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Chapter 9

0. Chapter 9. Patterns of Inheritance. Biology And Society: A Matter of Breeding. Genetics is the scientific study of heredity. Genetics explains why the offspring of purebred dogs are like their parents. Inbreeding of dogs makes some genetic disorders common.

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Chapter 9

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  1. 0 Chapter 9 Patterns of Inheritance Laura Coronado Bio 10 Chapter 9

  2. Biology And Society: A Matter of Breeding • Genetics is the scientific study of heredity. • Genetics explains why the offspring of purebred dogs are like their parents. • Inbreeding of dogs makes some genetic disorders common. • A dog’s behavior is determined by its • Genes • Environment Laura Coronado Bio 10 Chapter 9

  3. HERITABLE VARIATION AND PATTERNS OF INHERITANCE • Heredity is the transmission of traits from one generation to the next. • Gregor Mendel • Worked in the 1860s • Was the first person to analyze patterns of inheritance • Deduced the fundamental principles of genetics Laura Coronado Bio 10 Chapter 9

  4. In an Abbey Garden • Mendel studied garden peas because they • Are easy to grow • Come in many readily distinguishable varieties • Are easily manipulated • Can self-fertilize Petal Stamen (makes sperm- producing pollen) Carpel (produces eggs) Laura Coronado Bio 10 Chapter 9

  5. Definitions • A character is a heritable feature that varies among individuals. • Flower color • A trait is a variant of a character. • Purple or white flower color • Each of the characters Mendel studied occurred in two distinct forms. Laura Coronado Bio 10 Chapter 9

  6. Mendel • Created true-breeding varieties of plants • Purple & white pea flowers • Crossed two different true-breeding varieties • A monohybrid cross is a cross between parent plants that differ in only one character. • Hybrids are the offspring of two different true-breeding varieties. • The parental plants are the P generation. • Their hybrid offspring are the F1 generation. • A cross of the F1 plants forms the F2 generation. Laura Coronado Bio 10 Chapter 9

  7. White Removed stamens from purple flower. Stamens Transferred pollen from stamens of white flower to carpel of purple flower. Parents (P) Carpel Purple Pollinated carpel matured into pod. Planted seeds from pod. Offspring (F1) Laura Coronado Bio 10 Chapter 9 Figure 9.3-3

  8. Mendel’s Law of Segregation • Mendel performed many experiments. • He tracked the inheritance of characters that occur as two alternative traits. Laura Coronado Bio 10 Chapter 9

  9. Dominant Dominant Recessive Recessive Flower color Pod shape Constricted Inflated White Purple Pod color Flower position Green Yellow Stem length Axial Terminal Seed color Green Yellow Tall Dwarf Seed shape Wrinkled Round Laura Coronado Bio 10 Chapter 9 Figure 9.4

  10. 1 3 4 4 P Generation (true-breading parents) White flowers Purple flowers F1 Generation All plants have purple flowers Fertilization among F1 plants (F1 F1) F2 Generation of plants have purple flowers of plants have white flowers Figure 9.5-3

  11. Mendel developed four hypotheses from the monohybrid cross: 1. There are alternative versions of genes, called alleles. 2. For each character, an organism inherits two alleles, one from each parent. • An organism is homozygous for that gene if both alleles are identical. Called a homozygote. • An organism is heterozygous for that gene if the alleles are different. Called a heterozygote. Laura Coronado Bio 10 Chapter 9

  12. Mendel developed four hypotheses from the monohybrid cross 3. If two alleles of an inherited pair differ • The allele that determines the organism’s appearance is the dominant allele • The other allele, which has no noticeable effect on the appearance, is the recessive allele 4. Gametes carry only one allele for each inherited character. • The two members of an allele pair segregate (separate) from each other during the production of gametes. • This statement is the law of segregation. Laura Coronado Bio 10 Chapter 9

  13. 1 1 2 2 P Generation Genetic makeup (alleles) Purple flowers White flowers pp PP Alleles carried by parents p Gametes P All All F1 Generation (hybrids) Purple flowers Alleles segregate All Pp p P Gametes F2 Generation (hybrids) Sperm from F1 plant p P P PP Pp Eggs from F1 plant p pp Pp Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Laura Coronado Bio 10 Chapter 9 Figure 9.6-3

