Chapter 4 Mendel’s Work

1. Chapter 4Mendel’s Work Section 1

2. Gregor Mendel • Mid nineteenth century • Priest named Gregor Mendel • Worked in a garden at a monastery • Heredity: the passing of physical characteristics from parents to offspring

3. Why do pea plants have different characteristics? • Traits: physical characteristics • Examples: stem height (tall or short), seed color (green or yellow) • Pea plant TRAITS were often similar to those of their parents and sometimes they were different • His studies laid the foundation for GENETICS– the scientific study of heredity

4. Mendel’s Experiments • The flower’s petals surround the pistils and the stamens • Pistil: produces female sex cells, or eggs • Stamen: produce pollen, which contains male sex cells, or sperm • Fertilization: new organisms can form when the egg and the sperm join in this process

5. Self-Pollinating • Pea plants are usually self-pollinating • Pollen from a flower lands on a pistil of the same flower Cross-Pollinating • “crossed” • Mendel removed pollen from a flower on one plant, then brushed the pollen onto a flower on a second plant

6. Crossing Pea Plants • How are you going to study the inheritance of traits in pea plants? • Mendel decided to “cross” plants with contrasting traits • Example: tall plants with short plants • He started with purebred plants • Purebred: an organism is the offspring of many generations that have the same trait

7. The F1 and F2 Offspring

8. Results of a Cross F1 generation (both tall) P generation (one tall, one short) F2 generation (mix of tall and short)

9. What other traits did Mendel study? • Pod color, seed shape, flower position • What were the two forms of the seed shape? • Round and wrinkled

10. Genes and Alleles • Genes: factors that control a trait • Alleles: different forms of a gene • An organism’s traits are controlled by alleles it inherits from its parents. Some alleles are dominant, while others are recessive

11. Dominant Alleles vs. Recessive Alleles • Dominant Allele: an allele whose trait always shows up in the organism when the allele is present • Recessive Allele: an allele that is masked when a dominant allele is present

12. Let’s do some math….Examples for Stem Height • One recessive allele + one dominant allele = dominant traits (tall stem) • One recessive allele + one recessive allele = recessive traits (short stem)

13. Alleles in Mendel’s Crosses • Combination of Alleles: • Hybrid tall--- one allele for tall stems and one allele for short stems • Purebred tall--- two alleles for tall stems • Purebred short--- two alleles for short stems

14. Symbols for Alleles • Geneticists use letters to represent alleles • Dominant allele is represented by a capital letter • Example: tall stems = T • Recessive allele is represented by a lowercase letter • Example: short stems = t

15. 2 dominant alleles for tall stems = TT • 2 recessive alleles for short stems = tt • 1 recessive allele for short stems and 1 dominant allele for tall stems = Tt

16. Significance of Mendel’s Contribution • The importance of Mendel’s discovery was NOT recognized during his lifetime • Then, in 1900, three scientists discovered his work • They quickly realized the importance of his work • Mendel is often called the Father of Genetics