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Understanding Mendelian Genetics: Principles of Trait Inheritance

This chapter introduces Mendelian genetics, exploring how traits are passed down through generations. It discusses the historical context of Gregor Mendel’s experiments with garden peas, where he established foundational concepts such as the blending hypothesis versus the inheritance hypothesis. Mendel's methodology included self-fertilization and cross-fertilization, leading to the identification of dominant and recessive traits. Through carefully controlled experiments, he discovered key principles of heredity, including the law of segregation and the predictability of inheritance ratios, laying the groundwork for modern genetics.

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Understanding Mendelian Genetics: Principles of Trait Inheritance

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  1. Chapter 11 (Part 1) Introduction to Mendel Honors Genetics Ms. Gaynor

  2. Transmission (passing down) of Traits How? • One possible explanation of heredity is a “blending” hypothesis • genetic material contributed by two parents mixes

  3. Another Hypothesis • An alternative to the blending model is the hypothesis of inheritance (genes) • Parents pass on discrete heritable units (factors) called genes

  4. Figure 14.1 Gregor Johann Mendel1843 • Documented a mechanism of inheritance through his experiments with garden peas

  5. Gregor Johann Mendel • Mendel discovered the basic principles of heredity • By breeding garden peas in carefully planned experiments

  6. Mendel’s Experimental Method • Why use pea plants? • available in many varieties • easy to get • he could strictly control which plants mated with which • Grow quickly

  7. Pea Plant Fertilization • Self fertilization : mate with self  produce identical offspring • TRUE or PURE breeds • Cross fertilization : mate with another  can produce different offspring • HYBRIDS

  8. SELF Cross B A

  9. Procedure: Crossing pea plants

  10. Mendel’s Procedures • Mendel chose to track • Only those characteristics (traits) that varied in an “either-or” manner • Mendel also made sure that • He started his experiments with varieties that were “true-breeding”

  11. True Breeding vs. Hybridization • In a typical breeding experiment • Mendel mated 2 DIFFERENT, true-breeding varieties • called hybridization • The true-breeding parents (Pure breeding • made through true breeding mated two of the SAME varieties so these plants are “TRUE” for only 1 trait (can only make offspring w/ this trait)

  12. Mendel’s Generations • Original Parents • Are called the P generation • The hybrid (mixed) offspring of the P generation • Are called the F1 generation • When F1 individuals self-pollinate • The F2 generation is produced

  13. Generations (in general) • P = parental generation of a cross • F1 = the first generation after the parental (the results of the 1st cross) • F2= a cross between F1 individuals yields F2 (2nd cross)

  14. Mendel’s Results • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants • All of the offspring were purple • When Mendel crossed the F1 plants • Many of the plants had purple flowers, but some had white flowers

  15. P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation Mendel discovered • A ratio of about 3:1, purple to white flowers, in the F2 generation Where did the white color go? Figure 14.3

  16. Mendel’s Conclusions • Mendel reasoned that • In F1 plants, only purple flower factor was affecting flower color in the hybrids • Purple flower color was dominant, and white flower color was recessive • Some FACTOR was being transferred from parent to offspring

  17. Alleles • Alleles= alternative forms of genes (Mendel’s “factors”) • Each individual has 2 alleles for the same gene (because there are 2 homologous chromosomes) • Each parent passes one allele for each gene to his/her offspring

  18. Useful Genetic Vocabulary • Homozygous • A pair of IDENTICAL alleles for that gene • Exhibits true-breeding • aa = homozygous recessive (or just recessive) • HH = homozygous dominant • Heterozygous • A pair of alleles that are different for that gene • Aa or Hh

  19. Recessive is… • Recessive • an allele that does NOT produce a characteristic effect when present with a dominant allele • only expressed when the determining allele is present in the homozygous condition • aa or hh

  20. Dominant is… • Dominant • an allele that produces the same phenotypic effect whether inherited with a homozygous or heterozygous allele • Aa or AA, Hh or HH • The allele that is expressed • Not necessarily better, stronger, etc.

  21. Mendel’s Law of Segregation • The two alleles for a characteristic separate (segregate) during gamete formation (Anaphase I of meiosis) end up in different gametes • Each gamete gets 1 allele ONLY

  22. Mendel’s Model • Mendel developed a model to predict inheritance • Mendelian Genetics states • There will be a 3:1 inheritance pattern in his F2 offspring if 2 of his F1 offspring were crossed

  23. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Recall…Alleles Alternative versions of genes account for variations in inherited characters, which are now called alleles

  24. For each characteristic, • An organism inherits TWO alleles, one from each parent • A genetic locus is actually represented twice (once on each of the homologues)

  25. Mendelian GeneticsCOMPLETE DOMINANCE If the 2 alleles at a locus differ (heterozygous) • Then the dominant allele determines organism’s appearance • The recessive allele has no noticeable effect on organism’s appearance (it is hidden) • The 1st allele is “completely dominant” over the 2nd allele

  26. Mendelian GeneticsCOMPLETE DOMINANCE • Ex: Tongue rolling = trait • Tongue roller = dominant (T) • Non-tongue roller = recessive (t) • What is the phenotype of an individual with a genotype TT? tt? Tt?

  27. More Genetic Vocabulary • An organism’s phenotype • Is its physical appearance • Ex: Blue eyes, brown hair, etc • An organism’s genotype • Is its genetic makeup • A.k.a.-the allele combination • Ex: aa, Aa, AA • http://www.hippocampus.org/Biology;jsessionid=C83A30FA6EDF5E6BAEEDC6F71B0363E3

  28. Non-Mendelian GeneticsINCOMPLETE DOMINANCE Does not follow Mendel’s pattern… If 2 alleles at a locus differ (heterozygous) • Then the dominant allele “mixes with” the recessive allele to determines organism’s appearance • The recessive allele is NOT hidden • An “inbetween or mixed” phenotype is the result

  29. Non-Mendelian GeneticsINCOMPLETE DOMINANCE • Ex: Hair texture = trait • Curly = dominant (H) • Straight = recessive (h) • What would a “mix” of curly and straight be??? • What is the phenotype of an individual with a genotype HH? hh? Hh?

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