  14. Geneticists distinguish between an organism’s physical traits and its genetic makeup. • An organism’s physical traits are its phenotype. • An organism’s genetic makeup is its genotype. • Homologous chromosomes have • Genes at specific loci • Alleles of a gene at the same locus Laura Coronado Bio 10 Chapter 9

  15. Gene loci Dominant allele B a P Homologous chromosomes b P a Recessive allele Bb PP aa Genotype: Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous Laura Coronado Bio 10 Chapter 9 Figure 9.7

  16. Mendel’s Law of Independent Assortment • A dihybrid cross is the crossing of parental varieties differing in two characters. • What would result from a dihybrid cross? Two hypotheses are possible: 1. Dependent assortment 2. Independent assortment Laura Coronado Bio 10 Chapter 9

  17. Law of Independent Assortment • Mendel’s dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation. • Thus, the inheritance of one character has no effect on the inheritance of another. • Independent assortment is also seen in two hereditary characters in Labrador retrievers. Laura Coronado Bio 10 Chapter 9

  18. 1 1 1 1 2 2 2 2 1 1 1 1 1 1 1 1 4 4 4 4 4 4 4 4 9 16 3 16 3 16 1 16 (a) Hypothesis: Dependent assortment (b) Hypothesis: Independent assortment P Generation RRYY RRYY rryy rryy Gametes ry ry Gametes RY RY F1 Generation RrYy RrYy Sperm F2 Generation Ry ry rY RY Sperm ry RY RY RrYY RRYy RrYy RRYY RY rY Yellow round rrYY Eggs RrYy rrYy RrYY Eggs ry Green round Ry RRYy RrYy RRyy Rryy Yellow wrinkled ry rrYy RrYy Rryy rryy Green wrinkled Actual results (support hypothesis) Predicted results (not actually seen) Laura Coronado Bio 10 Chapter 9 Figure 9.8

  19. Blind dog Blind dog Chocolate coat, blind (PRA) Phenotypes Black coat, blind (PRA) Black coat, normal vision Chocolate coat, normal vision B_N_ bbnn Genotypes B_nn bbN_ (a) Possible phenotypes of Labrador retrievers Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Phenotypic ratio of offspring 1 chocolate coat, blind (PRA) 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision (b) A Labrador dihybrid cross Laura Coronado Bio 10 Chapter 9 Figure 9.9

  20. Using a Testcross to Determine an Unknown Genotype • A testcross is a mating between • An individual of dominant phenotype (but unknown genotype) • A homozygous recessive individual Laura Coronado Bio 10 Chapter 9

  21. Testcross B_ bb Genotypes Two possible genotypes for the black dog: Bb BB or Gametes b B B bb b Bb Bb b Offspring 1 black : 1 chocolate All black Laura Coronado Bio 10 Chapter 9 Figure 9.10

  22. The Rules of Probability • Mendel’s strong background in mathematics helped him understand patterns of inheritance. • The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events. Laura Coronado Bio 10 Chapter 9

  23. 1 1 1 1 4 4 4 4 1 1 1 1 1 1 2 2 2 2 2 2 F1 Genotypes Bb female Bb male Formation of sperm Formation of eggs F2 Genotypes Male gametes b B B B B b B (  ) Female gametes b B b b b Laura Coronado Bio 10 Chapter 9 Figure 9.11

  24. Mendel’s principles apply to the inheritance of many human traits. • A family pedigree • Shows the history of a trait in a family • Allows geneticists to analyze human traits • Dominant traits are not necessarily • Normal or More common • Wild-type traits are • Those seen most often in nature • Not necessarily specified by dominant alleles Laura Coronado Bio 10 Chapter 9

  25. DOMINANT TRAITS Freckles Widow’s peak Free earlobe RECESSIVE TRAITS No freckles Attached earlobe Straight hairline Laura Coronado Bio 10 Chapter 9 Figure 9.12

