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Introduction to Genetic Variation

Introduction to Genetic Variation. Or, lame photo montage thinly disguised as illustration of genetic variation. Key. Haploid gametes ( n = 23 ). Meiosis. Haploid ( n ). Egg ( n ). Diploid (2 n ). Sperm ( n ). MEIOSIS. FERTILIZATION. Testis. Ovary. Diploid zygote (2 n = 46).

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Introduction to Genetic Variation

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  1. Introduction to Genetic Variation Or, lame photo montage thinly disguised as illustration of genetic variation

  2. Key Haploid gametes (n = 23) Meiosis Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Testis Ovary Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

  3. Contributors to Genetic Variation • Independent assortment • Which chromosome does a gamete get? • Crossover events (“recombination”) • Chimeric alleles (remember chiasma formation?) • Random fertilization • Any sperm can fertilize any egg

  4. Independent assortment • Whose chromosome did I get in Meiosis I? • 50-50 shot at maternal or paternal per gamete • Independence of pairs • Each homologous pair is sorted independently from the others • For humans (n = 23) there are about 8 million possible combinations of chromosomes! Maternal Paternal n=2 chromosomes M1/M2 P1/P2 M1/P2 P1/M2

  5. Separation of Homologs Example: individual who is heterozygous at two genes Allele that contributes to red hued hair Allele that contributes to dark hair Allele that contributes to green eyes Allele that contributes to blue eyes Hair color gene Eye color gene During meiosis I, tetrads can line up two different ways before the homologs separate. OR Green eyes Red hues Blue eyes Red hues Green eyes Dark hair Blue eyes Dark hair

  6. Crossing Over- Genetic Recombination • Recombinant chromosomes • combine genes from each parent. • Prophase I • Chromosomes pair up gene by gene • Chiasma • Homologous portions of two nonsisterchromatids traded • In Humans • two to three times per chromosome pair. • New combinations of alleles • combinations that did not exist in each parent. • Independent assortment builds on this variability

  7. Key Events in Prophase of Meiosis I • Prophase I • 2 pairs of sister chromatids are held tightly together • Crossing over can occur at many locations • Swapping of segments between maternal and paternal chromosomes. Centromere Non-sister chromatids Sister chromatids Chromosomes Protein complex One homolog Synaptonemal complex Second homolog

  8. Fig. 13-12-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes

  9. Random Fertilization • Any sperm can fuse with any egg. • Humans (n=23) • Each ovum is one of 8 million possible chromosome combinations • Successful sperm is one of 8 million different possibilities • Zygote (diploid offspring) is 1 of 70 trillion possible combinations of chrms • Amazing how similar siblings/offspring can look! • Recombination adds even more variation to this. • Independent assortment builds on recombination • Mutations- ultimately create a population’s genetic diversity …or not!

  10. Gregor Mendel (1822-1884) • Lots of training • Augustine monk • Beekeeper • Physics teacher • Meteorologist • Monastery garden • Pea plants

  11. “Experiments on Plant Hybridization” • Published in 1866 • Before 20th century, cited 3 times • NOT cited in “The Origin of Species” (1859) • Rediscovered • Hugo de Vries • Better publicity

  12. Mendel and the Gene Idea • What he knew: • Heritable variations exist • Traits are transmitted from parents to offspring • Two main theories existed • Blending (mixing of traits) • Particulate inheritance (direct passage of one trait over another) • Where he started: • documented particulate inheritance with garden peas (Pisumsativum).

  13. Why Peas are Awesome Genetic Models for 1860s Phenotypes Trait Seed shape Round Wrinkled • Lots of visible traits (“phenotypes”) • flower color, seed shape, pod shape, etc. • Controlled mating • Hermaphroditic • sperm-producing organs (stamens) and egg-producing organs (carpels) • Cross-pollination (fertilization between different plants) can be done intentionally Seed color Yellow Green Pod shape Constricted Inflated Pod color Green Yellow Flower color Purple White Flower and   pod position Axial (on stem) Terminal (at tip) Stem length Tall Dwarf

  14. Mendel Focused on Particulate Inheritance • True-breeding varieties • Offspring of the same variety when they self-pollinate • Hybridization • mate two contrasting, true-breeding varieties • True-breeding parents P generation • Hybrid offspring of the P generation are called the F1 generation • F1 individuals self-pollinate, the F2 generation is produced

  15. TECHNIQUE How was this Technically Done? 1 • Peas normally self-fertilize • This is a problem… • Cut the stamen • Removes male gametes • Prevents selfing • Manually add pollen • Carpels fertilized by non-self plants • Forced outcrossing 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial gener- ation offspring (F1) 5

  16. Cross-Pollination (“Forced outcrossing”) Self-pollination Female organ (receives pollen) • Control over matings • Allows observations and predictions • Great approach for genetics at large SELF- POLLINATION Male organs (produce pollen grains, which produce male gametes) Eggs CROSS- POLLINATION Cross-pollination 1. Remove male organs from one individual. 3. Transfer pollen to the female organs of the individual whose male organs have been removed. 2. Collect pollen from a different individual.

  17. Particulate Inheritance: Dominant and Recessive Traits RR or Rr • Mendel’s outcrossed plants • Seed shapes were either round or wrinkled • No “chimeric” version • NOT 50-50; round seeds were more common • Dominant trait • Round seeds • Recessive trait • Wrinkled seeds • Writing convention for alleles: R vs. r • Capital letter = dominant allele; lowercase = recessive allele • Individuals with two copies of the same allele (RR or rr) are homozygous, and those with two different alleles (Rr) are heterozygous. If Dominant gene is present, offspring WILL have the trait without exception alwaysrr

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