Genetic Variation and Meiosis

Factors that Give Rise to Genetic Variation

• The behavior of chromosomes during meiosis and fertilization reshuffles alleles and chromosomes every generation
• Three mechanisms contribute to chromosomal genetic variation:
  • Independent assortment of chromosomes
  • Crossing over
  • Random fertilization

Independent Assortment

• The "2n rule" states that the number of possible chromosome sorting combinations is 2^n
• For humans (n = 23), there are 2^23 = 8,388,608 possible combinations of chromosomes based on independent assortment alone
• With n=2, there are 4 possibilities and 8 possible combinations of chromosomes

Random Fertilization

• 8,388,608 possible gametes x 8,388,608 possible gametes = >70 trillion possible offspring (excluding cross over)

Human Metaphase Plate

• Cell cycle is arrested at metaphase
• Chromatids are purified, Giemsa stained, and analyzed for number and rearrangements

Cross Over

• Early in Meiosis I, a pair of homologs is held together during synapsis
• A single crossing over event leads to 4 genetically unique daughter cells

When Things Go Wrong

• Aneuploidy: any deviation from the normal number of chromosomes, usually meaning a cell nucleus possessing too many or too few chromosomes
• Somatic aneuploidy: non-heritable and has a localized effect

Genetic Disorders

• Down Syndrome:
  • Low muscle tone
  • Single deep crease across center of palm
  • Looseness of joints
  • Small skin folds at the inner corners of the eyes
  • Excessive space between first and second toe
  • Mental retardation (mild to severe)
• Trisomy 21: results in characteristic facial and body patterns, varying mental abilities
• Kleinfelter's Syndrome:
  • Male child with an extra X chromosome
  • Development of breast tissue
  • Little body hair
  • Tall stature
  • Small testes
  • Infertility
  • Evidence of mental retardation may or may not be present
• Patau Syndrome (Trisomy 13):
  • Polydactyly
  • Deformed/rocker-bottom feet
  • Neurological problems
  • Microcephaly
  • Severe mental deficiency
  • Facial defects
• Edwards' Syndrome (Trisomy 18):
  • Low birth weight
  • Prominent occiput
  • Clubbed hands
  • Neurological problems
  • Severe development delays
  • Facial defects
• Turner Syndrome:
  • Nondisjunction of the X
  • Sterile females with webbed necks, shortened torso
  • 45 chromosomes - XO not XX

Characteristics of Turner Syndrome

• Short stature
• Failure of ovaries to develop (90-95%)
• Webbed neck (25%) or short neck (40%)
• Abnormal fingernails and toenails (70%)
• Low hairline at neck (40%)
• Heart defect (30%)
• Kidney or urinary tract defect (30%)
• Hearing disorders (50-90%)
• Frequent ear infections in childhood (75%)
• Shortening of bones in the hands (35%)
• Lower jaw smaller than normal (60%)
• Drooping eyelids (ptosis), wandering eyes (strabismus)

Terms to Know

• Mitosis: Clonal division - somatic cells
• Meiosis: Reductive division-gametes
• Somatic cell: all the cells of a multicellular organism except the gamete-forming cells
• Chiasmata: the structure that forms at the crossover points after genetic material is exchanged
• Crossing over: the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents
• Recombinant: describing something composed of genetic material from two sources
• Reductive division: a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus
• Synapsis: the formation of a close association between homologous chromosomes during prophase I
• Tetrad or Bivalent: two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I