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Chapter 9

Chapter 9. Patterns of Inheritance. Purebreds and Mutts-A Difference of Heredity Genetics is the science of heredity A common genetic background will produce offspring with similar physical and behavioral traits Purebred dogs show less variation than mutts

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Chapter 9

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  1. Chapter 9 Patterns of Inheritance

  2. Purebreds and Mutts-A Difference of Heredity • Genetics is the science of heredity • A common genetic background will produce offspring with similar physical and behavioral traits • Purebred dogs show less variation than mutts • True-breeding individuals are useful in genetic research • Behavioral characteristics are also influenced by environment

  3. MENDEL'S LAWS • 9.1 The science of genetics has ancient roots • Early attempts to explain heredity have been rejected by later science • Hippocrates' theory of Pangenesis • Particles from each part of the body travel to eggs or sperm and are passed on • Early 19th-century biologists' blending hypothesis • Traits from both parents mix in the offspring

  4. 9.2 Experimental genetics began in an abbey garden • Gregor Mendel hypothesized that there are alternative forms of genes, the units that determine heritable traits • Mendel crossed pea plants that differed in certain characteristics • Could control matings • Developed true-breeding varieties • Traced traits from generation to generation

  5. Terminology of Mendelian genetics • Self-fertilization: fertilization of eggs by sperm-carrying pollen of the same flower • Cross-fertilization (cross): fertilization of one plant by pollen from a different plant • True-breeding: identical offspring from self-fertilizing parents • Hybrid: offspring of two different varieties

  6. P generation: true-breeding parents • F1 generation: hybrid offspring of true-breeding parents • F2 generation: offspring of self-fertilizing F1 parents

  7. LE 9-2b Petal Stamen Carpel

  8. LE 9-2c Removed stamens from purple flower White Stamens Carpel Transferred pollen from stamens of white flower to carpel of purple flower Parents (P) Purple Pollinated carpel matured into pod Planted seeds from pod Offspring (F1)

  9. LE 9-2d Flower color Purple White Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Dwarf Stem length Tall

  10. 9.3 Mendel's law of segregation describes the inheritance of a single characteristic • From his experimental data, Mendel developed several hypotheses • There are alternative forms (alleles) of genes that account for variation in inherited characteristics • For each characteristic, an organism inherits two alleles, one from each parent • Homozygous: two identical alleles • Heterozygous: two different alleles

  11. If the two alleles of an inherited pair differ • The dominant allele determines the organism's appearance • The recessive allele has no noticeable effect on the organism's appearance • The law of segregation: A sperm or egg carries only one allele for each inherited trait, because allele pairs separate from each other during gamete production

  12. An organism's appearance does not always reveal its genetic composition • Phenotype: Expressed (physical) traits • Genotype: Genetic makeup

  13. LE 9-3a P generation (true-breeding parents)  Purple flowers White flowers F1 generation All plants have purple flowers Fertilization among F1 plants (F1 F1) F2 generation 3 4 1 4 of plants of plants have purple flowers have white flowers

  14. LE 9-3b Genetic makeup (alleles) P plants PP pp Gametes All P All p F1 plants (hybrids) All Pp Gametes 1 2 1 2 P p Sperm p P P PP Pp F2 plants Phenotypic ratio 3 purple : 1 white Eggs Genotypic ratio 1 PP : 2 Pp : 1 pp Pp pp p

  15. 9.4 Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene reside at the same locus on homologous chromosomes • Supports the law of segregation

  16. LE 9-4 Gene loci Dominant allele P a B P a b Recessive allele aa Bb PP Genotype: Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous

  17. 9.5 The law of independent assortment is revealed by tracking two characteristics at once • Dihybrid cross • Mate true-breeding parents differing in two characteristics • The F1 generation exhibits only the dominant phenotype • The F2 generation exhibits a phenotypic ratio of 9:3:3:1 • Mendel's law of independent assortment: each pair of alleles segregates independently of other allele pairs during gamete formation

  18. LE 9-5a Hypothesis: Independent assortment Hypothesis: Dependent assortment rryy RRYY rryy P generation RRYY rryy ry RY ry Gametes Gametes RY  RrYy RrYy F1 generation Sperm Sperm 1 4 1 4 1 4 1 4 RY rY Ry ry 1 2 1 2 ry RY 1 4 RY 1 2 RY RRYY RrYY RRYy RrYy F2 generation Eggs 1 4 rY 1 2 ry RrYY rrYY RrYy rrYy Eggs Yellow round 9 16 1 4 Ry RRYy RrYy RRyy Rryy Green round 3 16 1 4 ry Actual results contradict hypothesis Yellow wrinkled 3 16 RrYy rrYy Rryy rryy Actual results support hypothesis Green wrinkled 1 16

  19. LE 9-5b Blind Blind Phenotypes Genotypes Black coat, normal vision B_N_ Chocolate coat, normal vision bbN_ Black coat, blind (PRA) B_nn Chocolate coat, blind (PRA) bbnn Mating of heterozygotes (black, normal vision) BbNn  BbNn 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) Phenotypic ratio of offspring

  20. 9.6 Geneticists use the testcross to determine unknown genotypes • A testcross can reveal an unknown genotype • Mate an individual of unknown genotype and a homozygous-recessive individual • Each of the two possible genotypes (homozygous or heterozygous) gives a different phenotypic ratio in the F1 generation

  21. LE 9-6  Testcross: bb Genotypes B_ Two possibilities for the black dog: Bb BB or B B b Gametes b b Bb bb Bb All black 1 black : 1 chocolate Offspring

