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Chapter 14

Chapter 14. Mendel and the Gene Idea. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance. Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. LE 14-2. Removed stamens from purple flower.

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Chapter 14

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  1. Chapter 14 Mendel and the Gene Idea

  2. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

  3. LE 14-2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Stamens Carpel Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F1)

  4. Mendel chose to track only those characters that varied in an “either-or” manner • He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate)

  5. The Law of Segregation P Generation (true-breeding parents) • When any individual produces gametes, the copies of a gene separate, so that each gamete receives only one copy. Purple flowers White flowers PP x pp F1 Generation (hybrids) All plants had purple flowers Pp x Pp F2 Generation

  6. Mendel called the purple flower color a dominant trait and white flower color a recessive trait • “heritable factor” is what we now call a gene

  7. Mendel’s Model • Mendel developed four related concepts to explain the 3:1 inheritance pattern he observed in F2 offspring. • First Concept – alternate versions of genes, alleles, exist and explain for inherited variations. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

  8. P Generation (true-breeding parents) • Second concept - for each character an organism inherits two alleles, one from each parent Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

  9. P Generation (true-breeding parents) • Third concept-if the two alleles at a locus differ, then the dominant allele determines the organism’s appearance, and the recessiveallele has no noticeable effect on appearance Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

  10. P Generation (true-breeding parents) • Fourth concept – The Law of Segregation states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

  11. LE 14-5_2 P Generation Purple flowers PP White flowers pp Appearance: Genetic makeup: p P Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 1 p P 2 2 F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

  12. Useful Genetic Vocabulary • Homozygous - An organism with two identical alleles for a character. • Heterozygous - An organism that has two different alleles for a gene. • Unlike homozygotes, heterozygotes are not true-breeding

  13. LE 14-6 Genotype Phenotype PP (homozygous Purple 1 Pp (heterozygous 3 Purple 2 Pp (heterozygous Purple pp (homozygous White 1 1 Ratio 1:2:1 Ratio 3:1

  14. LE 14-7 Test Cross Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If Pp, then 1 2 offspring purple and 1 2 offspring white: If PP, then all offspring purple: p p p p P P Pp Pp Pp Pp P P pp pp Pp Pp

  15. The Law of Independent Assortment • Monohybrids - individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid cross Yy x Yy

  16. Dihybrids - heterozygous for two characters • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently YyRr x YyRr ?

  17. LE 14-8 P Generation YYRR yyrr Gametes yr YR YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment Sperm YR Yr yR yr 1 1 1 1 4 4 4 4 Sperm Eggs YR yr 1 1 2 2 YR 1 4 Eggs YYRR YYRr YyRR YyRr YR 1 2 F2 Generation (predicted offspring) YYRR YyRr Yr 1 4 YYRr YYrr YyRr Yyrr yr 1 2 YyRr yyrr yR 1 4 YyRR YyRr yyRR yyRr 3 1 4 4 yr 1 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 3 3 3 16 16 16 16 Phenotypic ratio 9:3:3:1

  18. Law of Independent Assortment - each pair of alleles segregates independently of other pairs of alleles during gamete formation • This law applies only to genes on different, non-homologous chromosomes

  19. Concept 14.2: The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability • The alleles of one gene segregate into gametes independently of another gene’s alleles

  20. The Multiplication and Addition Rules Applied to Monohybrid Crosses • Multiplication rule - the probability that two or more independent events will occur together is the product of their individual probabilities • Rr x Rr • Probability of gamete having: • RR? • rr?

  21. LE 14-9 Rr Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm R r 1 1 2 2 R R R r R 1 2 1 1 4 4 Eggs r r R r r 1 2 1 1 4 4

  22. Rule of addition - the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities • Rr x Rr • Probability of gamete having: • Rr?

  23. Solving Complex Genetics Problems with the Rules of Probability • A dihybrid cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • Cross: YyRr x YyRr • What is the probability of YYRR? • of Yyrr?

  24. Genetics Problems • PPYyRrTt x PpyyRrtt • What is the probability of PpyyrrTt?

  25. Genetics Problems • PpYyRr x Ppyyrr • What fraction of the offspring from this cross would exhibit the recessive phenotypes for at leasttwo of the three characteristics?

  26. Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive • When a gene has more than two alleles • When a gene produces multiple phenotypes

  27. The Spectrum of Dominance • Complete dominance • PP=Purple Pp=Purple • Codominance • BB=Black Fur WW=White Fur BW=Black & White Fur • Incomplete dominance • RR=Red WW=White RW=Pink

  28. P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation 1 1 Gametes CR CW 2 2 Sperm 1 1 CR CW 2 2 Eggs F2 Generation 1 CR 2 CRCR CRCW 1 CW 2 CRCW CWCW

  29. Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • For example, one baby out of 400 in the United States is born with extra fingers or toes • this unusual trait is dominant • Known as polydactyly • Natural Selection at work!

  30. Multiple Alleles • Most genes exist in populations in more than two allelic forms • Four phenotypes of the ABO blood group • three alleles for the enzyme (I): IA, IB, and i

  31. Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, the multiple symptoms of • cystic fibrosis and sickle-cell disease

  32. Epistasis • Epistasis - a gene at one locus alters the phenotypic expression of a gene at a second locus • In mice and many other mammals, coat color depends on two genes • One gene determines the pigment color • The other gene determines whether the pigment will be deposited in the hair

  33. LE 14-11 BbCc BbCc Sperm bC Bc 1 1 1 1 BC bc 4 4 4 4 1 BC BBCC BbCC BBCc BbCc 4 1 bC BbCC bbCC BbCc bbCc 4 BBcc Bbcc BBCc BbCc 1 Bc 4 bbcc Bbcc 1 bbCc bc BbCc 4 9 3 4 16 16 16

  34. Polygenic Inheritance AaBbCc AaBbCc • Quantitative characters are those that vary in the population along a continuum • usually indicates polygenic inheritance • Skin color in humans is an example of polygenic inheritance aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20/64 15/64 Fraction of progeny 6/64 1/64

  35. Nature and Nurture: The Environmental Impact on Phenotype • Norm of reaction - the phenotypic range of a genotype influenced by the environment • Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

  36. Concept 14.4: Many human traits follow Mendelian patterns of inheritance • Pedigree - a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees

  37. LE 14-14a First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak)

  38. LE 14-14b First generation (grandparents) Ff Ff ff Ff Second generation (parents plus aunts and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) ff FF or Ff Free earlobe Attached earlobe Recessive trait (attached earlobe)

  39. Pedigrees can also be used to make predictions about future offspring

  40. Dominantly Inheritance

  41. Recessively Inherited Disorders • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele but appear normal

  42. Recessively Inheritance

  43. X-Linked Recessive Inheritance

  44. X-Linked Dominant Inheritance

  45. Mitochondrial Inheritance

  46. Cystic Fibrosis • Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent • Common symptom - abnormal mucus buildup

  47. Sickle-Cell Disease • Sickle-cell disease affects one out of 400 African-Americans • Symptoms include physical weakness, pain, organ damage, and even paralysis

  48. Dominantly Inherited Disorders • Some human disorders are due to dominant alleles • One example is achondroplasia, a form of dwarfism that is lethal when homozygous for the dominant allele

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