Typical red-green color blindness in human patients is caused by mutations on genes located in the X chromosome. These mutations act in a recessive manner. Since females have two X chromosomes, the presence of a mutation in a single one of them does not normally result in color blindness. Males, in contrast, have a single X chromosome and therefore the presence of a mutation is likely to cause the disease.
About the other options: Down‘s syndrome is a numerical chromosomal anomaly, not related to sex. Human blood type is a codominant trait. Finally, tail length in dogs is a polygenic trait not amenable to classic Mendelian analysis.
Recessive<span> and </span>dominant alleles<span>. You will recall that genes have </span>different<span> forms called </span>alleles<span>. An </span>allele<span> can be </span>recessive<span> or </span>dominant<span>. A </span>recessive allele<span> only shows if the individual has two copies of the </span>recessive allele<span>.</span>
