BIO 325 Study Guide - Midterm Guide: Zygote, Chromosome, Gamete
Epistasis
• Epistasis – when the action of an allele at one gene hides traits normally caused by
alleles at another gene
• Epistasis – Alleles of one gene mask the effect of alleles of another gene
• Example: Bombay Phenotype
o Blood groups are determined by the presence or absence of sugars (A and B) on
the surface of red blood cells
o Genotype I determines what sugars are produced
o Substance H attaches sugar A and B to red blood cells
o Rare h allele prevents the production of substance H in homozygous
o hh individuals have no sugars attached to red blood cells, resulting blood O type;
therefore, h is epistatic to I
• Recessive Epistasis – homozygous recessive alleles of one gene mask the effect of both
alleles of another gene
o Recessive Epistasis = 9:3:4 ratio
• Dominant Epistasis – dominant allele of one gene masks the effect of both alleles of
another gene
o Dominant Epistasis = 12:3:1 ratio of phenotypes
• Redundant Genes – dominant A or B is required for maize leaf to development
o Redundant Genes = 15:1 ratio
Other Factors Affecting Phenotype
• Environment and Chance
o Temperature sensitive alleles-gene products function in a limited temperature
range
▪ Example: black and white cat
o Temperature dependent sex determination – same genes, different sexes
▪ Example: crocodiles
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