â€¢ Boys are affected by red-green deficiencies much more frequently than girls because the gene for red-green vision is located on the X chromosome. Defects are recessive, but because boys have only one X chromosome (and one Y) any defect shows up as a color deficiency. Girls, who have two X chromosomes, still see normally if one is faulty but the other is normal. For a girl to be red-green deficient both must be defective. Men cannot pass red-green color blindness on to their sons, although they can to their daughters. Women whose fathers were red-green deficient are carriers and may pass the defective gene on to their children. â€¢ The blue receptor gene is carried on a different chromosome, no. 7, of which everyone has two copies. Defects are dominant, so even if only one chromosome is defective it shows up as a blue-yellow deficiency. This affects males and females equally. â€¢ Babies see colors but it takes time before young children learn to name them properly. Doctors say children should be able to name a range of colors correctly by the age of four. â€¢ Since 2004, the Education Ministry has subsidized free eye tests through the medical funds for children in first grade. The ministry's web site says all first-graders should be tested as visual problems can cause learning difficulties, and adds that three to five percent of children have focusing problems, but it does not mention color vision. My experience is that if you want thorough color vision testing, you need to ask for it. â€¢ At school, teachers should always be told of any vision problems. Parents can help by labeling markers and pencils with the colors, by teaching children the correct colors of common objects, and by checking textbooks to see if pages are printed in hard-to-distinguish colors (e.g. green print on a red background), and photocopying those pages in black and white.