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Understanding Mendel's Patterns of Inheritance in Pea Plants

Chapter 12 explores Gregor Mendel's foundational contributions to genetics through his study of pea plants. Mendel chose peas due to their ability to produce hybrids, availability in various traits, and ease of cultivation. He conducted monohybrid and dihybrid crosses to reveal dominant and recessive traits, establishing the principles of segregation and independent assortment. This chapter also addresses extensions to Mendel’s model, including polygenic inheritance, incomplete dominance, and codominance, broadening the understanding of genetic inheritance.

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Understanding Mendel's Patterns of Inheritance in Pea Plants

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  1. Patterns of Inheritance Chapter 12

  2. Early Ideas of Heredity Gregor Mendel -chose to study pea plants because: 1. other research showed that pea hybrids could be produced 2. many pea varieties were available 3. peas are small plants and easy to grow 4. peas can self-fertilize or be cross-fertilized 5. Produce many “babies”(seeds) FAST!

  3. Monohybrid Crosses Monohybrid cross: a cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits -each trait had 2 alternate forms (variations) -Mendel cross-fertilized the 2 true-breeding strains for each trait

  4. Monohybrid Crosses F1 generation (1st filial generation): offspring produced by crossing 2 opposite (Tall crossed with short) pure-bred strains All F1 plants resembled only 1 parent -no plants with intermediate forms between the 2 parents were produced (Example: no medium heights)

  5. Monohybrid Crosses F2 generation: offspring resulting from the self-fertilization of F1 plants F2 plants exhibited both forms of the trait: ¾ plants with the dominant form ¼ plant with the recessive form Mendel discovered the ratio is actually: 1 pure-bred dominant plant 2 hybrid dominant plants 1 pure-bred recessive plant Ratio = 3 : 1.

  6. Monohybrid Crosses dominant: the form of each trait expressed in the F1 plants (Capital Letters ) recessive: the form of the trait not seen in the F1 plants (Small-case Letters)

  7. Monohybrid Crosses - definitions gene: information for a trait passed from parent to offspring alleles: alternate forms of a gene homozygous: having 2 of the same allele (TT – tt – RR – rr – BB – bb) heterozygous: having 2 different alleles (Tt – Rr – Bb)

  8. Monohybrid Crosses - definitions genotype: total set of alleles of an individual (Genes - letters) PP = homozygous dominant Pp = heterozygous pp = homozygous recessive phenotype: outward appearance of an individual (Physical)

  9. Monohybrid Crosses – Mendel’s conclusions Principle of Segregation: Two alleles (homologous chromosomes) separate during gamete (Sperm or Egg) formation Meiosis Proves Mendel was correct!

  10. Monohybrid Crosses – Mendel’s conclusions Principle of Independent Assortment: the alleles of each gene divide into gametes independently of each other Mendel was mostly right about this (show on board)

  11. Probability – Predicting Results Product Rule: the probability of 2 independent events occurring is the PRODUCT of their individual probabilities. Rr Yy x RrYy, probability of obtaining rr yy offspring is: probability of rr = ¼ probability of yy = ¼ probability of rr yy = ¼ x ¼ =1/16

  12. Punnett Squares • Punnett squares can be used to predict the outcome of a genetic cross

  13. Pedigree analysis is used to track inheritance patterns in families.

  14. Testcross Testcross: a cross used to determine the genotype of an individual with dominant phenotype -cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) -the phenotypic ratios of offspring are different, depending on the genotype of the unknown parent

  15. Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross -for example: RR YY x rryy The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

  16. Dihybrid Crosses The F2 generation shows all four possible phenotypes in a set ratio: 9 : 3 : 3 : 1

  17. Extensions to Mendel Mendel’s model of inheritance assumes that: -each trait is controlled by a single gene -each gene has only 2 alleles -there is a clear dominant-recessive relationship between the alleles Many genes do not meet these criteria!

  18. Extensions to Mendel (1) Polygenic inheritance- multiple genes control the phenotype of a trait. These traits show continuous variation Examples: Human height, Eyecolor and Skin Color - Lab Retrievers

  19. Extensions to Mendel (2) Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes.

  20. Red X white 4/4 pink

  21. Extensions to Mendel (3) Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes. (4) Multiple alleles: more than 2 possible alleles for a gene

  22. P generation F1 generation F1 generation F2 generation

  23. Multiple alleles: gene with more than 2 possible alleles • Each individual inherits only 2 • Examples: Blood types (A B o) + Rabbit Coats (Cchh c )

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