Chapter 11 (Part 1)

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

10. SELF Cross B A

11. Procedure: Crossing pea plants

12. 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”

13. 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)

14. 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

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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.

24. 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

25. 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

26. 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

27. 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)

28. 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

29. 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?

30. 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

31. 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

32. 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?