Mendelian Genetics

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  • Figure 2.2

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  • 1. Genetics
    • Mendel and the Gene Idea
  • 2. Heredity
    • What genetic principles account for the transmission of traits from parents to offspring?
    • One possible explanation of heredity is a “blending” hypothesis - The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green
    • An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units, genes
  • 3. Gregor Mendel
    • Documented a particulate mechanism of inheritance through his experiments with garden peas
    Fig. 2.2 Gregor Mendel’s monastery garden.
  • 4. Mendelian Genetics
    • Gregor Johann Mendel (1822-1884)
      • Augustinian monk, Czech Republic
      • Foundation of modern genetics
      • Studied segregation of traits in the garden pea ( Pisum sativum ) beginning in 1854
      • Published his theory of inheritance in 1865. “Experiments in Plant Hybridization”
      • Mendel was “rediscovered” in 1902
      • Ideas of inheritance in Mendel’s time were vague. One general idea was that traits from parents came together and blended in offspring. Thus, inherited information was predicted to change in the offspring, an idea that Mendel showed was wrong. “Characters,” or what we now call alleles, were inherited unchanged. This observation and the pattern of inheritance of these characters gave us the first definition of a gene
  • 5. Themes of Mendel’s work
    • Variation is widespread in nature
    • Observable variation is essential for following genes
    • Variation is inherited according to genetic laws and not solely by chance
    • Mendel’s laws apply to all sexually reproducing organisms
  • 6. Mendel’s Experimental, Quantitative Approach
    • 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
    • Mendel chose to work with the garden pea (Pisum sativum)
      • Because they are available in many varieties, easy to grow, easy to get large numbers
      • Because he could strictly control which plants mated with which
  • 7. Crossing Pea Plants 1 5 4 3 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) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 1 )
  • 8. Mendel’s experimental design
    • Statistical analyses:
      • Worked with large numbers of plants
      • counted all offspring
      • made predictions and tested them
    • Excellent experimentalist
      • controlled growth conditions
      • focused on traits that were easy to score
      • chose to track only those characters that varied in an “either-or” manner
  • 9. Mendel’s Studied Discrete Traits
  • 10. Genetic Vocabulary
    • Character: a heritable feature, such as flower color
    • Trait: a variant of a character, such as purple or white flowers
    • Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father
    • Alternative forms of traits are called alleles
  • 11. Antagonistic traits Dominant Recessive
  • 12. Mendel’s experimental design
    • Mendel also made sure that he started his experiments with varieties that were “true-breeding”
    X X X X X X
  • 13. Genetic Vocabulary
    • Phenotype – observable characteristic of an organism
    • Genotype – pair of alleles present in and individual
    • Homozygous – two alleles of trait are the same (YY or yy)
    • Heterozygous – two alleles of trait are different (Yy)
    • Capitalized traits = dominant phenotypes
    • Lowercase traits= recessive phenotypes
  • 14. Genetic Vocabulary
    • Generations:
      • P = parental generation
      • F1 = 1st filial generation, progeny of the P generation
      • F2 = 2nd filial generation, progeny of the F1 generation (F3 and so on)
    • Crosses:
      • Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait.
      • Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.
