Chromosomal Abnormalities Summary

Chromosomal Abnormalities

Review of Basic Chromosome Knowledge

• A typical somatic human cell contains $46$ chromosomes, organized into $23$ pairs.
• $22$ pairs are autosomes, and $1$ pair are sex chromosomes.
• Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
• Each gamete (sperm or egg) contains $22$ autosomal chromosomes and $1$ sex chromosome.

Causes of Chromosomal Abnormalities

• Nondisjunction: This is the failure of homologous chromosomes to separate during anaphase I or sister chromatids during anaphase II of meiosis.
• Nondisjunction leads to aneuploidy, which is an abnormal number of chromosomes.
• Trisomy: The presence of three copies of a chromosome instead of the usual two.
• Monosomy: The presence of only one copy of a chromosome instead of the usual two.
• Most embryos with an incorrect number of chromosomes do not survive and cease development early on.
• Nondisjunction occurs more frequently during egg production than sperm production.

Examples of Chromosomal Abnormalities

Missing or Extra Sex Chromosomes

• Klinefelter Syndrome
  • Karyotype: XXY
  • Affected individuals are sterile males.
  • They do not produce testosterone from the testes.
  • Lack secondary male sexual characteristics.
• Turner Syndrome
  • Karyotype: XO
  • This is the only viable monosomy in humans, resulting in individuals with $45$ chromosomes.
  • Affected individuals are sterile females.
  • They lack secondary female sexual characteristics.
  • Physical characteristics include short stature, a broad chest, and a "webbed" neck.
• Trisomy X (Meta-female)
  • Karyotype: XXX
  • Individuals typically do not have unique physical attributes.
  • They may experience difficulty getting pregnant.
  • It can cause developmental delays and learning disabilities.
• Jacobs Syndrome
  • Karyotype: XYY
  • Affected boys are often very tall and have chronic acne.
  • It can cause learning disabilities and behavioral problems.

Missing or Extra Autosomes

• Down Syndrome
  • Also known as Trisomy 21.
  • Always associated with some degree of mental impairment, which varies greatly among individuals.
  • Unique facial features include an eyelid fold, large tongue, rounded head, and short neck.
  • Individuals may have congenital heart problems.
  • Prevalence in women over 40: $1/80$
• Edwards Syndrome
  • Also known as Trisomy 18.
  • Infants usually die in infancy.
  • Severe problems with organs.
  • Physical characteristics include crossed fingers, low-set ears, and a small head.
  • Severe intellectual disabilities.
• Patau Syndrome
  • Also known as Trisomy 13.
  • Infants usually die in infancy.
  • Problems with the heart, eyes, brain, and spinal cord.
  • Severe intellectual disabilities.
  • Cleft lip and/or palate are common.

Missing or Extra Pieces of Chromosomes

• Cri du Chat Syndrome
  • Caused by the deletion of part of chromosome 5.
  • Infants have a narrow trachea, resulting in a characteristic "mewing" cry.
  • Severe intellectual inability.
  • Physical characteristics include a rounded head, small chin, wide-set eyes, and a fold of skin on the upper eyelids.
• Translocation Down Syndrome
  • Accounts for 5% of Down syndrome cases.
  • Involves the translocation of chromosome 21 to chromosome 14.
  • It is an inherited disorder.
• Williams Syndrome
  • Caused by the deletion of part of chromosome 7.
  • Poor academic skills.
  • Cardiovascular problems and premature aging of the skin.
  • Unique facial features including a turned-up nose, wide mouth, small chin, and large ears.
  • Individuals often have excellent verbal and musical abilities.