The Chromosomal Basis of Heredity

1
The Chromosomal Basis of Heredity

• The Cell Cycle
• Mitosis
• Meiosis and Mendels Laws
• Sex Linkage and Sex Determination
• Statistical Analysis of Genetic Data

2
Chromosome nomenclature

• Centromere - site of attachment of spindle fibers
  during mitosis and meiosis
• Chromosome - all genetic material attached to a
  single centromere
• Chromatid - one copy of a chromosome replication
  results in the production of two identical sister
  chromatids
• bivalent - homologous chromosomes paired at
  meiosis
• tetrad - the four products of meiosis

3
Summary of Mitosis

• Interphase - nondividing cell S phase occurs
  here chromosomes dispersed
• Prophase - chromosomes condense centrioles
  migrate to poles
• Metaphase - nuclear envelope dissolves
  chromosomes align on equatorial (metaphase) plate
• Anaphase - spindle fibres contract centromeres
  divide
• Telophase - nuclear envelope reforms
• Cytokinesis - cytoplasmic cell division

4
Results of Mitosis
Two identical daughter nuclei are formed,
each with a complete diploid set of chromosomes
5
Summary of Meiosis I - The Reductional Division

• Prophase I - Paired chromosomes condense
  chiasmata form
• Meiosis I - paired chromosomes align on metaphase
  plate
• Anaphase I - spindle fibres contract, separating
  paired chromosomes. Centromeres do NOT divide
• Telophase I - nuclear envelope forms, resulting
  in the production of two haploid nucleus (however
  each chromosome has two chromatids

6
Summary of Meiosis II - The Equational Division

• Similar to Mitosis, in that
• Prophase - condensation of replicated chromosomes
• Metaphase - alignment on metaphase plate
• Anaphase - division of centromeres
• Telophase - reformation of nuclear envelope

7
Results of Meiosis
Four haploid nuclei are formed, that are NOT
genetically identical
Meiosis I provides the cellular basis
for Mendels Laws
8
What Makes a Male in Humans and Flies

• Humans - presence or absence of Y chromosome
• XO - Turners syndrome Female
• XXY - Klinefelters Syndrome Male
• XYY - Criminal Chromosome? Not!!
• Drosophila - number of X chromosomes relative to
  number of sets of autosomes
• XO - Sterile Male
• XXY - Fertile Female