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David has very light skin and hair. Sometimes he has a distinct “musty” odor to his sweat and urine. He is prone to seizures. Doctors were unsure for a long time what caused his conditions until they realized he had a disease caused PKU. This diseased phenotype can include all of these symptoms due to one defective enzyme.
Jim inherited recessive alleles for red hair color. He also inherited a gene that causes complete baldness.
Certain butterflies have different pigmentation based on the environment in where they are hatched. It appears to be a trait passed down from parent insects but it is actually driven by the temperature.
In the above pedigree, there is ample evidence for autosomal dominant inheritance: • The disease is passed from the father (II-3) to the son (III-5), this never happens with X-linked traits. • The disease occurs in three consecutive generations, this never happens with recessive traits. • Males and females are affected, with roughly the same probability. • However, II-1 does not express the disease. He must have inherited the mutant allele because he passed it on to two children, III-1 and III-3. II-1 is a classical example of incomplete penetrance, he has the allele for the disease but he does not express it.
Marfan syndrome: • May appear pleiotropic • Can also be an example of variable expressivity • Some people have all of the characteristic symptoms while others may just have one or two.