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Patterns of Inheritance Chapter 9 Part 2 9.4 Complex Variations in Traits A trait that is influenced by the products of multiple genes often occurs in a range of small increments of phenotype (continuous variation) The more genes and other factors that influence a trait, the more continuous the distribution of phenotype Continuous Variation Continuous variation • A range of small differences in a shared trait Bell curve • Curve that typically results when range of variation in a continuous trait is plotted against frequency in a population Continuous Variation (Bell Curve) Fig. 9-12a, p. 165 Fig. 9-12b, p. 165 Animation: Continuous variation in height Environmental Effects on Phenotype Enzymes and other gene products control steps of most metabolic pathways Mutations, interactions among genes, and environmental conditions can affect one or more steps, and contribute to variation in phenotypes Environmental Effects on Animal Phenotypes Snowshoe hare in summer and winter Daphnia without and with predators Fig. 9-13a, p. 166 Fig. 9-13b, p. 166 Fig. 9-13c, p. 166 Environmental Effects on Plant Phenotypes Height (centimeters) 60 60 60 0 0 0 A Plant grown at high elevation (3,060 meters above sea level) B Plant grown at mid-elevation (1,400 meters above sea level) C Plant grown at low elevation (30 meters above sea level) Fig. 9-14, p. 166 Animation: Coat color in the Himalayan rabbit 9.5 Human Genetic Analysis Inheritance patterns in humans are studied by following inherited genetic disorders through generations and graphing results as pedigrees Pedigree • Chart showing the pattern of inheritance of a gene in a family Pedigree: Polydactyly male female marriage/mating offspring individual showing trait being studied sex not specified generation A Standard symbols used in pedigrees *Gene not expressed in this carrier. B A pedigree for polydactyly, which is characterized by extra fingers, toes, or both. The black numbers signify the number of fingers on each hand; the blue numbers signify the number of toes on each foot. Fig. 9-15, p. 167 male female marriage/mating offspring individual showing trait being studied sex not specified generation A) Standard symbols used in pedigrees *Gene not expressed in this carrier. B) A pedigree for polydactyly, which is characterized by extra fingers, toes, or both. The black numbers signify the number of fingers on each hand; the blue numbers signify the number of toes on each foot. Stepped Art Fig. 9-15, p. 167 Animation: Pedigree diagrams Genetic Abnormalities and Disorders Genetic abnormality • An uncommon version of a heritable trait that does not result in medical problems Genetic disorder • A heritable condition that results in a syndrome of mild or severe medical problems Syndrome • Set of symptoms characterizing a genetic disorder 9.6 Human Genetic Disorders Some dominant or recessive alleles on autosomes or the X chromosome are associated with genetic abnormalities or disorders An autosomal dominant allele is expressed in homozygotes and heterozygotes An autosomal recessive allele is expressed only in homozygotes Some Autosomal Dominant Disorders Autosomal Dominant Inheritance normal mother affected father meiosis and gamete formation affected child normal child disorder-causing allele (dominant) Fig. 9-16a, p. 168 Fig. 9-16b, p. 168 Animation: Autosomal-dominant inheritance Some Autosomal Recessive Disorders Autosomal Recessive Inheritance carrier mother carrier father meiosis and gamete formation affected child carrier child normal child disorder-causing allele (recessive) Fig. 9-17a, p. 169 Animation: Autosomal-recessive inheritance X-Linked Recessive Disorders Alleles on the X chromosome are inherited and expressed differently in males and females Males cannot transmit a recessive X-linked allele to their sons Females pass X-linked alleles to male offspring • Example: Queen Victoria of England and hemophilia Some X-Linked Recessive Disorders X-Linked Recessive Inheritance Fig. 9-18 (top), p. 170 Fig. 9-18 (a-d), p. 170 Animation: X-linked inheritance An X-Linked Trait: Queen Victoria and Hemophilia Animation: Morgan’s reciprocal crosses Video: ABC News: Genetic testing: Screening embryos for disease 9.7 Changes in Chromosome Number Chromosome number can change permanently, usually resulting from nondisjunction Nondisjunction • Failure of duplicated chromosomes (sister chromatids) or homologous chromosomes to separate during meiosis or mitosis Nondisjunction During Meiosis Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Fig. 9-20a, p. 172 Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Stepped Art Fig. 9-20b, p. 172 Aneuploidy Aneuploidy • A chromosome abnormality in which a cell has too many or too few copies of a particular chromosome (trisomy, monosomy) The most common aneuploidy in humans, trisomy 21, causes Down syndrome Some Disorders Caused by Aneuploidy Polyploidy Autosomal aneuploidy is usually fatal in humans, but many flowering plants, and some insects, fishes and other animals are polyploid Polyploid • Having three or more of each type of chromosome characteristic of the species Autosomal Change and Down Syndrome Trisomy 21 (Down syndrome) • The only autosomal trisomy that allows humans to survive to adulthood • Affected individuals tend to have certain physical features and impairments Nondisjunction leading to trisomy 21 increases with age of the mother Autosomal Change and Down Syndrome Fig. 9-20b, p. 172 Fig. 9-20c, p. 172 Sex Chromosome Abnormalities Usually associated with learning difficulties, speech delays, and motor skill impairment Female sex chromosome abnormalities: • Turner syndrome (XO), XXX syndrome Male sex chromosome abnormalities: • Klinefelter syndrome (XXY); XYY syndrome Turner’s Syndrome (XO) Affects development of ovaries, sexual maturity Animation: Nondisjunction 9.8 Some Prospects in Human Genetics Geneticists estimate the chance that a couple’s offspring will inherit a genetic abnormality or disorder Potential parents who may be at risk of transmitting a harmful allele to offspring have screening or treatment options Prenatal Diagnosis Tests performed before birth • Amniocentesis • Chorionic villi sampling (CVS) • Fetoscopy Carry risks of miscarriage or injury to fetus Amniocentesis Risks of puncture, infection, loss of amniotic fluid Animation: Amniocentesis Fetoscopy Increased risk of miscarriage Preimplantation Diagnosis A single cell taken from an embryo produced by in vitro fertilization is tested before implantation Genetic Screening and Phenotypic Treatments Genetic screening • Hospitals routinely screen newborns for certain genetic disorders such as PKU (Phenylketonuria) Phenotypic treatments • Surgery, prescription drugs, hormone replacement therapy, or dietary controls can minimize symptoms of some genetic disorders 9.9 Impacts/Issues Revisited The cystic fibrosis (CF) allele is very common in some populations The CF allele is lethal in homozygotes, but offers heterozygotes some protection against bacterial diseases such as typhoid fever Digging Into Data: Cystic Fibrosis and Typhoid Fever