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Chapter 19 Heredity Lesson 1 - Monohybrid Cross and Test Cross
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Chapter 19 Heredity Lesson 1 - Monohybrid Cross and Test Cross

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Chapter 19 Heredity Lesson 1 - Monohybrid Cross and Test Cross Chapter 19 Heredity Lesson 1 - Monohybrid Cross and Test Cross Presentation Transcript

  • Heredity Chapter 19
  • Lesson Objectives
    • At the end of the lesson, you should be able to
    • (a) Define the gene as a unit of inheritance and distinguish clearly between the terms genes and allele
    • (b) Explain the terms dominant , recessive , co-dominant , homozygous , heterozygous , phenotype and genotype
    • (c) Predict the results of simple crosses with expected ratios if 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation
    • (d) Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny
    • (e) Use genetic diagrams to solve problems involving monohybrid inheritance
  • Whose child is it?
  • Heredity
    • Passing of characteristics/traits e.g. eye colour, height and intelligence to us from our parents
    • As traits can be inherited, we could selectively breed varieties of animals and plants with desirable qualities
    • e.g. cows that produced more milk OR rice plants that produced more rice
  • http:// alltopix.com/gallery/index.php?n =13
  • Relationship between DNA, Chromosomes and Genes
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  •  
  • Gregor Mendel
  • Background information
    • Gregor Mendel was a monk who lived during the mid-late 1800’s, and he is considered the “father” of genetics.
    • Mendel was interested in discovering how traits are inherited in the offspring of sexually-reproducing organisms.
    • Mendel devised what is now called a monohybrid cross to answer the first question of how a single trait is passed from parent to offspring.
  • Monohybrid Cross
    • Inheritance involving only ONE pair of contrasting characters
    • 1) tallness or shortness of the plant
    • 2) plants that had either red or white flowers
    • 3) plants that produced seeds that were either yellow or green OR round or wrinkled
  • Mendel’s Insight into Patterns of Inheritance
    • Mendel did his experiments with the garden pea plant
    • - Pollen (male gamete) released from anthers at ends of stamens
    • - Egg (female gamete) is in ovule at base of carpel
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  •  
  • Terms to familiarise
    • Phenotype : the expressed trait, the outward appearance or visible character of an organism e.g. tallness and dwarfness of a plant
    • Genotype : the genetic make-up of an organism e.g. dwarf pea plants have genotype tt while a tall plant may have genotype either TT or Tt
    • Dominant : An allele that is almost always expressed , even if only one copy is present e.g. TT or Tt
    • Recessive : A gene that is phenotypically manifest in the homozygous state e.g. tt but is masked in the heterozygote by the presence of a dominant allele
    • Homozygous : having the same two alleles at a given locus on homologous chromosomes (e.g. TT , tt )
    • Heterozygous : Having two different alleles at a given locus on a pair of homologous chromosomes e.g. Tt
  • Examples:
    • Let T represent the allele for tall plants. (dominant)
    • And t represent the allele for dwarf plants. (recessive)
    Phenotype Tall plant Pure-bred tall plant Homozygous tall plant Heterozygous tall plant Dwarf plant Genotype TT or Tt TT TT Tt tt
  • Can we determine the offspring of a genetic cross?
    • Doing genetic crosses using the Punnett Square
    a A a aa A a A aa Aa A a a A aa Aa Aa A a a A aa Aa Aa AA A a female gametes 1. 2. 3. 4. male gametes
  •  
  • Mendel’s Monohybrid Cross
  • Mendel’s Monohybrid Cross
  • ***Explaining Mendel’s Monohybrid Cross
    • Parental phenotype:
    • Parental genotype:
    • Gametes:
    • Random fertilisation:
    • (using the Punnett Square)
    • F 1 genotype:
    • F 1 phenotype:
    purple x white TT x tt All Tt All purple T t T T t t Tt Tt Tt Tt T t Let T represent the alleles of the pea plant with purple flower. Let t represent the alleles of the pea plant with white flower.
  • ***Explaining Mendel’s Monohybrid Cross
    • F 1 phenotype:
    • F 1 genotype:
    • Gametes:
    • Random fertilisation:
    • (using the Punnett Square)
    • F 2 genotype:
    • F 2 phenotype:
    • Phenotypic ratio:
    purple x purple Tt x Tt TT, Tt, Tt, tt purple, purple, purple, white 3:1 (purple:white) t t T t T t TT Tt tt Tt T T
  • Traits/ character Phenotypic ratio of 3:1 Condition : Only when the sampling size is large then will we obtain a phenotypic ratio of 3:1
  • Summary of Mendel’s monohybrid crosses
  • Question:
    • A homozygous red flowered hibiscus is crossed with a homozygous white flowered hibiscus. What is the phenotypic ratio that will result from the cross between the offspring resulting from the F1 generation, given that red is dominant.
    • Parental phenotype:
    • Parental genotype:
    • Gametes:
    • Random fertilisation:
    • (using the Punnett Square)
    • F 1 genotype:
    • F 1 phenotype:
    red x white TT x tt All Tt All red T t T T t t Tt Tt Tt Tt T t Let T represent the alleles of the hibiscus with red flower. Let t represent the alleles of the hibiscus with white flower.
    • F 1 phenotype:
    • F 1 genotype:
    • Gametes:
    • Random fertilisation:
    • (using the Punnett Square)
    • F 2 genotype:
    • F 2 phenotype:
    • Phenotypic ratio:
    red x red Tt x Tt TT, Tt, Tt, tt red, red, red, white 3:1 (red:white) t t T t T t TT Tt tt Tt T T
  • *** Testcross to figure out an unknown genotype