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Mendelian Genetics

Mendelian Genetics. Genetics and heredity. For a long time, general ideas of inheritance were known. =. +. +. What was really lacking was a quantitative understanding of how particular traits were passed down to their offspring. +. =. Gregor Mendel (1822-1884). German monk born in 1822

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Mendelian Genetics

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  1. Mendelian Genetics

  2. Genetics and heredity For a long time, general ideas of inheritance were known = + +

  3. What was really lacking was a quantitative understanding of how particular traits were passed down to their offspring. + =

  4. Gregor Mendel (1822-1884) • German monk born in 1822 • Lived in what is now the Czech Republic • Tended the garden at his monastery, conducted experiments with pea plants • Studied peas for 7 years, published results 1866; ignored until 1900 • Now considered the “Father of Genetics”

  5. What is a trait? • A trait is a characteristic that can vary from one individual to the next (e.g., eye color) • Mendel chose 7 pea plant traits to study: • seed shape, seed color, flower color, pod shape, pod color, flower position, plant height • He was lucky that each trait happened to be located on different chromosomes (people didn’t know about chromosomes back then)

  6. Mendel's experiments (Normally done by bees… Mendel used scissors and a paint brush) • First, he created true (pure) breeding pea plants • Then he experimented with different traits (one at a time) • He cross-pollinated plants with different traits, and observed the offspring

  7. Important vocabulary • True (pure) breeding = offspring always have same trait as parent (every time, no matter what!) • Self-pollination = plant fertilizes itself • Cross-pollination = one plant fertilizes another • P generation = Parent generation • F1 generation = 1st generation offspring (“filius” is Latin for “son”; offspring of P generation) • F2 generation = 2nd generation offspring (offspring of F1 generation)

  8. Mendel’s crosses • When Mendel crossed true-breeding tall plants with true-breeding dwarf plants (P generation)… …what size were the offspring (F1 generation)?

  9. The F1 generation were all tall

  10. Mendel was surprised! • Mendel had expected the traits to blend: tall + dwarf = medium • Instead, the tallness of parent had masked the shortness of the other parent tall + short = tall????

  11. …what size were the offspring (F2 generation)? • Mendel then let the F1 generation self-pollinate to create the F2 generation

  12. Some of the F2 offspring were tall, and some were dwarf • The ratio was always 3 tall to 1 dwarf (3:1)

  13. Mendel's findings • When Mendel crossed true-breeding plants with different traits, all of the F1 plants showed only one trait (e.g., all tall); the F2 plants showed a 3:1 ratio • He did not get “medium” plants! • He called the traits dominant (showed in F1 generation) or recessive (didn’t show up in the F1 generation)

  14. Mendel’s conclusions Mendel realized that: • Inheritance of traits is controlled by genes contributed by each parent (e.g., flower color)

  15. Mendel’s conclusions • These genes come in different forms, called alleles (e.g., purple or white flowers)

  16. Mendel’s conclusions • Some alleles are dominant while others are recessive • When both types of alleles are present, dominant alleles mask (cover up) recessive alleles Mendel’s Discoveries & Results

  17. Mendel’s Law of Dominance: • Generally, one allele is dominant to another • Dominant allele always expressed, even if combined with recessive allele • written as uppercase letter of the trait • e.g., tall = dominant: TT, tT, or Tt • Recessive allele expressed only if dominant allele is not present. • written as a lowercase letter of the dominant trait • e.g., dwarf = recessive: tt

  18. More vocabulary… • Genotype = genetic combination of alleles (e.g., TT, Tt, tt) • Phenotype = physical characteristic or trait (e.g., tall, short) • Homozygous = both alleles are the same (e.g., TT or tt) → “true breeding” • Heterozygous = alleles are different (e.g., Tt) • Hybrid = offspring of crosses of parents with different traits (e.g., offspring of TT and tt)

  19. Mendel’s Law of Segregation • When gametes form, the alleles from each parent are segregated (separated) from each other • Each gamete carries a single allele for each gene

  20. 1 2 tt TT 3 4 5 6 T T t t Tt Tt Tt Tt 7 8 t T t T 9 10 TT Tt Tt tt 11 12 13 14 Parents (P) gametes F1 gametes Probability of F2 generation being TT? F2 TT homozygous tall no P n/a T n/a yes n/a Tt no tall F1 heterozygous tt short F2 no homozygous

  21. Punnett Squares • Instead of doing all of that hard work for each of the possible outcomes under the Law of Segregation, we can do a Punnett Square (geneticist Reginald Punnett created it) Mother’s Gametes • There is a 1 in 4 chance of the offspring being TT • There is a 2 in 4 chance of the offspring being Tt • There is a 1 in 4 chance of the offspring being tt • 3 out of 4 plants = tall • 1 out of 4 plants = dwarf • Tall to dwarf = 3:1 T t T TT Tt Father's Gametes Tt t Tt

  22. Mendel’s Law of Independent Assortment • Genes for two different traits are inherited independently • There is no connection between them (e.g., plant height and seed color) Animation

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