Expression of some genes depends on environment. Affected individuals show jerky, uncontrolled movements of head and limbs, progressive mental deterioration. View karyotype of normal human. Occurs far more frequently in males than females. Every female gets two X chromosomes, so even if one has color-blind allele, other could be normal and compensate.
All daughters are carriers.
View pedigree showing hemophilia in the royal families of Europe. Most genetic disorders are rare. Some are frequent in certain populations. Carriers are heterozygous individuals for recessive diseases. They are not affected, but can potentially have children Frequency of heterozygotes in population.