Chapter 19 Heredity Lesson 1 - Monohybrid Cross and Test Cross

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Chapter 19 Heredity Lesson 1 - Monohybrid Cross and Test Cross

  1. 1. Heredity Chapter 19
  2. 2. Lesson Objectives <ul><li>At the end of the lesson, you should be able to </li></ul><ul><li>(a) Define the gene as a unit of inheritance and distinguish clearly between the terms genes and allele </li></ul><ul><li>(b) Explain the terms dominant , recessive , co-dominant , homozygous , heterozygous , phenotype and genotype </li></ul><ul><li>(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 </li></ul><ul><li>(d) Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny </li></ul><ul><li>(e) Use genetic diagrams to solve problems involving monohybrid inheritance </li></ul>
  3. 3. Whose child is it?
  4. 4. Heredity <ul><li>Passing of characteristics/traits e.g. eye colour, height and intelligence to us from our parents </li></ul><ul><li>As traits can be inherited, we could selectively breed varieties of animals and plants with desirable qualities </li></ul><ul><li>e.g. cows that produced more milk OR rice plants that produced more rice </li></ul>
  5. 5. http:// alltopix.com/gallery/index.php?n =13
  6. 6. Relationship between DNA, Chromosomes and Genes
  7. 9. Gregor Mendel
  8. 10. Background information <ul><li>Gregor Mendel was a monk who lived during the mid-late 1800’s, and he is considered the “father” of genetics. </li></ul><ul><li>Mendel was interested in discovering how traits are inherited in the offspring of sexually-reproducing organisms. </li></ul><ul><li>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. </li></ul>
  9. 11. Monohybrid Cross <ul><li>Inheritance involving only ONE pair of contrasting characters </li></ul><ul><li>1) tallness or shortness of the plant </li></ul><ul><li>2) plants that had either red or white flowers </li></ul><ul><li>3) plants that produced seeds that were either yellow or green OR round or wrinkled </li></ul>
  10. 12. Mendel’s Insight into Patterns of Inheritance <ul><li>Mendel did his experiments with the garden pea plant </li></ul><ul><li>- Pollen (male gamete) released from anthers at ends of stamens </li></ul><ul><li>- Egg (female gamete) is in ovule at base of carpel </li></ul>
  11. 15. Terms to familiarise <ul><li>Phenotype : the expressed trait, the outward appearance or visible character of an organism e.g. tallness and dwarfness of a plant </li></ul><ul><li>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 </li></ul><ul><li>Dominant : An allele that is almost always expressed , even if only one copy is present e.g. TT or Tt </li></ul><ul><li>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 </li></ul><ul><li>Homozygous : having the same two alleles at a given locus on homologous chromosomes (e.g. TT , tt ) </li></ul><ul><li>Heterozygous : Having two different alleles at a given locus on a pair of homologous chromosomes e.g. Tt </li></ul>
  12. 16. Examples: <ul><li>Let T represent the allele for tall plants. (dominant) </li></ul><ul><li>And t represent the allele for dwarf plants. (recessive) </li></ul>Phenotype Tall plant Pure-bred tall plant Homozygous tall plant Heterozygous tall plant Dwarf plant Genotype TT or Tt TT TT Tt tt
  13. 17. Can we determine the offspring of a genetic cross? <ul><li>Doing genetic crosses using the Punnett Square </li></ul>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
  14. 19. Mendel’s Monohybrid Cross
  15. 20. Mendel’s Monohybrid Cross
  16. 21. ***Explaining Mendel’s Monohybrid Cross <ul><li>Parental phenotype: </li></ul><ul><li>Parental genotype: </li></ul><ul><li>Gametes: </li></ul><ul><li>Random fertilisation: </li></ul><ul><li>(using the Punnett Square) </li></ul><ul><li>F 1 genotype: </li></ul><ul><li>F 1 phenotype: </li></ul>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.
  17. 22. ***Explaining Mendel’s Monohybrid Cross <ul><li>F 1 phenotype: </li></ul><ul><li>F 1 genotype: </li></ul><ul><li>Gametes: </li></ul><ul><li>Random fertilisation: </li></ul><ul><li>(using the Punnett Square) </li></ul><ul><li>F 2 genotype: </li></ul><ul><li>F 2 phenotype: </li></ul><ul><li>Phenotypic ratio: </li></ul>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
  18. 23. 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
  19. 24. Summary of Mendel’s monohybrid crosses
  20. 25. Question: <ul><li>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. </li></ul>
  21. 26. <ul><li>Parental phenotype: </li></ul><ul><li>Parental genotype: </li></ul><ul><li>Gametes: </li></ul><ul><li>Random fertilisation: </li></ul><ul><li>(using the Punnett Square) </li></ul><ul><li>F 1 genotype: </li></ul><ul><li>F 1 phenotype: </li></ul>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.
  22. 27. <ul><li>F 1 phenotype: </li></ul><ul><li>F 1 genotype: </li></ul><ul><li>Gametes: </li></ul><ul><li>Random fertilisation: </li></ul><ul><li>(using the Punnett Square) </li></ul><ul><li>F 2 genotype: </li></ul><ul><li>F 2 phenotype: </li></ul><ul><li>Phenotypic ratio: </li></ul>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
  23. 28. *** Testcross to figure out an unknown genotype

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