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Human Biology Sylvia S. Mader Michael Windelspecht Chapter 20 Patterns of Genetic Inheritance Lecture Outline Part 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 20.4 Beyond Simple Inheritance Patterns Incomplete dominance • Occurs when the heterozygote phenotype is intermediate between phenotypes of the 2 homozygotes • Example: (curly hair) CC x SS (straight hair) CS (wavy hair) 2 20.4 Beyond Simple Inheritance Patterns Familial hypercholesterolemia • 2 _________ alleles lack LDL-cholesterol receptors. • 1 mutated allele has half the normal number of receptors. • 2 normal alleles have the usual number of receptors. • When receptors are completely absent, excessive cholesterol is deposited in various places in the body, including under the skin. 3 20.4 Beyond Simple Inheritance Patterns Familial hypercholesterolemia Plasma cholesterol (milligrams/deciliter) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homozygote 1000 900 800 700 600 Heterozygote 500 400 300 Normal cholesterol deposits 200 100 0 © Mediscan/ Medical-On-Line Figure 20.17 The inheritance of familial hypercholesterolemia. 4 20.4 Beyond Simple Inheritance Patterns Codominance • Occurs when the alleles are equally expressed in a heterozygote • Example: (Type A blood) AA x BB (Type B blood) AB (Type AB blood that has characteristics of both blood types) 5 20.4 Beyond Simple Inheritance Patterns Multiple allele inheritance • The gene exists in several allelic forms. • A person only has 2 of the possible alleles. • A good example is the ___________________. • A and B are ______________ alleles. • The O allele is recessive to both A and B; therefore, to have this blood type, you must have 2 recessive alleles. 6 20.4 Beyond Simple Inheritance Patterns Multiple allele inheritance What type of blood would each of the following individuals have in a cross between Ao and Bo? Possible genotypes: AB Bo Ao oo Phenotypes: Type AB blood Type B blood Type A blood Type O blood 7 20.4 Beyond Simple Inheritance Patterns Blood type inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents × IBi IAi sperm eggs IA i IB IAIB I Bi i IAi ii Offspring Key Blood type A Blood type B Blood type AB Blood type O Phenotypic Ratio 1:1:1:1 1 1 1 1 8 Figure 20.18 The inheritance of ABO blood types. 20.5 Sex-Linked Inheritance Sex-linked inheritance • Traits are controlled by genes on the sex chromosomes. • X-linked inheritance – the allele is carried on the X chromosome • Y-linked inheritance – the allele is carried on the Y chromosome • Most sex-linked traits are X-linked. 9 20.5 Sex-Linked Inheritance X-linked inheritance: Color blindness Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents XBY Possible offspring: XBXB normal vision female XBXb normal vision female XBY normal vision male XbY normal vision male XBY × XBXb eggs XB XB Xb XBXB XBXb Key XB = Normal vision Xb = Color-blind Normal vision Color-blind Phenotypic Ratio Females All sperm Cross: XBXb x Y XBY XbY Males 1:1 1 1 Offspring Figure 20.19 Results of an X-linked cross. 10 20.5 Sex-Linked Inheritance X-linked disorders • These are more often found in ________ than __________ because recessive alleles are always expressed. • Most X-linked disorders are recessive. – Color blindness is most often characterized by redgreen color blindness. – Duchenne muscular dystrophy is characterized by wasting of muscles and death by age 20. – Fragile X syndrome is the most common cause of inherited mental impairment . – Hemophilia is characterized by the absence of particular clotting factors; blood clots very slowly or not at all. 11 20.5 Sex-Linked Inheritance Duchenne muscular dystrophy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. fibrous tissue abnormal muscle normal tissue (left, right): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center; (center): Courtesy Muscular Dystrophy Association Figure 20.21 Muscular dystrophy. 12 20.5 Sex-Linked Inheritance X-linked disorders Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. XBXB XBY XbY XBXb daughter grandfather XBY XbXb XbY XBY XBXB XBXb XbY grandson Key XBXB = Unaffected female XBXb = Carrier female XbXb = Color-blind female XBY = Unaffected male XbY = Color-blind male Figure 20.20 X-linked recessive disorder pedigree. X-linked Recessive Disorders • More males than females are affected. • An affected son can have parents who have the normal phenotype. • For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier. • The characteristic often skips a generation from the grandfather to the grandson. • If a woman has the characteristic, all of her sons will have it. 13 20.5 Sex-Linked Inheritance X-linked disorders: Hemophilia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Unaffected male Unaffected female Victoria Edward Hemophiliac male Carrier female Victoria 1 Alice Louis IV 3 Alexandra ? Olga Albert 2 Nicholas II ? ? 10 4 Leopold Beatrice Mary ? 7 ? Helena 8 ? Marie Alexi Tatiana Anastasia ? 9 ? ? ? Juan Carlos 5 All were assassinated 6 1. Victoria 2. Edward VII 3. Irene 4. George V 6. Margaret 7. Victoria 8. Alfonso XIII 9. Juan Philip Elizabeth II 12 Kate Figure 20B The royal families’ X-linked pedigree. William 11 13 16 14 15 10. Alexandra 11. Charles 12. Diana 13. Andrew 14. Edward 15. Anne 16. Sarah Harry (queen): © Stapleton Collection/Corbis; (prince): © Huton Archive/Getty Images 14