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Transmission of Genes From Generation to Generation Chapter 3 3.1 Heredity: How are Traits Inherited?

Transmission of Genes From Generation to Generation Chapter 3 3.1 Heredity: How are Traits Inherited?. Why do we begin examining inheritance by discussing Gregor Mendel and pea plants?

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Transmission of Genes From Generation to Generation Chapter 3 3.1 Heredity: How are Traits Inherited?

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  1. Transmission of Genes From Generation to GenerationChapter 3 3.1 Heredity: How are Traits Inherited? • Why do we begin examining inheritance by discussing Gregor Mendel and pea plants? • Before Mendel experimented with the inheritance of traits in garden peas there was no clear understanding of how traits were inherited and passed from one generation to the next. • Mendel used experimental genetics to uncover the fundamental principles of inheritance.

  2. 3.2 Mendel’s Experimental Design • Mendel’s experiments established the foundation for the science of genetics and defined the ideal properties of an experimental organism: • It should have a number of different traits that can be studied • The plant should be self-fertilizing and a flower that reduces accidental pollination • Offspring of self-fertilized plants should be fully fertile so more crosses can be made • His experiments were carried out on a large scale to reduce errors due to small sample size: he used more than 28,000 plants in his experiments

  3. Mendel’s Experiments in the Monastery Garden Fig. 3-2, p. 47

  4. Traits Selected for Study by Mendel Fig. 3-1, p. 46

  5. 3.3 Crossing Pea Plants: Single Traits • Mendel’s initial crosses studied the inheritance of a single trait such as shape or seed color • When plants with smooth peas were crossed with plants having wrinkled peas, the offspring all produced smooth peas • When the plants from these smooth seeds were crossed with each other, the offspring yielded 7,324 peas. 5,474 were smooth and 1,850 were wrinkled.

  6. Mendel’s Terminology • P1 = parental generation • F1 = first generation • F2 = second generation • So in this experiment • P1: smooth x wrinkled • F1: offspring all smooth • F2: offspring smooth (5,474) and wrinkled (1,850)

  7. Results of Mendel’s Crosses Fig. 3-4, p. 48

  8. Results of Mendel’s Crosses Fig. 3-3, p. 48

  9. Mendel’s’ Conclusions • Only one of the parental traits was expressed in the F1 plants • Genetic factors can be hidden (unexpressed) • Despite identical appearance, P1 and F1plants must have different genetic factors • F1 plants must carry hidden genetic factors • The trait not expressed in the F1 plants reappeared in 25% of the F2 plants • F1 plants must carry two factors, one from each parent • Traits were not blended as they passed though parents

  10. Mendel’s’ Conclusions • In these crosses there were two inherited elements or factors that were responsible for the trait (these factors are now referred to as genes). • In the F1 generation factors (genes) were present but not expressed.

  11. Recessive and Dominant Traits • Dominant Trait • Trait expressed in the F1 (heterozygous) condition • Recessive Trait • Trait repressed in the F1 but re-expressed in some members of the F2 generation

  12. Phenotype and Genotype • Phenotype • Observable properties of an organism • Genotype • The specific genetic constitution of an organism

  13. Keep In Mind • Some traits can appear in offspring even when the parents don’t have the trait

  14. Phenotype and Genotype Fig. 3-5, p. 49

  15. Mendel’s Principle of Segregation • Pairs of factors (genes) separate from each other during gamete formation and exhibit dominant/recessive relationships

  16. Gene Pairs Segregate during Gamete Formation • Genes • Fundamental units of heredity • Segregation • Members of a gene pair separate from each other during gamete formation

  17. Ss Ss Ss Ss Smooth Smooth s s S S Gamete formation by F1 parents F1 cross Genotype Phenotype 3/4 Smooth 1 SS 2 Ss 1/4 wrinkled 1 ss F2 ratio

  18. Combinations of Gene Forms (Alleles) • Allele • One possible alternative form of a gene • Usually distinguished by its phenotypic effects • Homozygous • Having identical alleles for one or more genes • Heterozygous • Having two different alleles for one or more genes

  19. Punnett Square Fig. 3-6, p. 50

  20. Exploring Genetics:Ockham’s Razor • “Entities must not be multiplied without necessity”William of Ockham (1300-1349) • Principle of parsimony • When constructing an argument, use the smallest number of steps or the simplest assumptions • The simplest assumption is usually the easiest to prove or disprove by scientific experiments

  21. 3.4 More Pea Plants, Multiple Traits: The Principle of Independent Assortment • Mendel’s later experiments showed that the members of one gene pair separate or segregate independently of other gene pairs

  22. Crosses Involving Two Traits to Follow Different Alleles Fig. 3-8, p. 52

  23. Analysis of a Dihybrid Cross Fig. 3-9, p. 52

  24. Mendel’s Principle of Independent Assortment • Independent assortment • The random distribution of alleles into gametes during meiosis • Leads to formation of all possible combinations of gametes with equal probability in a cross between two individuals

  25. Punnett Square Illustrates Independent Assortment in a Dihybrid Cross

  26. P1 cross P1 cross ssyy ssYY SSyy SSYY Smooth Yellow x wrinkled green x wrinkled Yellow Smooth green Gamete formation Gamete formation sy sY Sy SY Fertilization Fertilization SsYy F1 = Smooth Yellow F1 cross SsYy SsYy Fig. 3-10, p. 53

