Overview
This lecture explains genetic disorders caused by abnormal chromosome numbers, including Down syndrome, Turner syndrome, and Klinefelter syndrome, their causes, and key features.
Genetic Disorders and Chromosome Number
- Genetic disorders can result from changes in the DNA sequence, known as mutations.
- Disorders may arise due to gene mutations, chromosome number changes, or chromosome structure changes.
- Abnormal chromosome number stems from errors during meiosis, specifically nondisjunction.
- Aneuploidy refers to having too many or too few chromosomes, often causing nonviable offspring or altered phenotypes.
- Polyploidy is the presence of more than two chromosome sets; common in plants but leads to miscarriage in humans.
- Triploid (3 sets) or tetraploid (4 sets) organisms often have larger cells and can be seen in seedless bananas and watermelons.
Examples of Chromosomal Disorders
- Down Syndrome (Trisomy 21): Caused by a third copy of chromosome 21; symptoms include characteristic facial features, broad hands, cognitive delays, and variable severity.
- Down syndrome occurs in about 1 in 800 births and is more likely with increasing maternal age, especially over 35.
- Down syndrome can also result from translocation, where a chromosome piece fuses with another.
- Turner Syndrome: Occurs in females receiving only one X chromosome; features include short stature, abnormal ovarian development, infertility, neck skin folds, kidney issues, swelling, and heart defects.
- Klinefelter Syndrome: Occurs in males with two X chromosomes and one Y (XXY); causes impaired sexual development, possible female physical traits, low testosterone, and infertility.
Key Terms & Definitions
- Genetic Disorder — Disease caused in whole or part by abnormal DNA sequence.
- Mutation — Change in DNA sequence.
- Nondisjunction — Failure of chromosomes or chromatids to separate during meiosis.
- Aneuploidy — Presence of an abnormal number of chromosomes.
- Polyploidy — More than two complete sets of chromosomes in a cell.
- Trisomy — Condition of having three copies of a chromosome (e.g., Trisomy 21).
- Translocation — A piece of one chromosome attaches to another chromosome.
Action Items / Next Steps
- Review symptoms and causes of Down, Turner, and Klinefelter syndromes.
- Understand the definitions for aneuploidy, polyploidy, and nondisjunction.