  26. First generation (grandparents) ff Ff Ff Ff Second generation (parents, aunts, and uncles) ff Ff ff FF ff Ff or Ff Third generation (brother and sister) ff FF or Ff Female Male Attached Free Laura Coronado Bio 10 Chapter 9 Figure 9.13

  27. Human Disorders Controlled by a Single Gene • Many human traits • Show simple inheritance patterns • Are controlled by single genes on autosomes • Recessive Disorders • Most human genetic disorders are recessive. • Individuals who have the recessive allele but appear normal are carriers of the disorder. Laura Coronado Bio 10 Chapter 9

  28. Laura Coronado Bio 10 Chapter 9 Table 9.1

  29. Parents Hearing Hearing Dd Dd Offspring d D Sperm Dd DD Hearing (carrier) D Hearing Eggs Dd dd Hearing (carrier) d Deaf Laura Coronado Bio 10 Chapter 9 Figure 9.14

  30. Cystic fibrosis • Is the most common lethal genetic disease in the United States • Is caused by a recessive allele carried by about one in 25 people of European ancestry • Prolonged geographic isolation of certain populations can lead to inbreeding, the mating of close relatives. • Inbreeding increases the chance of offspring that are homozygous for a harmful recessive trait. Laura Coronado Bio 10 Chapter 9

  31. Dominant Disorders • Some human genetic disorders are dominant. • Huntington’s disease, which leads to degeneration of the nervous system, does not begin until middle age. • Achondroplasia is a form of dwarfism. • The homozygous dominant genotype causes death of the embryo. • Thus, only heterozygotes have this disorder. Laura Coronado Bio 10 Chapter 9

  32. Parents Dwarf (achondroplasia) Normal (no achondroplasia) Dd dd d d Sperm Dd Dd D Dwarf Dwarf Eggs dd dd d Normal Normal Zachary Jake Amy Jeremy Molly Jo Matt Laura Coronado Bio 10 Chapter 9 Figure 9.16

  33. The Process of Science: What Is the Genetic Basis of Hairless Dogs? • Observation: Dogs come in a wide variety of physical types. • Question: What is the genetic basis for the hairless phenotype? • Hypothesis: A comparison of genes of coated and hairless dogs would identify the gene or genes responsible. • Prediction: A mutation in a single gene accounts for the hairless appearance. • Experiment: Compared DNA sequences of 140 hairless dogs from 3 breeds with 87 coated dogs from 22 breeds. • Results: Every hairless dog, but no coated dogs, had a single change in a single gene. Laura Coronado Bio 10 Chapter 9

  34. Laura Coronado Bio 10 Chapter 9 Figure 9.17

  35. Genetic Testing • Today many tests can detect the presence of disease-causing alleles. • Most genetic testing is performed during pregnancy. • Amniocentesis collects cells from amniotic fluid. • Chorionic villus sampling removes cells from placental tissue. • Genetic counseling helps patients understand the results and implications of genetic testing. Laura Coronado Bio 10 Chapter 9

  36. VARIATIONS ON MENDEL’S LAWS • Some patterns of genetic inheritance are not explained by Mendel’s laws. • In incomplete dominance in plants and people, F1 hybrids have an appearance in between the phenotypes of the two parents. Laura Coronado Bio 10 Chapter 9

  37. 1 1 1 1 1 1 2 2 2 2 2 2 P Generation White Red RR rr Gametes r R F1 Generation Pink Rr Gametes r R Sperm F2 Generation r R R RR Rr Eggs r rr Rr Laura Coronado Bio 10 Chapter 9 Figure 9.18-3

  38. Hypercholesterolemia • Is characterized by dangerously high levels of cholesterol in the blood. • Is a human trait that is incompletely dominant. • Heterozygotes have blood cholesterol levels about twice normal. • Homozygotes have blood cholesterol levels about five times normal. Laura Coronado Bio 10 Chapter 9

  39. Hh HH hh Homozygous for ability to make LDL receptors Heterozygous Homozygous for inability to make LDL receptors GENOTYPE LDL LDL receptor PHENOTYPE Cell Normal Mild disease Severe disease Laura Coronado Bio 10 Chapter 9 Figure 9.19

