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Notes 5-1 & 5-2

Notes 5-1 & 5-2. Mendel and Punnett Squares. Mendel’s Pea Plants. Mendel was a geneticist who studied pea plants He began his experiments by crossing 2 purebred organisms. Crossing Pea Plants.

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Notes 5-1 & 5-2

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  1. Notes 5-1 & 5-2 Mendel and Punnett Squares

  2. Mendel’s Pea Plants • Mendel was a geneticist who studied pea plants • He began his experiments by crossing 2 purebred organisms

  3. Crossing Pea Plants • Gregor Mendel crossed pea plants that had different traits. The illustrations show how he did this.

  4. Mendel’s Experiments • In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants.

  5. Dominant and Recessive Alleles • Mendel studied several traits in pea plants.

  6. Genes are factors that control a trait • Each gene has 2 contrasting forms called alleles • Some alleles are dominant (represented by a capital letter) • Some alleles are recessive (represented by a lower case letter) • If a gene has both dominant alleles, it is homozygous dominant (TT) • If a gene has both recessive alleles, it is homozygous recessive (tt) • If a gene has 1 dominant and 1 recessive allele, it is heterozygous (Tt)

  7. A Punnett Square • In this cross, both parents are heterozygous for the trait of seed shape. R represents the dominant round allele, and r represents the recessive wrinkled allele.

  8. Probability and Genetics • In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

  9. Phenotypes and Genotypes • An organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic makeup, or allele combinations.

  10. What Are the Genotypes? • Mendel allowed several F1 pea plants with yellow seeds to self-pollinate. The graph shows the approximate numbers of the F2 offspring with yellow seeds and with green seeds.

  11. Yellow–6,000; green–2,000 Reading Graphs: How many F2 offspring had yellow seeds? How many had green seeds? What Are the Genotypes?

  12. 8,000; 75% have yellow peas and 25% have green peas. Calculating: Use the information in the graph to calculate the total number of offspring that resulted from this cross. Then calculate the percentage of the offspring with yellow peas, and the percentage with green peas. What Are the Genotypes?

  13. Both parents probably had the genotype Bb. Inferring: Use the answers to Question 2 to infer the probable genotypes of the parent plants. (Hint: Construct Punnett squares with the possible genotypes of the parents.) What Are the Genotypes?

  14. Codominance • In codominance, the alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring.

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