Unit 5.3 - AP Biology

Degrees of dominance

Complete dominance- occurs when phenotypes of the heterozygous and dominant homozygote are the same heterozygote and dominant homozygote are the same

incomplete dominance- occurs when phenotype of hybrid offspring is somewhere in between two parental phenotypes

Codominance- occurs when two dominant alleles affect the phenotype in separate, distinguishable ways

Recessively Inherited Disorders

• create “carriers” who are heterozygous but phenotypically normal, for example albinism

Multiple alleles (more than 2)

• most genes actually exist in more than 2 forms

  for example, 4 phenotypes of the ABO blood groups determined by 3 alleles of the gene:

  • Ia and Ib (codominant) and I (recessive)

Specific carbs added to the surface of blood cells; Ia adds the A carb, Ib adds the B carb; I adds neither, IaIb adds both

Pleiotropy (pleiotropic genes)

• genes that have multiple phenotypic effects

• Can be responsible for multiple symptoms of certain hereditary diseases (ex. Cystic fibrosis and sickle-cell disease)

Epistasis (epistatic genes)

• gene at 1 locus alters phenotypic expression of a gene at a 2nd locus

• Ex. Labrador retrievers coat color depends on 2 genes

  • one gene determines the pigment color (w/ alleles B for black and b for brown) and the other gene (w/ alleles E for color and e for no color m) determines whether the pigment will be deposited in the hair

Polygenic Inheritance

• quantitative characters vary along a continuum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of 2 or more genes on a single phenotype

• Ex. Human skin color

Nature and nurture: environmental impact on phenotype

• another departure from Mendelian heretics arises when the phenotype for a character depends on environment as well as genotype

• Phenotypic range is generally broadest for polygenic traits, which are called multi factorial because genetic and environmental factors collectively influence phenotype