  40. ABO Blood Groups: An Example of Multiple Alleles and Codominance • The ABO blood groups in humans are an example of multiple alleles. • The human blood type alleles IA and IB exhibit codominance: Both alleles are expressed in the phenotype. • The immune system produces blood proteins called antibodies that can bind specifically to blood cell carbohydrates. • Blood cells may clump together if blood cells of a different type enter the body. • The clumping reaction is the basis of a blood-typing lab test. Laura Coronado Bio 10 Chapter 9

  41. Blood Group (Phenotype) Antibodies Present in Blood Reactions When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Red Blood Cells Genotypes A B AB O Carbohydrate A IAIA or IAi Anti-B A Carbohydrate B IBIB or IBi B Anti-A — AB IAIB Anti-A Anti-B ii O Laura Coronado Bio 10 Chapter 9 Figure 9.20

  42. Pleiotropy and Sickle-Cell Disease • Pleiotropy is the impact of a single gene on more than one character. • Sickle-cell disease • Exhibits pleiotropy • Results in abnormal hemoglobin production • Causes disk-shaped red blood cells to deform into a sickle shape with jagged edges Laura Coronado Bio 10 Chapter 9

  43. Individual homozygous, for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes into long flexible chains, causing red blood cells to become sickle-shaped. Colorized SEM Sickled cells can lead to a cascade of symptoms, such as weakness, pain, organ damage, and paralysis. Figure 9.21

  44. Polygenic Inheritance • Polygenic inheritance is the additive effects of two or more genes on a single phenotype. Laura Coronado Bio 10 Chapter 9

  45. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 1 1 1 20 20 15 15 15 6 6 6 64 64 64 64 64 64 64 64 64 64 64 P Generation aabbcc (very light) AABBCC (very dark) F1 Generation AaBbCc AaBbCc Sperm F2 Generation Eggs Fraction of population Skin pigmentation Laura Coronado Bio 10 Chapter 9 Figure 9.22

  46. The Role of Environment • Many human characters result from a combination of heredity and environment. • Only genetic influences are inherited. Laura Coronado Bio 10 Chapter 9

  47. THE CHROMOSOMAL BASIS OF INHERITANCE • The chromosome theory of inheritance states that • Genes are located at specific positions on chromosomes • The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns • It is chromosomes that undergo segregation and independent assortment during meiosis and thus account for Mendel’s laws. Laura Coronado Bio 10 Chapter 9

  48. 1 1 1 1 4 4 4 4 P Generation Wrinkled-green seeds (rryy) Round-yellow seeds (RRYY) y Y r R r R Y y MEIOSIS FERTILIZATION Gametes r y Y R F1 Generation All round-yellow seeds (RrYy) R y r Law of Independent Assortment: Follow both the long and the short chromosomes. Law of Segregation: Follow the long chromosomes (carrying R and r) taking either the left or right branch. Y MEIOSIS Metaphase I (alternative arrangements) r r R R They are arranged in either of two equally likely ways at metaphase I. The R and r alleles segregate in anaphase I of meiosis. y Y y Y Metaphase II r r R R Only one long chromosome ends up in each gamete. They sort independently, giving four gamete types. y Y y Y y Y y Y Y Y y y Gametes R R r r R r r R ry Ry rY RY Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. Fertilization recombines the r and R alleles at random. FERTILIZATION AMONG THE F1 PLANTS F2 Generation 9 : 1 : 3 : 3 Figure 9.24-4

  49. Linked Genes • Linked genes • Are located close together on a chromosome • May be inherited together • Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined that some genes were linked based on the inheritance patterns of their traits. Laura Coronado Bio 10 Chapter 9

  50. Dihybrid testcross Black body, short wings (mutant) Gray body, long wings (wild-type) GgLl ggll Female Male Results Offspring Gray-long GgLl Black-short ggll Gray-short Ggll Black-long ggLl 944 185 965 206 Recombinant phenotypes 17% Parental phenotypes 83% Laura Coronado Bio 10 Chapter 9 Figure 9.25-2

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