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Exceptions to Mendel’s Rules Genetics
Some Examples • Multiple Alleles: more than 2 in a population • Co-dominance: both alleles fully expressed • Incomplete Dominance: both exert some influence • Others: epistasis, pleitropy, polygenic inheritance, multifactorial
Pleitropy • Single gene pair exerts multiple effects • Symptoms seem unrelated and are therefore hard to connect and diagnosis Some historians believe Lincoln had Marfan Syndrome, an autosomal dominant disorder of the connective tissue
Epistasis • One gene pair influences the expression of another gene pair • In Labrador retrievers, two gene pairs determine coat color • One gene pair determines amount of melanin, the other determines distribution
Polygenic Inheritance • Many genes; one effect • usually produce a range of phenotypes • Example: skin color • Controlled by 5 gene pairs • AABBCCDDEE = very dark • Aabbccddee = very light • Gene pairs influence the metabolic pathway
Polygenic Inheritance • Probability is that more phenotypes will be in the middle • 50% chance of A or a if parent is heterozygous • Most are “average” • Bell shaped curve • May also be influenced by environment
Bell Shaped Curve Height, skin color, intelligence, hair color, eye color are examples of CONTINUOUS VARIATION
Also playing a role in continuous variation is environment • Himalayan rabbit fur color is influenced by temperature • An enzyme required to make melanin only works at low temperatures • How does environment influence human skin color?