Chapter 15

• Wild type


  Normal phenotypes that were common in the fly populations
• Mutant phenotypes


  Traits alternative to the wild type
• Morgan's finding supported the chromosome theory of inheritance
• A female fruit fly whose male parent has white eyes must have a gene for white eyes
• Short segments at the ends of the Y chromosomes are homologous with the X, allowing the two to behave like homologs during meiosis in males
• A gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo
• Sex-linked gene


  A gene that is located on either sex chromosome
• 1. linked genes


  Genes on the Y chromosome
• Only 78 genes, coding for about 25 proteins, have been identified on the human Y chromosome
• 1. linked genes


  Genes on the X chromosome
• The human X chromosome contains about 1,100 genes
• Some disorders caused by recessive alleles on the X chromosome in humans

  • Color blindness (mostly X-linked), Duchenne muscular dystrophy, Hemophilia, Male pattern baldness
• X Inactivation in Female Mammals

  In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development. The inactive X condenses into a Barr body.
• All daughters will be heterozygous and all sons will be colorblind if a colorblind woman marries a man with normal color vision
• Morgan noted that these genes do not assort independently and reasoned that they were on the same chromosome
• Recombination of Unlinked Genes: Independent Assortment of Chromosomes

  Nonparental phenotypes were also produced in the testcross. This involves genetic recombination, the production of offspring with combinations of traits differing from either parent. Offspring with a phenotype matching one of the parental (P) phenotypes are called parental types. Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants. That mechanism was the crossing over of homologous chromosomes.
• Recombinant chromosomes bring alleles together in new combinations in gametes. Random fertilization increases even further the number of variant combinations that can be produced. This abundance of genetic variation is the raw material upon which natural selection works.
• If the paternal chromosome has alleles L, M, and n and the maternal chromosome has l, m, and N, then the chromosome that cannot be produced by crossing over is lmn.
• Dihybrid testcross


  1. Wild-type F1

  2. Homozygous recessive

  3. Replication of chromosomes

  4. Meiosis I

  5. Meiosis I and II

  6. Meiosis II

  7. Recombinant chromosomes
• Testcross offspring


  • Wild type (gray normal)
  • Black-vestigial
  • Gray-vestigial
  • Black-normal
• Recombination frequency = 2,300 total offspring, Recombinant offspring × 100 = 17%
• Possible genotypes


  • b vg
  • b + vg
  • + b + vg
  • b vg +
  • b + vg +
  • + b vg
  • + b + vg
  • b vg b vg
• New Combinations of Alleles: Variation for Natural Selection

  • Recombinant chromosomes bring alleles together in new combinations in gametes
  • Random fertilization increases even further the number of variant combinations that can be produced
  • This abundance of genetic variation is the raw material upon which natural selection works
• Alterations of chromosome number or structure

  • Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
  • Plants tolerate such genetic changes better than animals do
• Nondisjunction


  1. Meiosis I

  2. Meiosis II
• Aneuploidy


  • Results from the fertilization of gametes in which nondisjunction occurred
  • A monosomic zygote has only one copy of a particular chromosome
  • A trisomic zygote has three copies of a particular chromosome
• Polyploidy


  • Triploidy (3n) is three sets of chromosomes
  • Tetraploidy (4n) is four sets of chromosomes
  • Polyploidy is common in plants, but not animals
  • Polyploids are more normal in appearance than aneuploids
• Alterations of Chromosome Structure

  • Deletion
  • Duplication
  • Inversion
  • Translocation
• Chromosome structure alterations


  • Deletion
  • Duplication
  • Inversion
  • Translocation
• Human Disorders Due to Chromosomal Alterations

  • Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond
  • These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
• Down Syndrome (Trisomy 21)


  • An aneuploid condition that results from three copies of chromosome 21
  • Affects about one out of every 830 children born in the United States
  • The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained
• Aneuploidy of Sex Chromosomes

  • Klinefelter syndrome (XXY)
  • XYY males
  • XXX females
  • Turner syndrome (X0 females)
• Cri du chat syndrome

  • Results from a specific deletion in chromosome 5
  • A child born with this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood
• Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes
• Genomic Imprinting


  • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits
  • Genomic imprinting involves the silencing of certain genes depending on which parent passes them on
• Genomic Imprinting


  • Normal-sized mouse (wild type)
  • Dwarf mouse (mutant)
• Inheritance of Organelle Genes

  • Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm
  • Extranuclear genes are inherited maternally because the zygote's cytoplasm comes from the egg
• The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant