosome with only a red gene would have deuteranopia. Modern gene sequencing shows significant variation in the red and green gene numbers on the X chromosome in the population. Crossing over can also split genes, creating hybrid or chimeric genes, affecting the light sensitivity range of the pigment.
Recombination often results in chimeric genes, leading to variation in red and green sensitivity. While normal individuals might have multiple green genes, usually only the red and the first green gene are expressed. Unequal recombination can shift green genes, leading to conditions like deuteranopia if only a red gene is present. Hybrid genes, split during recombination, can behave as 'green-like' or 'red-like' pigments, affecting the resultant opsin pigment's spectral sensitivity. The variation in these hybrid genes is the basis for red-green color blindness.
Color deficiency terms are classified by severity. Monochromats have one cone and see in grayscale. Dichromats have two functional cones, allowing minimum color distinction. Trichromats have three cones, enabling rich color perception. Tetrachromats, found in some animals and possibly humans, have four. More specific terms categorize which cone is missing: protanope (red cone), deuteranope (green cone), and tritanope (blue cone). Anomalous trichromats have three cone types, but one is partially functioning (e.g., protanomaly with a green-like pigment, deuteranomaly with a red-like pigment).
Protan and deutan deficiencies fall under red-green deficiency, which is the most common, especially in males (about 1% for missing a red or green cone). Missing a blue cone (tritan deficiency) is much rarer. This video built on previous information about color vision and genetics, explaining how inherited color deficiency results from missing cones or having hybrid cones. Future videos will delve deeper into color science and simulate what the world looks like with these deficiencies.