Multiple-allele series only partly follow simple Mendelian genetics . MD is characterized by a gradual, irreversible wasting of skeletal muscle. It is a sex-linked trait most often passed on to sons by their mothers. The most common form, Duchene MD, begins to weaken the legs of boys by age 3 and inevitably gets worse with each passing year. There is no cure for this disorder, and it usually results in death before the age of 30.
Symptoms range from numbness and tingling to paralysis.
This is the only mechanism of evolution that can produce new alleles of a gene. The collective groups of symptoms known as sickle-cell trait is an example. Simple Mendelian rules of dominance do not apply to the complex interaction of these genes. As a result, phenotypes may appear as apparent blends or intermediate expressions. Human skin and hair color are polygenic traits.