Chromosomes

  • Morgan and D. melanogaster
    • Chromosome theory of inheritance
    • Genes have loci on chromosomes, where chromosomes segregate through independent assortment.
    • Chromosomes: location of Mendel’s heritable factors
    • Fruits flies are convenient organisms used in studies: can produce 100’s offspring, eggs every 2 weeks, 4 pairs chromosomes, 3 pairs autosomes, 1 pair sex chromosomes
    • Wild type: phenotype most in population (red eyes: W+)
    • Mutant type: alternate to wild type (white eyes: w)
  • Correlating Allelic/Chromosome behavior
    • White eyed F x red eyed M
    • Expected to be all red but actually 50% red-eyed F and 50% white-eyed M
    • Sex-linked inheritance pattern (either sex chromosome)
  • Sex chromosomes
    • Autosomes: 22 pairs in humans
    • Homomorphic: same shape
    • Sex chromosomes: 1 pair
    • Heteromorphic: not morphologically similar, meiosis -> some regions synapse
    • Humans: 2 sex chromosomes X and Y
    • Males are heterogametic; XY genotype, half of sperm get X and other get Y
    • Females are homogametic; XX genotype, all eggs with X
    • Alternate systems of sex determinism like IW in reptiles
    • Haplodiploidy: not always chromosomal
  • X-linked genes
    • Sex-linked genes: either sex chromosome
    • Y-linked genes: on Y(few) most are related to sex determination(sex)
    • Only passed down male family members
    • X-linked genes: on X, are many genes as in autosome, not all determine sex
    • X-linked recessive disorders: more common in M than F
    • Males are hemizygous (carry only 1 X)
    • Alleles on X expressed, recessive alleles can’t be masked
    • Females are homozygous (2 X)
    • Recessive alleles can be masked by other X
  • X-inactivation
    • Females deal with 2 X chromosomes, are functionally hemizygous
    • Barr body: 1 X randomly inactivated during embryonic development
    • Inactive X condenses: inactivation involves DNA methylation(C-H)/histone proteins
    • Color in cats X-linked
    • If females are heterozygous (X^A X^a)
    • Some cells X^A is active, others have active X^a
  • Violations of independent assortment
    • Linked genes
    • Close genes tend to be inherited together
    • Linkage: genes inherited together
      • 100% daughters have same alleles/gametes as parents, no recombination (if parents had DE and de gametes, so will daughters)
    • Genes are on same chromosome close together, violate law of independent assortment
    • If unlinked: 50% offspring like parents and 50% recombinants
    • Recombinant mechanisms
    • Parental type: gene combo parents have (AB/ab)
    • If no crossing over, 100% daughter same genes as parent
    • Genes recombined during prophase I
    • Parental type AB and ab
      • Recombination: form of gen. Combinations not seen in parents
      • 25% AB, 25% ab, 25% aB, 25% Ab
      • 50% like parents, 50% recombinants