  • 15. Phenotype vs Genotype Figure 14.6 3 1 1 2 1 Phenotype Purple Purple Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1
  • 16. Phenotype vs Genotype Dominant & recessive alleles (Fig. 10.7):
  • 17. Mendel’s Experimental Design
    • In a typical breeding experiment Mendel mated two contrasting, true-breeding varieties, a process called hybridization
    • The true-breeding parents are called the P generation
    • The hybrid offspring of the P generation are called the F1 generation
    • When F1 individuals self-pollinate the F2 generation is produced
  • 18. Mendel’s Observations
    • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants all of the offspring were purple
    • When Mendel crossed the F1 plants, many of the plants had purple flowers, but some had white flowers
    • A ratio of about three to one, purple to white flowers, in the F2 generation
    P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by  ). The resulting F 1 hybrids were allowed to self-pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
  • 19. Mendel’s Rationale
    • In the F1 plants, only the purple trait was affecting flower color in these hybrids
    • Purple flower color was dominant, and white flower color was recessive
    • Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring
    • There are four related concepts that are integral to this hypothesis
  • 20. Heredity Concepts
    • Alternative versions of genes account for variations in inherited characters, which are now called alleles
    • For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice
    • If the two alleles at a locus differ, the dominant allele determines the organism’s appearance
    • The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
    Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
  • 21. Law of Segregation
    • Mechanism of gene transmission
    Fertilization: alleles unite Gametogenesis: alleles segregate
  • 22. Mendelian Genetics
    • Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?
    • We can answer this question using a Punnett square
    Parental P 0 cross
  • 23. Mendelian Genetics
    • Classical Punett's Square is a way to determine ways traits can segregate
    Parental P 0 cross F 1 cross Determine the genotype and phenotype
  • 24. Mendelian Genetics
    • Classical Punett's Square is a way to determine ways traits can segregate
    Parental P 0 cross F 1 cross Determine the genotype and phenotype
  • 25. Mendelian Genetics
    • Classical Punett's Square is a way to determine ways traits can segregate
    Parental P 0 cross F 1 cross Determine the genotype and phenotype
  • 26. Mendel’s Law Of Segregation, Probability And The Punnett Square P Generation F 1 Generation F 2 Generation P p P p P p P p Pp PP pp Pp Appearance: Genetic makeup: Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: Gametes: F 1 sperm F 1 eggs 1 / 2 1 / 2  Each true-breeding plant of the parental generation has identical alleles, PP or pp . Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F 1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. 3 : 1 Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F 2 generation. This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F 1  F 1 ( Pp  Pp ) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.
  • 27. Mendel’s Monohybrid Cross Purple (Pp) Purple ( PP ) P p p p P P P p F 1 generation All purple White (pp) Purple (Pp) Pp Pp PP Gametes F 2 generation ¾ purple, ¼ white Gametes Gametes Gametes Pp Pp Pp Pp pp
  • 28. Smooth and wrinkled parental seed strains crossed.
    • Punnett square
      • F1 genotypes: 4/4 Ss
      • F1 phenotypes: 4/4 smooth
  • 29. Mendel Observed The Same Pattern In Characters
  • 30. The Testcross
    • In pea plants with purple flowers the genotype is not immediately obvious
    • A testcross
      • Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
      • Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
  • 31. Test Cross
    • To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:
    PP P P pp p p Recessive phenotype Dominant Phenotype Gametes Gametes Alternative 1 – Plant with dominant phenotype is homozygous ? Pp P p p p pp Recessive phenotype ? Alternative 2 – Plant with dominant phenotype is heterozygous Dominant Phenotype Gametes Gametes
  • 32. Test Cross
    • To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:
    PP If all offspring are purple; unknown plant is homozygous. P P pp p p Pp Pp Pp Pp Recessive phenotype Dominant Phenotype Gametes Gametes Alternative 1 – Plant with dominant phenotype is homozygous ? Pp If half of offspring are white; unknown plant is heterozygous. P p p p pp pp pp Pp Pp Recessive phenotype ? Alternative 2 – Plant with dominant phenotype is heterozygous Dominant Phenotype Gametes Gametes
  • 33. The Testcross  Dominant phenotype, unknown genotype: PP or Pp ? Recessive phenotype, known genotype: pp If PP , then all offspring purple: If Pp , then 1 ⁄ 2 offspring purple and 1 ⁄ 2 offspring white: p p P P Pp Pp Pp Pp pp pp Pp Pp P p p p APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. RESULTS
  • 34. The Law of Independent Assortment
    • Mendel derived the law of segregation by following a single trait
      • 2 alleles at a single gene locus segregate when the gametes are formed
      • The F1 offspring produced in this cross were monohybrids, heterozygous for one character
    • Mendel identified his second law of inheritance by following two characters at the same time
      • Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently
      • Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
  • 35. Dihybrid Cross
    • With his monohybrid crosses, Mendel determined that the 2 alleles at a single gene locus segregate when the gametes are formed.
