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Chapter 14

Chapter 14. Mendel and the Gene Idea. Gregor Mendel. Monk Pea Plants  many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants. P Generation (true-breeding parents). . Purple flowers. White flowers. F 1 Generation

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Chapter 14

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  1. Chapter 14 Mendel and the Gene Idea

  2. Gregor Mendel Monk Pea Plants  many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants

  3. P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation Experiments • P generation = true breeding parents • Purple x White • F1 generation = hybrids from P generation cross • All purple plants • F2 generation = offspring from F1 self-pollination • 3:1 ratio of purple to white plants

  4. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Figure 14.4 Mendel’s Conclusions • Alternate versions of genes • ALLELES • Found on Homologous Chromosomes • Represented with letters

  5. Each trait is controlled by 2 alleles – one contributed from each parent

  6. The Law of Segregation • The 2 alleles for a trait segregate during gamete formation (meiosis) and end up in different gametes

  7. Mendel’s Other Law: • The Law of Independent Assortment • Genes located on different chromosomes are inherited independently • Exceptions: genes far away on same chromosome (many map units apart) allowing crossing over to occur can be inherited independently • * Genes located close together and are usually inherited together are called “linked” ex: red hair, freckles

  8. Some alleles are dominant while others are recessive • Dominant alleles “mask” recessive • Dominant alleles = capital letters • Recessive alleles = lowercase letters

  9. Vocab • Homozygous = 2 of the same alleles • Homozygous Dominant = DD • Shows dominant trait • Homozygous Recessive = dd • Shows recessive trait • Heterozygous = 1 dominant, 1 recessive allele • Heterozygous = Dd • Shows dominant trait and masks recessive • Genotype = allele make up (ex: DD) • Phenotype = physical trait

  10. Determine Probabilities • Punnett Squares • Aka Monohybrid Cross  1 trait • Explains Law of Segregation

  11. Testcross • Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype • Is the organism DD or Dd?! • Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

  12. Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. If PP, then all offspring purple: If Pp, then 1⁄2 offspring purple and 1⁄2 offspring white: p p p p RESULTS P P Pp Pp Pp Pp P p Pp pp Pp pp • The testcross Figure 14.7

  13. Dihybrid Cross Typical Outcome for 2 heterozygous individuals: 9:3:3:1 • Shows inheritance of two traits • Ex: seed color AND seed shape

  14. Types of Dominance • Complete Dominance • Phenotype of Dd is the same as DD • Codominance • Shows 2 dominant traits • Ex: BB = black, WW = white, BW = black AND white • Incomplete Dominance • Shows blended pattern • Ex: CRCR = Red, CwCw = White, CRCW = Pink

  15. Codominance vs. Incomplete Dominance

  16. Table 14.2 Multiple Alleles • Most traits are controlled by 2 alleles (ex: DD) • Some traits have more than 2 alleles • ABO Blood Type • IA, IB, i are the 3 alelles

  17. Pleiotropy • 1 gene has multiple phenotypic effects • Ex: Sickle Cell Anemia

  18. AaBbCc AaBbCc aabbcc Aabbcc AABBCc AABBCC AaBbcc AaBbCc AABbCc 20⁄64 15⁄64 Fraction of progeny 6⁄64 1⁄64 Multiple Genes (Polygenic) Ex: Hair, Skin, Eye Color Additive Effects

  19. Epistasis Outcome differs from 9:3:3:1 • A gene at one locus alters the phenotypic expression of a gene at a second locus (masks the other gene) • Ex: Laborador Retrievers • 2 genes: (E,e) & (B,b) • pigment (E) or no pigment (e) • pigment concentration: black (B) to brown(b)

  20. Multifactorial Traits Controlled by genetics and environment Ex: Skin color in humans, hydrangea color Nature vs. Nurture

  21. Pedigrees Circles = Females Squares = Males Shaded = Expresses phenotype Half-Shaded = Carriers Parents joined by horizontal lines Offspring listed below parents in birth order

  22. Human Genetic Disorders • Most are recessive (require homozygous recessive genotype) • Carriers = heterozygous individuals that do not show trait, but “carry” the gene for it (Ex: Aa) • Cystic Fibrosis: defective chloride channel in cell membranes lead to build up of thickened mucus • Tay-Sach’s disease: brain cells cannot metabolize lipids • Sickle-Cell Anemia: misshaped blood cells

  23. Human Genetic Disorders • Huntington’s Disease: Dominant, degenerative disease of nervous system • Achondroplasia: Dominant form of dwarfism (“AA” is lethal) • Many disorders are multifactorial • Heart disease • Cancer • Diabetes

  24. Carrier Recognition • Fetal Testing • Amniocentesis: withdrawl of fluid from amniotic sac to collect cells for viewing and karyotyping • Chorionic villus sampling (CVS): small amount of tissue suctioned from placenta • Ultrasound: non-invasive, reveals structures • Newborn Screening • Blood: test for PKU (Phenylketonuria) • Recessive disorder that leads to mental retardation • Requires a change in diet

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