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Cecie Starr Christine Evers Lisa Starr www.cengage.com/chemistry/starr Chapter 14 Human Inheritance (Sections 14.1 - 14.4) Albia Dugger • Miami Dade College Variation in Human Skin Color • Fraternal twins Kian and Remee inherited different alleles of genes for skin color from their mixed-race parents, who must be heterozygous for those alleles 14.1 Shades of Skin • Like most human traits, skin color has a genetic basis • Minor differences in alleles for melanin synthesis and deposition of melanosomes affect skin color • Differences probably evolved as a balance between vitamin D production and protection against harmful UV radiation, which increases risk of birth defects Variation in Human Skin Color • More than 100 gene products are involved in melanin synthesis, and melanosome formation and deposition • Light-skinned people of European descent carry a mutation in gene SLC24A5 that encodes a transport protein in melanosome membranes All in the Family: Mixed Race Twins 14.2 Human Genetic Analysis • Geneticists study inheritance patterns in humans by tracking genetic disorders and abnormalities through families • A genetic abnormality is an uncommon version of a heritable trait that does not result in medical problems • A genetic disorder is a heritable condition that sooner or later results in mild or severe medical problems Pedigrees • Charting genetic connections with pedigrees reveals inheritance patterns of certain traits: • Dominant and recessive alleles • Alleles on autosomes or sex chromosomes • Probability that a trait will recur a family or population • pedigree • Chart showing the pattern of inheritance of a trait through generations in a family Charting Pedigrees • Standard symbols used in pedigrees Example: Polydactyly • Polydactyly is characterized by extra fingers, toes, or both • Black numbers on the pedigree = number of fingers on each hand • Red numbers = number of toes on each foot Pedigree for Polydactyly Charting Pedigrees male female marriage/mating offspring individual showing trait being studied sex not specified generation Gene not expressed in this carrier. Fig. 14.2ab, p. 204 Animation: Pedigree diagrams To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Pedigree for Huntington’s Disease Types of Genetic Variation • Single genes that follow Mendelian inheritance patterns govern more than 6,000 genetic abnormalities and disorders • Most human traits are polygenic (influenced by multiple genes) and often have environmental factors as well • Alleles that give rise to severe genetic disorders are rare Types of Genetic Variation • We will look at 6 main patterns of inheritance for genetic abnormalities and disorders: • Autosomal dominant inheritance pattern • Autosomal recessive inheritance pattern • X-linked recessive inheritance pattern • X-linked dominant inheritance pattern • Changes in chromosome number • Changes in chromosome structure Key Concepts • Tracking Traits in Humans • Inheritance patterns in humans are determined by following traits through generations of family trees • Types of traits followed in such studies include genetic abnormalities or syndromes associated with a genetic disorder ANIMATION: Human Sex Determination To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 14.3 Autosomal Inheritance Patterns • An allele is inherited in an autosomal dominant pattern if the trait it specifies appears in homozygous and heterozygous people • An allele is inherited in an autosomal recessive pattern if the trait it specifies appears only in homozygous people The Autosomal Dominant Pattern • An autosomal dominant trait appears in every generation. • When one parent is heterozygous, and the other is homozygous recessive, each child has a 50% chance of inheriting the dominant allele and displaying the trait Autosomal Dominant Inheritance • A dominant allele (red) is fully expressed in heterozygous people Autosomal Dominant Inheritance normal mother affected father meiosis and gamete formation affected child normal child disorder-causing allele (dominant) Fig. 14.3, p. 206 Autosomal Dominant Inheritance normal mother affected father meiosis and gamete formation affected child normal child disorder-causing allele (dominant) Stepped Art Fig. 14.3, p. 206 ANIMATION: Autosomal-recessive inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Autosomal Dominant Disorders Disorder Main Symptoms Achondroplasia One form of dwarfism Aniridia Defects of the eyes Camptodactyly Rigid, bent fingers Hypercholesterolemia High cholesterol level Huntington’s disease Degeneration of nervous system Marfan syndrome Abnormal connective tissue Polydactyly Extra fingers, toes, or both Progeria Drastic premature aging Neurofibromatosis Tumors of nervous system, skin Achondroplasia • Achondroplasia interferes with formation of the embryonic skeleton Hutchinson–Gilford Progeria • A mutation causes defects in transcription, mitosis, and division • Symptoms of premature