On the 6th September, 1766, in the Quaker village of Eaglesfield in Cumberland, John Dalton was born.... and why Colour Blind Awareness Day now falls on this day. Dalton was the son of a handloom weaver, largely self-taught, teaching others by the age of twelve. He would become the chemist who gave modern chemistry its first workable atomic theory; and the first person to give colour vision deficiency a systematic scientific description. In several languages the condition is still called Daltonism. The story begins with ordinary objects that would not behave. He once bought his mother stockings he thought were a sober Quaker grey. They were scarlet. In autumn 1792 he watched a pink geranium that looked sky-blue by day and, by candlelight, a colour he called red. Other people saw no change. His brother Jonathan did. That was the clue: the peculiarity ran in the family. On 31st October 1794 he stood before the Manchester Literary and Philosophical Society and read Extraordinary Facts Relating to the Vision of Colours. Published in 1798, it is the first systematic account of what we now call colour vision deficiency or colour blindness... He mapped the solar spectrum as he saw it; two or three distinctions where others counted six, and guessed, wrongly, that the jelly of his eye was tinted blue and filtered the light. He asked that his eyes be examined after death. On 28th July 1844, the day after he died, they were. The humours were “perfectly colourless.” One hundred and fifty-one years later, in 1995, DNA taken from his preserved eye tissue settled the diagnosis: deuteranopia. He lacked the gene for the middle-wavelength (green) photopigment. The man who weighed atoms had been missing one of the proteins that turn light into colour. The same decade that produced that paper produced the work he is more famous for. From meteorology and the behaviour of mixed gases he derived the law of partial pressures. In 1803 he told the Manchester society he had been measuring “the relative weights of the ultimate particles of bodies.” In 1808 A New System of Chemical Philosophy set out the theory: matter is made of indivisible atoms; atoms of the same element are alike in mass; different elements have different masses; compounds form in simple whole-number ratios. Some of his formulas were wrong; he took water as HO, not H₂O, but the architecture held. Chemistry became quantitative. Today is Colour Blind Awareness Day because a UK charity of that name, founded in 2010, chose Dalton’s birthday when it launched the observance in 2015. Estimates commonly put the number of people with some form of colour vision deficiency at around 300 million. The most-quoted figures for the common red-green types, especially in populations of Northern European ancestry, remain about 1 in 12 men and 1 in 200 women. he most common forms; red-green deficiencies (protan and deutan types, including Dalton’s deuteranopia) are X-linked recessive. The relevant opsin genes sit on the X chromosome. A male has one X. If that X carries the variant, he has the condition. Inheritance is from his mother. A female has two X chromosomes. She is affected only if both parents carry a variant. That is why the rates diverge so sharply: roughly 8% of men versus 0.5% of women in the populations where those figures were first established. Fathers cannot pass the X-linked trait to their sons; sons get the father’s Y. Other forms do not follow that pattern at all. Blue-yellow (tritan) defects are inherited in an autosomal pattern and affect males and females equally; they are much rarer, on the order of fewer than 1 in 10,000. Complete absence of colour vision (achromatopsia) is also rare and not the same sex-linked story. Colour vision can also be acquired later in life from disease, injury, medication or ageing; and that can happen to anyone. Dalton himself illustrated the common pattern: he and his brother both had it; their sister is not recorded as affected. That is exactly what X-linked recessive inheritance looks like in a family. That is the thread that runs from a birth in 1766, geranium in 1792, through a paper in Manchester, a pair of eyes left to science, a DNA test in 1995, and a day marked every 6th September...