Key Definitions to Know

What is epistasis?

A gene or genes at one genetic locus hide(s) the expression of genes at another, different genetic locus

What are the three different types of epistasis?

1. Recessive epistasis

2. Duplicate recessive epistasis

3. Dominant epistasis

What is the ratio for recessive epistasis?

9:3:4

What is the ratio for duplicate recessive epistasis?

9:7

What is the ratio for dominant epistasis?

12:3:1

What is recessive epistasis?

The presence of a pair of recessive alleles at one genetic locus prevents the expression of dominant OR recessive alleles at a second (different) genetic locus

What are hypostatic genes?

Genes whose expression is hidden or masked by other non-allelic genes

What is duplicate recessive epistasis?

The dominant phenotype can ONLY appear when TWO DIFFERENT dominant alleles are both present at the same time. In the absence of either one of the dominant alleles, a different, common phenotype will arise.

What is dominant epistasis?

The presence of a dominant allele at one genetic locus will prevent the expression of dominant or recessive alleles at a second, different genetic locus.

What is pleiotropy?

One gene affects more than one, seemingly-unrelated phenotypic characteristic or trait

What are some examples of a pleiotropic effect?

1. Lethal allele

2. mei-1 mutant in Drosophila

3. Sickle cell anemia

4. Marfan syndrome

What is a lethal allele?

Capable of causing the death of an organism (often in addition to other effects on the phenotype)

What are dominant lethal alleles?

Cause lethality when only one copy of the gene is present (AA or Aa would be lethal)

What are recessive lethal alleles?

are lethal only when homozygous

What is penetrance?

Percentage of individuals with a given genotype who exhibit the phenotype normally associated with that genotype

What is complete penetrance?

Everyone having the appropriate genotype expresses the expected phenotype

What is reduced penetrance?

A certain percentage of individuals who inherit the appropriate genotype fail to exhibit the expected phenotype

What is a non-penetrant gene?

When individuals who have the appropriate genotype fail to express the trait (describe individuals from reduced penetrance)

What is expressivity?

A measure of the degree to which a given allele is expressed at the phenotypic level

Basically measures the intensity or severity of the phenotype

Variable expressivity

Occurs when a phenotype is expressed to a different degree among individuals having the same genotype

What are s*x-limited traits?

Traits that appear ONLY in members of one s*x even though members of both s*xes can possess the genotype associated with the trait

(AUTOSOMAL NOT X-LINKED)

What are s*x-influenced traits?

Autosomal traits that are expressed differently in males and females

Relationship between genotype and phenotype differs in males and females

What is linkage?

The tendency of two genes that are located near to each other on the SAME chromosome to be inherited together

What is syntenic?

Refers to genes that are physically located on the same chromosome, regardless of whether they do or do not exhibit linkage