  22. 9.7 Mendel's laws reflect the rules of probability • Events that follow probability rules are independent events • One such event does not influence the outcome of a later such event • The rule of multiplication: The probability of two events occurring together is the product of the separate probabilities of the independent events • The rule of addition: The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways

  23. LE 9-7 F1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 2 1 2 B b B B B b 1 2 B 1 4 1 4 F2 genotypes b B b b 1 2 b 1 4 1 4

  24. CONNECTION • 9.8 Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel's laws • The dominant phenotype results from either the heterozygous or homozygous genotype • The recessive phenotype results from only the homozygous genotype • Family pedigrees can be used to determine individual genotypes

  25. LE 9-8a Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe

  26. LE 9-8b Dd Joshua Lambert Dd Abigail Linnell D ? John Eddy D ? Hepzibah Daggett D ? Abigail Lambert dd Jonathan Lambert Dd Elizabeth Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing

  27. CONNECTION • 9.9 Many inherited disorders in humans are controlled by a single gene • Thousands of human genetic disorders follow simple Mendelian patterns of inheritance • Recessive disorders • Most genetic disorders • Can be carried unnoticed by heterozygotes • Range in severity from mild (albinism) to severe (cystic fibrosis) • More likely to occur with inbreeding

  28. Dominant disorders • Some serious, but nonlethal, disorders (achondroplasia) • Lethal conditions less common than in recessive disorders • Cannot be carried by heterozygotes without affecting them • Can be passed on if they do not cause death until later age (Huntington's disease)

  29. LE 9-9a Parents Normal Dd Normal Dd  Sperm D d Dd Normal (carrier) DD Normal D Eggs Offspring Dd Normal (carrier) dd Deaf d

  30. CONNECTION • 9.10 New technologies can provide insight into one's genetic legacy • New technologies can provide insight for reproductive decisions • Identifying carriers • Tests can distinguish parental carriers of many genetic disorders • Fetal testing • Amniocentesis and chorionic villus sampling (CVS) allow removal of fetal cells to test for genetic abnormalities

  31. LE 9-10a Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Suction tube inserted through cervix to extract tissue from chorionic villi Needle inserted through abdomen to extract amniotic fluid Ultrasound monitor Fetus Fetus Placenta Placenta Chorionic villi Uterus Cervix Cervix Uterus Centrifugation Amniotic fluid Fetal cells Fetal cells Biochemical tests Several weeks Several hours Karyotyping

  32. Fetal imaging • Ultrasound imaging uses sound waves to produce a picture of the fetus • Newborn screening • Some genetic disorders can be detected at birth by routine tests • Ethical considerations • How will genetic testing information be used? Video: Ultrasound of Human Fetus 1

  33. VARIATIONS ON MENDEL'S LAWS • 9.11 The relationship of genotype to phenotype is rarely simple • Mendel's principles are valid for all sexually reproducing species • However, most characteristics are inherited in ways that follow more complex patterns

  34. 9.12 Incomplete dominance results in intermediate phenotypes • Complete dominance • Dominant allele has same phenotypic effect whether present in one or two copies • Incomplete dominance • Heterozygote exhibits characteristics intermediate between both homozygous conditions • Not the same as blending

  35. LE 9-12a P generation White rr Red RR  R r Gametes F1 generation Pink Rr 1 2 1 2 R r Gametes Sperm 1 2 1 2 R r Red RR Pink rR 1 2 R F2 generation Eggs 1 2 Pink Rr White rr r

  36. LE 9-12b Genotypes: HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors Phenotypes: LDL LDL receptor Cell Mild disease Severe disease Normal

  37. 9.13 Many genes have more than two alleles in the population • In a population, multiple alleles often exist for a single characteristic • Example: human ABO blood group • Involves three alleles of a single gene • AB blood group is an example of codominance-both alleles are expressed in heterozygotes

  38. LE 9-13 Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Blood Group (Phenotype) Antibodies Present in Blood Genotypes O A B AB Anti-A Anti-B O ii IAIA or IAi Anti-B A IBIB or IBi B Anti-A AB IAIB

  39. 9.14 A single gene may affect many phenotypic characteristics • Pleiotropy: a single gene may influence multiple characteristics • Example: sickle cell disease • Allele causes production of abnormal hemoglobin in homozygotes • Many severe physical effects • Heterozygotes generally healthy

  40. Most common inherited disorder among people of African descent • Allele persists in population because heterozygous condition protects against malaria

  41. LE 9-14 Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle-cells 5,555 Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Accumulation of sickled cells in spleen Physical weakness Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Anemia Impaired mental function Pneumonia and other infections Kidney failure Paralysis Rheumatism

  42. 9.15 A single characteristic may be influenced by many genes • Polygenic inheritance is the additive effects of two or more genes on a single phenotypic characteristic • Example: human skin color • Controlled by at least three genes

  43. LE 9-15 P generation  aabbcc (very light) AABBCC (very dark) F1 generation  AaBbCc AaBbCc 1 64 6 64 15 64 20 64 15 64 6 64 1 64 Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 20 64 F2 generation 1 8 1 8 15 64 1 8 1 8 Fraction of population Eggs 1 8 6 64 1 8 1 8 1 64 1 8 Skin color

  44. 9.16 The environment affects many characteristics • Many characteristics result from a combination of genetic and environmental factors • Nature vs. nurture is an old and hotly contested debate • Only genetic influences are inherited

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