Chapter 4: Modification of Mendelian Ratios Allele *Wild-type allele *Mutant allele Conventional symbols for alleles: recessive allele- initial letter of the name of the recessive trait, lowercased and italicized dominant allele- same letter in uppercase Genetic nomenclature is extremely diverse!

Incomplete or Partial Dominance Cross between parents with contrasting traits: Red flowers or white flowers Offspring with an intermediate phenotype: pink flowers

Codominance: Example: MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

Examples: *Table 4.1: over 100 alleles at a given locus in Drosophila *ABO Blood group in humans Multiple Alleles: *Characterized by the presence of glycoprotein antigens on the surface of red blood cells *Distinct from the M and N antigens *Also exhibits codomiance

Lethal Alleles: Example: Coat color in mice *A = agouti = wild-type allele *A Y = yellow = mutant allele

Combining modified modes of inheritance:

Gene interaction: *Epistasis Example: *In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome *Coat color in mice *Black (B) is dominant to brown (b) *Second gene responsible for allowing pigment to be deposited in hair C = presence, c = absence (colorless)

*Novel phenotypes due to gene interaction Example: disc-shaped fruit (AABB) X long fruit (aabb) pepper color