Genetics

Exception

Definition

Example

Multiple Alleles

A gene for a specific trait exists in more than two forms

Blood type: type AB, type A (AO or AA), type B (BO or BB), or type O (OO)

Codominance

Different alleles, both fully expressed in the heterozygote

C represents stripes, O represents spots, individual with CO has both spots and stripes

Incomplete Dominance

Heterozygote phenotype is an intermediate between either homozygote

AA= red flower, aa=white flower, Aa= pink flower

Lethal Allele Combinations

A gene that causes an early death

Achondroplasia: a homozygous individual for this disorder will result in still birth

Pleiotropy

One gene, many effects, often seeming unrelated

PKU: can cause mental retardation and hair and skin discoloration

Phenocopy

An environmentally caused condition that appears to be inherited

A woman uses a skin bleaching cream with a chemical that darkens her fingertips, a symptoms similar to those of Alkaptonuria

Heterogenity

Different genes can cause the same phenotype

Mutations in any of at least three genes cause familial ALS

Epistasis

One gene masks or affects the expression of another

A man would have a widow’s peak, but instead he is bald

Penetrance

Percentage of individuals with a genotype who have an associated phenotype

A woman with neurofibromatosis shows many symptoms. A genetic test shows her son has the disease causing allele, but he has no symptoms. 

Expressivity

Degree of severity of a phenotype

75% of individuals with “the blue gene” show dark blue color, 25% show light blue color