Karyotypes & Chromosomal Disorders Study Notes

Karyotypes & Chromosomal Disorders

Human Karyotype

  • Definition: A photograph of one’s chromosomes, grouped in pairs of homologous chromosomes by size.
  • Composition:   - Humans have a total of 46 chromosomes: 1 pair of sex chromosomes (XX or XY) and 22 pairs of autosomes (chromosomes that are not sex chromosomes).

Chromosome Abnormalities

  • Origin of Abnormalities: All chromosomal abnormalities stem from an error during meiosis, known as nondisjunction.
  • Definition of Nondisjunction: The failure of homologous chromosomes to separate properly during meiosis, leading to gametes with abnormal chromosome numbers.
Nondisjunction of Chromosomes
  • Outcome of Nondisjunction:   - If nondisjunction occurs:     - One gamete receives both chromosomes of a pair, while the other receives none, potentially leading to chromosomal disorders in offspring.

Analyzing Karyotypes

  1. Chromosome Count: Are there 46 chromosomes?
  2. Chromosome Pair Identity: Are there 2 identical chromosomes in each pair of autosomes and 2 sex chromosomes?
  3. Structural Changes: Are there any rearrangements between chromosomes or large, obvious deletions?

Limitations of Karyotypes

Karyotypes do not provide information about:

  1. Individual DNA strands or genes or the DNA sequence.
  2. The number of genes contained within a chromosome.
  3. The presence of specific gene mutations.

Chromosome Disorders Caused by Nondisjunction

  1. Down Syndrome (Trisomy 21)
  2. Edwards Syndrome (Trisomy 18)
  3. Klinefelter's Syndrome (Trisomy XXY)
  4. XYY Syndrome (Trisomy XYY)
  5. XXX Syndrome (Trisomy X)
  6. Turner Syndrome (Monosomy XO)
  • Note: Nearly all other nondisjunctions result in the death of the embryo or child before reaching adulthood.

Down Syndrome (Trisomy 21)

  • Definition: An extra copy of chromosome 21.
  • Chromosome Count: 47 chromosomes.
  • Survival: One of the few trisomy disorders where the individual has a high probability of living to adulthood.
  • Symptoms:   - Short stature.   - Mental retardation.   - Reduced life span.   - Characteristic physical features.

Klinefelter Syndrome (XXY)

  • Definition: Trisomy involving the sex chromosomes; males with an extra X chromosome.
  • Chromosome Count: 47 chromosomes.
  • Characteristics:   - Typically sterile.   - Low testosterone levels.   - Taller than average height.   - Many individuals remain undiagnosed unless tested.

XYY Syndrome

  • Definition: Males with an extra Y chromosome.
  • Chromosome Count: 47 chromosomes.
  • Characteristics:   - Taller than average height.   - Normal development often goes unnoticed unless testing is performed.   - Possible sterility issues.

XXX Syndrome

  • Definition: Females with an extra X chromosome.
  • Chromosome Count: 47 chromosomes.
  • Characteristics:   - Taller than average height.   - Generally develop normally and may be undiagnosed unless tested.   - Possible sterility issues.

Turner Syndrome (XO)

  • Definition: Females with only one X chromosome (missing a second sex chromosome).
  • Chromosome Count: 45 chromosomes.
  • Unique Quality: This is the only monosomy in which the embryo survives; other missing chromosomes typically result in embryo/fetus loss.
  • Characteristics:   - Sterile.   - Shorter than average height.   - Various characteristic symptoms indicative of the syndrome.