Lecture on DNA and Down Syndrome

Overview of DNA and Chromosomes

• DNA is the blueprint for making a human, packaged into chromosomes.
• Humans have 46 chromosomes, organized into 23 pairs (one from each parent).
• Reproduction involves both parents contributing 23 chromosomes each.

Down Syndrome

Chromosomal Causes

• Occurs when one parent contributes an extra chromosome, resulting in 47 chromosomes.
• Specifically, Down syndrome involves an extra copy of chromosome 21, known as Trisomy 21.

Meiosis and Nondisjunction

• Meiosis produces sex cells (sperm and egg) each with 23 chromosomes.
• Nondisjunction:
  • Accounts for 95% of Down syndrome cases.
  • Chromosomes fail to separate properly during meiosis, leading to an extra chromosome in some cells.
  • Can occur during the first or second division stage of meiosis.

Robertsonian Translocation

• Accounts for 4% of Down syndrome cases.
• Involves the translocation of chromosome 21 to chromosome 14 or another chromosome.
• Results in hybrid chromosomes, which can lead to Trisomy 21 or other chromosomal imbalances.

Mosaic Down Syndrome

• A mix of cells with 46 and 47 chromosomes.
• Occurs due to nondisjunction during mitosis after conception.
• Results in varying percentages of affected cells based on when nondisjunction occurs.

Effects of Down Syndrome

• Affects nearly every organ system due to overexpression of genes on the extra chromosome 21.
• Common issues:
  • Heart defects (septal defects)
  • Duodenal atresia
  • Increased risk of leukemia
  • Cognitive impairments and increased risk of Alzheimer's
  • Physical characteristics (e.g., flat facial profile, epicanthal folds)

Risk Factors

• Major risk factor: maternal age.
  • Incidence increases with maternal age, from 1 in 1500 (<20 years) to 1 in 25 (>45 years).

Prenatal Screening and Diagnosis

• Screening tests include measurements of:
  • Alpha-fetoprotein (AFP)
  • Unconjugated estriol (UE3)
  • Human chorionic gonadotropin (HCG)
  • Inhibin A
• Ultrasound can assess nuchal translucency.
• Not definitive, but indicates the risk level.

Quality of Life and Support

• Improved medical care has enhanced the longevity and quality of life for those with Down syndrome.
• Many individuals lead full, thriving lives.

Supporting Research and Awareness

• Support can be provided through donations, subscriptions, and spreading information.