  27. SsYy SsYy F1 cross x sy Sy sY SY SY SSYY Smooth Yellow SSYy Smooth Yellow SsYY Smooth Yellow SsYy Smooth Yellow F2G e n e r a t I o n Sy SSYy Smooth Yellow SSyy Smooth green SsYy Smooth Yellow Ssyy Smooth green SsYY Smooth Yellow SsYy Smooth Yellow ssYY wrinkled Yellow SsYy Smooth Yellow sY ssYy wrinkled Yellow Ssyy Smooth green ssYy wrinkled Yellow ssyy wrinkled green sy F2 Genotypic ratios F2 Phenotypic ratios 1/16 SSYY 2/16 SSYy 9/16 Smooth Yellow SsYY 2/16 SsYy 4/16 SSyy 1/16 3/16 Smooth green Ssyy 2/16 ssYY 1/16 3/16 wrinkled Yellow 2/16 ssYy 1/16 ssyy 1/16 wrinkled green Fig. 3-10, p. 53

  28. Phenotypic and Genotypic Ratios in a Dihybrid Cross Fig. 3-11, p. 54

  29. Exploring Genetics: Evaluating Results: The Chi Square Test • A statistical test to determine whether the observed distribution of individuals in phenotypic categories is as predicted or occurs by chance X2= ∑ d2/E • Acceptable probability depends on the number of phenotypic classes df=n-1

  30. Chi-Square Analysis of Mendel’s Data Table 3-3, p. 56

  31. Self-crossing of F2 Plants Confirms Mendel’s Predictions Fig. 3-7, p. 51

  32. Mendel’s Contribution • Mendel’s principle of segregation and principle of independent assortment are fundamental to our understanding of the science of heredity (genetics)

  33. Keep In Mind • We can identify genetic traits because they have a predictable pattern of inheritance worked out by Gregor Mendel

  34. 3.5 Meiosis Explains Mendel’s Results: Genes are on Chromosomes • Genes pairs (alleles) are located on chromosome pairs • The position occupied by a gene on a chromosome is referred to as a locus • The behavior of chromosomes in meiosis causes segregation and independent assortment of alleles

  35. Genes, Chromosomes, and Meiosis Table 3-2, p. 55

  36. Segregation and Independent Assortment in Meiosis Fig. 3-12, p. 55

  37. 3.6 Mendelian Inheritance in Humans • Segregation and independent assortment occur with human traits • Inheritance of certain human traits is predictable based on Mendelian inheritance patterns

  38. Segregation of Albinism Fig. 3-13, p. 58

  39. Independent Assortment of Two Traits Can be Followed using a Punnett Square Fig. 3-15, p. 59

  40. Pedigrees • Traits in humans are traced by constructing pedigrees that follow traits through generations • Pedigree • A diagram listing the members and ancestral relationships in a family • Used in the study of human heredity

  41. Pedigree Construction • Pedigree construction • Uses a standard form and symbols • Uses family history to show how a trait is inherited and estimate risk for family members • Provides genetic counseling for those at risk of having children with genetic disorders

  42. Aborted or stillborn offspring Male Female Deceased offspring Mating Mating between relatives (consanguineous) or Unaffected individual Parents and children. Roman numerals symbolize generations. Arabic numbers symbolize birth order within generation (boy, girl, boy) or Affected individual I II Proband; first case in family that was identified or 1 2 3 P P Monozygotic twins Known heterozygotes or Carrier of X-linked recessive trait Dizygotic twins Offspring of unknown sex Infertility Fig. 3-16, p. 60

  43. Numbering System used in Pedigree Construction • Each generation is identified by a Roman numeral • Each individual within a generation is identified by an Arabic number • The first affected family member who seeks medical attention for a genetic disorder in referred to as the proband

  44. Following a Trait Through a Pedigree Fig. 3-17, p. 61

  45. Keep In Mind • Pedigrees are constructed to follow the inheritance of human traits

  46. 3.7 Variations from Mendel • Alleles can interact in ways other than dominant/recessive • Incomplete dominance • Codominance • Multiple alleles • Different genes can interact • Epistasis

  47. Incomplete Dominance • The expression of a phenotype that is intermediate to those of the parents. • An example is the inheritance of flower color in snapdragons: • R1R1 (red) x R2R2 (white) = R1R2 (pink) Fig. 3-18, p. 62

  48. Codominance • Full phenotypic expression of both members of a heterozygous gene pair • An example is the inheritance of the MN blood group in humans: GENOTYPE BLOOD TYPE (PHENOTYPE) LMLM M LMLN MN LNLN N

  49. Multiple Alleles • Genes that have more than two alleles • An example if the inheritance of the ABO blood types in humans - in this case the alleles are co-dominant Fig. 3-19, p. 63

  50. Genes Can Interact in Complex Ways to Produce Phenotypes • Epistasis • A form of gene interaction in which one gene masks or prevents expression of another gene • An example is the Bombay blood type in humans. • Bombay gene, unrelated to the ABO blood type gene, when mutated, can block expression of blood types A and B.

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