Some phenotypes, such as colour-blindness, are caused by sex-linked genes. Explain why colour-blindness is more common in men than in women
Sex-linked genes are located on the sex-chromosomes, X and Y. Females have two X-chromosomes, XX, and males have one X and one Y-chromosome, XY. The X-chromosome is much larger than the Y-chromosome and so contains a greater number of genes, including the gene which codes for colour-blindness. Colour-blindness is a recessive allele disorder, meaning that if there is a dominant allele for normal-colour vision within the genotype of the individual then they do not have colour-blindness. An individual with homozygous recessive phenotype will have colour-blindness. This means that females require two copies of the recessive allele for them to have colour-blindness, whilst males only require one copy of the recessive allele. This means men have a 1/2 chance of having colour-blindness, whilst women have a 1/4 chance, such that colour-blindness is much more common in men.
B = dominant allele for normal-colour visionb = recessive allele for colour-blindness
Genotype - corresponding phenotype:XBXB - normal colour-vision femaleXBXb - normal colour-vision female (carrier of colour-blindness)XbXb - colour-blind femaleXBY - normal colour-vision maleXbY - colour-blind male