    • With his dihybrid crosses, Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently.
  • 36. Dihybrid Cross
    • For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y).
    • Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.
  • 37.
    • For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y).
    • Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.
    Dihybrid Cross
  • 38. Dependent Segregation
    • If dependent segregation (assortment) occurs:
      • Alleles at the 2 gene loci segregate together, and are transmitted as a unit.
      • Therefore, each plant would only produce gametes with the same combinations of alleles present in the gametes inherited from its parents:
    Parents Parental Gametes F 1 Offspring R Y F 1 Offspring’s Gametes R R Y Y r r y y r y R r Y y R Y r y What is the expected phenotypic ratio for the F 2 ?
  • 39. R Y R Y F 2 With Dependent Assortment: r y r y Ratio is 3 round, yellow : 1 wrinkled, green r r y y R r Y y R r Y y R R Y Y
  • 40. Independent Segregation
    • Alleles at the 2 gene loci segregate (separate) independently, and are NOT transmitted as a unit. Therefore, each plant would produce gametes with allele combinations that were not present in the gametes inherited from its parents:
    Parents Parental Gametes F 1 Offspring R Y F 1 Offspring’s Gametes R R Y Y r r y y r y R r Y y R y r y What is the expected phenotypic ratio for the F 2 ? R Y r Y
  • 41. Mendelian Genetics Dihybrid cross - parental generation differs in two traits example-- cross round/yellow peas with wrinkled/green ones Round/yellow is dominant What are the expected phenotype ratios in the F 2 generation? round, yellow = round, green = wrinkled, yellow = wrinkled, green =
  • 42. RY RY ry ry Phenotypic ratio is 9 : 3 : 3 : 1 F 2 with independent assortment: R y r Y r Y R y rr Y y R r Y y rr yy R r yy R r yy RR yy RR Y y R r Y y R r Y y RR Y y R r YY RR YY rr Y y R r Y y rr YY R r YY
  • 43. A Dihybrid Cross
    • How are two characters transmitted from parents to offspring?
      • As a package?
      • Independently?
    • A dihybrid cross
      • Illustrates the inheritance of two characters
      • Produces four phenotypes in the F2 generation
    YYRR P Generation Gametes YR yr  yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1 ⁄ 2 YR YR yr 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 yr YYRR YyRr yyrr YyRr 3 ⁄ 4 1 ⁄ 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 ⁄ 4 Yr 1 ⁄ 4 yR 1 ⁄ 4 yr 1 ⁄ 4 9 ⁄ 16 3 ⁄ 16 3 ⁄ 16 1 ⁄ 16 YYRR YYRr YyRR YyRr Yyrr YyRr YYrr YYrr YyRR YyRr yyRR yyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yR yr 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 Sperm CONCLUSION The results support the hypothesis ofindependent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color ( Y ) and round shape ( R ) are dominant.
  • 44. Dihybrid cross
    • F 2 generation ratio: 9:3:3:1
  • 45. Law of Independent Assortment
    • Mendel’s dihybrid crosses showed a 9:3:3:1 phenotypic ratio for the F2 generation.
    • Based on these data, he proposed the Law of Independent Assortment, which states that when gametes form, each pair of hereditary factors (alleles) segregates independently of the other pairs.
  • 46. Laws Of Probability Govern Mendelian Inheritance
    • Mendel’s laws of segregation and independent assortment reflect the rules of probability
      • The multiplication rule
        • States that the probability that two or more independent events will occur together is the product of their individual probabilities
      • The rule of addition
        • States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
  • 47. Laws Of Probability - Multiplication Rule
    • The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself
    • Following the self-hybridization of a heterozygous purple pea plants ( P p), what is the probability that a given offspring will be homozygous for the production of white flowers (pp)?