aging begin before age two The Autosomal Recessive Pattern • An autosomal allele is inherited in a recessive pattern if it is expressed only in homozygous people, so recessive traits may skip generations • People heterozygous for the allele are carriers – they have the allele but not the trait • Each child of two carriers has a 25% chance of being homozygous and having the trait Autosomal Recessive Inheritance • Two parents who are carriers of a recessive autosomal allele (red) • Each child has a 25% chance of being homozygous for the trait Autosomal Recessive Inheritance carrier mother carrier father meiosis and gamete formation affected child carrier child normal child A disorder-causing allele (recessive) Fig. 14.5a, p. 207 Autosomal Recessive Inheritance carrier mother carrier father meiosis and gamete formation affected child carrier child normal child disorder-causing allele (recessive) Stepped Art Fig. 14.5a, p. 207 Animation: Autosomal-dominant inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Autosomal Recessive Disorders Disorder Albinism Methemoglobinemia Cystic fibrosis damage Ellis–van Creveld Fanconi anemia Galactosemia Hemochromatosis Phenylketonuria (PKU) Sickle-cell anemia Tay–Sachs disease Main Symptoms Absence of pigmentation Blue skin coloration Abnormal glandular secretions leading to tissue and organ Dwarfism, heart defects, polydactyly Abnormalities, bone marrow failure Brain, liver, eye damage Iron overload , joint & organ damage Mental impairment Adverse pleiotropic effects Deterioration of mental and physical abilities; early death Albinism • Albinism, a lack of melanin, occurs in people homozygous for recessive alleles that code for a defective form of the enzyme tyrosinase Tay–Sachs Disease • Mutations cause gangliosides to accumulate to toxic levels in nerve cells • Affected children, such as Conner Hopf, die before age five Key Concepts • Autosomal Inheritance • Many human traits can be traced to dominant or recessive alleles on autosomes • These alleles are inherited in characteristic patterns: dominant alleles tend to appear in every generation; recessive ones can skip generations 14.4 X-Linked Inheritance Patterns • An allele is inherited in an X-linked pattern when it occurs on the X chromosome • Most X-linked inheritance disorders are recessive, because Xlinked dominant alleles tend to be lethal in male embryos XX and XY • X-linked recessive disorders tend to appear in men more often than in women • Men (XY) have only one X chromosome • Women have two X chromosomes (XX), so they can be heterozygous for a recessive allele • Men can transmit an X-linked allele to daughters, but not to sons – only a woman can pass an X-linked allele to a son X-Linked Recessive Inheritance • In this case, the mother carries a recessive allele on one of her two X chromosomes (red) X-Linked Recessive Inheritance carrier mother normal father meiosis and gamete formation normal daughter or son carrier daughter affected son recessive allele on X chromosome Fig. 14.6, p. 208 X-Linked Recessive Inheritance carrier mother normal father meiosis and gamete formation normal daughter or son carrier daughter affected son recessive allele on X chromosome Stepped Art Fig. 14.6, p. 208 Animation: X-linked inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE X-Linked Recessive Disorders Disorder Main Symptoms Androgen insensitivity XY individual but having some syndrome female traits; sterility Red-green color Inability to distinguish red from blindness green Hemophilia Impaired blood clotting ability Muscular dystrophies Progressive loss of muscle function X-linked anhidrotic Mosaic skin (patches with or without dysplasia sweat glands); other effects Red–Green Color Blindness • Most genes involved in proper function of pigment-containing receptors in the eyes are on the X chromosome • Color blindness includes a range of conditions in which an individual cannot distinguish among some or all colors • Some types of color blindness confuse red and green colors, others see green as shades of gray, but perceive blues and yellows quite well Red–Green Color Blindness Tests for Color Blindness Hemophilia A • Hemophilia A, an X-linked recessive disorder that interferes with blood clotting, involves factor VIII, a protein product of a gene on the X chromosome • In the 19th century, hemophilia was relatively common in royal families of Europe and Russia, probably because of the common practice of inbreeding Hemophilia in Descendants of Queen Victoria Duchenne Muscular Dystrophy • A gene on the X chromosome encodes dystrophin, a protein essential in muscle and nerve cells • Boys with DMD are in a wheelchair by age 12, and die from a heart disorder or respiratory failure before age 30 Key Concepts • Sex-Linked Inheritance • The X chromosome holds about 10 percent of all human genes, so many traits are affected by alleles on this chromosome • Inheritance patterns of such X-linked alleles tend to differ between males and females