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AP Biology

AP Biology. Lecture #27 Exceptions to Mendelian Genetics. Extending Mendelian genetics. Mendel worked with a simple system peas are genetically simple most traits are controlled by a single gene each gene has only 2 alleles, 1 of which is completely dominant to the other

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AP Biology

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  1. AP Biology Lecture #27 Exceptions to Mendelian Genetics

  2. Extending Mendelian genetics • Mendel worked with a simple system • peas are genetically simple • most traits are controlled by a single gene • each gene has only 2 alleles, 1 of which is completely dominant to the other • The relationship between genotype & phenotype is rarely that simple

  3. Complete Dominance – the heterozygote is indistinguishable from one of the two homozygotes. • Rh factor – a protein found on the surface of the blood cells in some people. • Two Alleles • Rh+ - possess the Rh factor • Rh- - lacks the Rh factor • Heterozygote Rh+,Rh- - possess the Rh factor and has positive type blood.

  4. Incomplete dominance • appearance between the phenotypes of the 2 parents. Ex: carnations • Heterozygote shows an intermediate, blended phenotype • example: • RR = red flowers • rr = white flowers • Rr = pink flowers • make 50% less color • Incomplete dominance in carnations: red, pink, white

  5. Partial Dominance Hypercholesterolemia – defect in LDL receptor mediated endocytosis. Two alleles Hc+ - functional LDL receptor Hc- defective LDL receptor Homozygotes Hc-,Hc- super high serum cholesterol early onset of heart disease. Hc+,Hc+ normal serum cholesterol- typical risk of heart disease. Hc+,Hc- high serum cholesterol – high risks of heart disease.

  6. Co-dominance • 2 alleles affect the phenotype equally & separately • not blended phenotype • human ABO blood groups • 3 alleles • IA, IB, i • IA & IB alleles are co-dominant • glycoprotein antigens on RBC • IAIB = both antigens are produced • i allele recessive to both

  7. Multiple alleles: • more than 2 possible alleles for a gene. Ex: human blood types

  8. Pleiotropy: • genes with multiple phenotypic effect. • one gene affects more than one phenotypic character • Ex: sickle-cell anemia • Normal and sickle red blood cells

  9. Pleiotropy • Most genes are pleiotropic • one gene affects more than one phenotypic character • 1 gene affects more than 1 trait • dwarfism (achondroplasia) • gigantism (acromegaly)

  10. Inheritance pattern of Achondroplasia Aa x aa Aa x Aa dominantinheritance a a A a Aa Aa AA Aa A A dwarf dwarf lethal a a aa aa Aa aa 50% dwarf:50% normal or1:1 67% dwarf:33%normalor2:1

  11. Epistasis • One gene completely masks another gene • coat color in mice = 2 separate genes • C,c:pigment (C) or no pigment (c) • B,b:more pigment (black=B) or less (brown=b) • cc = albino, no matter B allele • 9:3:3:1 becomes 9:3:4 B_C_ B_C_ bbC_ bbC_ _ _cc _ _cc

  12. Epistasis in Labrador retrievers • 2 genes: (E,e) & (B,b) • pigment (E) or no pigment (e) • pigment concentration: black (B) to brown(b) eebb eeB– E–bb E–B–

  13. Polygenic inheritance • Some phenotypes determined by additive effects of 2 or more genes on a single character • phenotypes on a continuum • human traits • skin color • height • weight • intelligence • behaviors

  14. Multifactoral traits – traits influenced by both the genetic and environment. Examples Flower color in hydrangea Skin pigmentation in humans Milk yield in cows

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