    • Probability that a pollen seed will carry p: ½
    • Probability that an egg will carry p: ½
    • Probability that the offspring will be pp:
    • 1/2 X 1/2 = 1/4
  • 48. Laws Of Probability - Addition Rule
    • The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities
    • Following the self-hybridization of a heterozygous purple pea plant ( P p), what is the probability that a given offspring will be purple ?
    • Probability of maternal P uniting with paternal P : 1/4
    • Probability of maternal p uniting with paternal P : 1/4
    • Probability of maternal P uniting with paternal p: 1/4
    • Probability that the offspring will be purple :
    • 1/4 + 1/4 + 1/4 = 3/4
  • 49. Probability In A Monohybrid Cross
    • Can be determined using these rules
     Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 1 ⁄ 2 r r R r r Sperm  Eggs
  • 50. Mendel’s conclusions
    • Genes are distinct entities that remain unchanged during crosses
    • Each plant has two alleles of a gene
    • Alleles segregated into gametes in equal proportions, each gamete got only one allele
    • During gamete fusion, the number of alleles was restored to two
  • 51. Summary of Mendel’s Principles
    • Mendel’s Principle of Uniformity in F1:
      • F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents.
      • Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (alleles).
    • Mendel’s Law of Segregation:
      • Recessive characters masked in the F1 progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny.
      • Two members of a gene pair segregate (separate) from each other during the formation of gametes.
    • Mendel’s Law of Independent Assortment:
      • Alleles for different traits assort independently of one another.
      • Genes on different chromosomes behave independently in gamete production.
  • 52. Exceptions To Mendel’s Original Principles
    • Incomplete dominance
    • Codominance
    • Multiple alleles
    • Polygenic traits
    • Epistasis
    • Pleiotropy
    • Environmental effects on gene expression
    • Linkage
    • Sex linkage
  • 53. Incomplete dominance
    • Neither allele is dominant and heterozygous individuals have an intermediate phenotype
    • For example, in Japanese “Four o’clock”, plants with one red allele and one white allele have pink flowers:
    P Generation F 1 Generation F 2 Generation Red C R C R Gametes C R C W  White C W C W Pink C R C W Sperm C R C R C R C w C R C R Gametes 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 Eggs 1 ⁄ 2 C R C R C R C W C W C W C R C W
  • 54. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F 1 generation F 2 generation C R C R All C R C W C W C W 1 : 2 : 1 Gametes Gametes C R C W C R C W C R C R C R C W C R C W C W C W Incomplete Dominance
  • 55. Codominance
    • Neither allele is dominant and both alleles are expressed in heterozygous individuals
    • Example ABO blood types
  • 56. Polygenic Traits
    • Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits.
    • Polygenic traits often show continuous variation, rather then a few discrete forms:
  • 57. Epistasis
    • Type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus.
    • Labrador retrievers one gene locus affects coat color by controlling how densely the pigment eumelanin is deposited in the fur.
    • A dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat
    • However, a second gene locus controls whether any eumelanin at all is deposited in the fur. Dogs that are homozygous recessive at this locus (ee) will have yellow fur no matter which alleles are at the first locus:
  • 58. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ee No dark pigment in fur eebb eeB_ Yellow fur Yellow fur E_ Dark pigment in fur E_bb E_B_ Brown fur Black fur Epistasis
  • 59. Pleiotropy
    • This is when a single gene locus affects more than one trait.
    • For example, in Labrador retrievers the gene locus that controls how dark the pigment in the hair will be also affects the color of the nose, lips, and eye rims.
  • 60. Environmental Effects on Gene Expression
    • The phenotype of an organism depends not only on which genes it has (genotype), but also on the environment under which it develops.
    • Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of debate and controversy about the relative importance of these 2 factors, particularly for complex human traits.
  • 61.