SImple Inheritance Patterns

Incomplete Penetrance

When a dominant phenotype is not expressed even though the individual carries the dominant allele.

Molecular Level (Incomplete Penetrance)

Protein coded by the gene may not exert an effect, due to environmental influences or other genes that counteract the protein.

Incomplete Dominance

When a heterozygote has a phenotype that is intermediate between either corresponding homozygote.

Molecular Level (Incomplete Dominance)

50% of the protein, produced by a single copy of the functional allele in the heterozygote, is not sufficient to produce the same trait as in a homozygote making 100% of that protein.

Heterozygote Advantage (Overdominance)

When the heterozygote has a trait that confers a greater level of reproductive success than either homozygote.

Example of Heterozygote Advantage

Sickle Cell Anemia.

Codominance

When heterozygote expresses both alleles simultaneously without forming an intermediate phenotype.

Molecular Level (Codominance)

The codominant alleles code proteins that function slightly differently from each other, and the function of each protein in the heterozygote affects the phenotype uniquely.

X-Linked Inheritance

The inheritance of genes that are located on the X chromosome.

Molecular Level (X-Linked Inheritance)

If a pair of X-linked alleles shows a simple dominant/recessive relationship, 50% of the protein, produced by a single copy of the dominant allele in a heterozygous female, is sufficient to produce the dominant trait.

Sex-Influenced Inheritance

The effect of sex on the phenotype of an individual.

Molecular Level (Sex-Influenced Inheritance)

Sex hormones regulate gene expression, which influences phenotype.

Sex-Limited Inheritance

The trait only occurs in one sex.

Molecular Level (Sex-Limited Inheritance)

Sex hormones only produced in one sex are essential for an individual to display a phenotype.

Lethal Alleles

Allele that has the potential to kill the individual.

Molecular Level (Lethal Alleles)

Most commonly loss-of-function alleles that code proteins necessary for survival.