Chromosomal Inheritance

• Mendel's "hereditary factors" were genes
• Genes are located on chromosomes
• Locating genes along chromosomes

  Tagging isolated chromosomes with a fluorescent dye that highlights the gene
• Chromosome theory of inheritance

  • Mendelian genes have specific loci (positions) on chromosomes
  • Chromosomes undergo segregation and independent assortment

  
• Mitosis and meiosis were first described in the late 1800s
• The behavior of chromosomes during meiosis can account for Mendel's laws of segregation and independent assortment
• Fruit flies


  • They produce many offspring
  • A generation can be bred every two weeks
  • They have only four pairs of chromosomes
• Wild type


  Normal phenotypes common in the fly populations
• Mutant phenotypes


  Traits alternative to the wild type
• Correlating behavior of a gene's alleles with behavior of a chromosome pair

  1. Mating male flies with white eyes (mutant) with female flies with red eyes (wild type)

  2. F1 generation all had red eyes

  3. F2 generation showed 3:1 red:white eye ratio, but only males had white eyes
• The white-eyed mutant allele must be located on the X chromosome
• Morgan's finding supported the chromosome theory of inheritance
• Sex chromosomes


  X chromosome and Y chromosome

  
• The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features
• Methods of sex determination

  • X-Y system
  • X-0 system
  • Z-W system
  • Haplo-diploid system

  
• Sex-linked gene


  A gene located on either sex chromosome

  
• 1. linked genes


  Genes on the Y chromosome
• 1. linked genes


  Genes on the X chromosome
• Recessive X-linked trait


  Expressed in females who are homozygous and in males who are hemizygous
• 1. linked recessive disorders in humans

  • Color blindness
  • Duchenne muscular dystrophy
  • Hemophilia
• X inactivation in female mammals


  One of the two X chromosomes in each cell is randomly inactivated during embryonic development
• If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

  
• Linked genes


  Genes located on the same chromosome that tend to be inherited together
• Investigating how linkage affects inheritance

  Crossing flies that differed in traits of body color and wing size
• Body color and wing size are usually inherited together in specific combinations (parental phenotypes)
• Genes on the same chromosome do not assort independently
• Genetic recombination


  The production of offspring with combinations of traits differing from either parent
• Parental types


  Offspring with a phenotype matching one of the parental phenotypes
• Recombinant types/recombinants


  Offspring with nonparental phenotypes (new combinations of traits)
• A 50% frequency of recombination is observed for any two genes on different chromosomes
• Crossing over


  The physical connection between genes on the same chromosome is occasionally broken
• Recombinant chromosomes bring alleles together in new combinations in gametes
• Random fertilization increases the number of variant combinations that can be produced
• This abundance of genetic variation is the raw material upon which natural selection acts
• Gray body, normal wings

  • F1 dihybrid
• Black body, vestigial wings

  • Double mutant
• Replication of chromosomes


  1. Meiosis I

  2. Meiosis II
• Meiosis I and II

  Processes that produce recombinant chromosomes
• Recombinant chromosomes


  • b+ vg+
  • b vg+
  • b+ vg
  • b vg
• Eggs


  • b+ vg+
  • b vg
  • b+ vg+
  • b+ vg+
  • b+ vg+
  • b+ vg
  • b vg+
  • b vg
  • b vg
  • b vg
  • b vg
  • b vg
  • b vg
  • b vg
  • b vg
  • b vg
  • b+ vg+