The frequency with which two genes are inherited together or linked depends on the length of the DNA between the two genes. Another example of the role of genes in health is gene therapy. These are classified according to the chromosome on which they are found: autosomal, X-linked or Y-linked and are covered in more detail in the following section: Basic genomic concepts.
Annual Review of Ecology and Systematics. Human disorders attributable to sex-linked recessive inheritance trait conspicuous signs hemophilia A bleeding tendency with joint involvement Duchenne muscular dystrophy progressive muscle weakness Lesch-Nyhan syndrome cerebral palsy, self-mutilation fragile-X syndrome mental retardation, characteristic facies.
X-linked recessive. Moreover, close relatives of more severely affected individuals e. Red-green colorblindness is a common inherited trait in humans. Among these are such disorders that result from triplet repeat expansions within or near specific genes e.
Pedigree for Color Blindness.
New York: Schuman, p— If there is a higher risk of disease in family members of an affected person than in the general population, this suggests a genetic component. Identifying these genes can help improve understanding of disease aetiology, help predict who is at risk, assist with targeting disease prevention and treatment, and help identify new targets for therapies.
The "a" recessive allele will not be expressed in her phenotype. Queen Victoria of England was a carrier of the gene for hemophilia. X-linked genes are also responsible for a common form of baldness referred to as "male pattern baldness".
Alport syndrome Dent's disease X-linked nephrogenic diabetes insipidus. PCSK9 variant and cancer e. Pedigree charts allow us to follow the history in a family of a genetic condition and draw conclusions about inherited traits as illustrated in the figure below.