BIO 30 2nd exam

• Independent Assortment


  The expected ratio is 1:1:1:1
• No Linkage/Independent Assortment


  50% R: 50% P
• Linkage in sweet peas


  -William Bateson, Edith Saunders, and Reginald C. Punnet

  -exception to the Principle of Independent Assortment
• complete Linkage


  When genes are so closely associated that they are always inherited together, linkage between them is complete,.
• Testcross


  Experimental type of cross with unknown genotype & homozygous recessive genotype
• Phenotypes in testcross

  • Recessive phenotype
  • Dominant phenotype
• Parentals in testcross

  • RRTT
  • rrtt
• F2 via Test Cross


  • Rr Tt
  • rrtt
• If parentals are 50% and recombinants are 50%


  Genes are on different homologous chromosomes
• If parentals are >50% and recombinants are >50%

  Genes are located on the same chromosome
• Linkage


  Linear arrangement of non-allelic genes on the same chromosome
• Complete Linkage


  • Applies to Housefly, Humans, and Rice
  • If the number of genes exceeds the number of chromosomes, there is LINKAGE
  • Genes are very close to each other and thus, are transmitted together
  • All parental types (100%); no recombinant types (0%)
• Complete Linkage


  • Parentals: prt va x pr+ vgt
  • Gametes: Prt vat, pr vg, pr vg, prt vat
• Incomplete Linkage


  • Genes are located far from each other so crossing over is possible
  • RECOMBINANT TYPES are obtained by crossing over; >50% parental and <50% recombinant types
• Incomplete Linkage


  • Parentals: prt vat x pr va
  • Gametes: prt vat, pr vg, pr vat, prt vg
• Crossing Over Events

  • Single Crossover at AB
  • Two-Strand Double Crossover
  • Three-Strand Double Crossover
  • Four-Strand Double Crossover
• Linkage Group


  • Physical association of genes on a chromosome
  • Number of linkage groups is equal to the p (haploid number) chromosome #
• Linkage in Drosophila was published by Thomas Hunt Morgan and confirmed the Chromosome Theory
• 1. linked Traits in Drosophila

  • wt (wild type, red eyes)
  • w+ (white eyes)
• Genotypes in Drosophila


  • XX-female
  • XY-male
• Alfred Sturtevant, Herman Muller, Calvin Bridges were students of Thomas Hunt Morgan and studied the phenomenon of crossing-over
• Genetic or Linkage Map


  • Linear arrangement of non-allelic genes on the chromosome
  • Distance between genes is based on % Recombination
• Steps for 3-point Test Cross

  1. Make a 3-point Test cross AaBbCc x aabbe

  2. Examine the test cross progeny

  3. Establish the Proper Gene Order
• Proper Gene Order

  • Parental types = most frequent
  • Double Cross Over (DCO) = least frequent
  • Underline the 2 genes that are always together in the parentals or DCOs, make the non-underlined gene the middle gene
• Compute for the distance between genes

  1. % crossover at Region I = (Scol + DCO) / Total Progeny x 100

  2. % Crossover at Region II = (Scol + DCO) / Total Progeny x 100
• Diploid 3-point Testcross


  • Genotypes and Frequencies
• Coefficient of Coincidence (CC)


  • Measure of strength of linkage
  • CC = ADCO / EDCO
• Interference (I)


  • I = 1 - CC
  • If I = 1, there is complete interference; Genes are close to each other
  • If I = 0, there is no interference; Genes are far from each other
• Compute for the frequency of DCO, SCOI, SCOLL, and parentals

  1. DCO = CC x Col x Coll x Total Progeny

  2. SCOL = Col x Total - DCO

  3. SCOLL = Coll x Total - DCO

  4. Parentals = Total Progeny - (DCO + SCOL + SCOLL)
• Sex Linkage


  Genes for sex determination are located on sex chromosomes
• Examples of X-linked Inheritance

  • Colorblindness
  • Hemophilia
  • Absence of central incisors
  • Congenital deafness
  • Congenital cataract
• Types of Sex Determination

  • Genetic Sex Determination
  • Environmental Sex Determination
  • Chromosomal Sex Determination
• Mendelian Concept of the gene was as a Mendelian Factor
• R.A. Fisher's viewpoints on the gene

  • Hypothetical entity
  • Chemical compound
• Chemical Composition of the Chromosome

  • Lipids
  • Proteins (Histones, Non-Histone)
  • Nucleic Acids (DNA, RNA)
• Characteristics of genes

  • Can duplicate itself with extra fidelity
  • Stable Molecular structure
  • Very low frequency of mutation
  • Mutation is duplicated faithfully
  • Can carry all necessary biological information
  • Can transmit information from generation to generation
  • Stored information must be decoded and translated into action
• Proteins were thought to be the most important molecule, but DNA was later identified as the genetic material
• Key events leading to DNA discovery

  • Friedrich Miescher isolated "nuclein" from pus cells
  • Ernst Haeckel discovered the nucleus as the most obvious cellular component
  • Edmund Wilson studied karyokinesis and identified DNA as the important nuclear element
  • Frederick Griffith's transformation experiment
  • Avery, MacLeod and McCarty identified DNA as the Transforming Principle
  • Hershey and Chase's blender experiment proved DNA as the genetic material in phages
  • Zinder and Lederberg's transduction experiment
• Key scientists involved in elucidating DNA structure

  • Francis Crick
  • James D. Watson
  • Maurice Wilkins
  • Rosalind Franklin
• Alfred Hershey, Martha Chase

  Proved that DNA was the genetic material in bacterial viruses (phages)