Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 15.4 Alterations of Chromosome Number or Structure Cause Some Genetic Disorders
Large-scale chromosomal changes—such as abnormal numbers or structural rearrangements—can significantly affect development and cause genetic disorders. These alterations typically arise from errors during meiosis or exposure to damaging agents like radiation.
Abnormal Chromosome Number: Aneuploidy
- Nondisjunction: chromosomes fail to separate during meiosis I or II
- Results in gametes with too many or too few chromosomes
- Aneuploidy: abnormal chromosome number in the zygote
- Monosomy (2n – 1): missing one chromosome
- Trisomy (2n + 1): one extra chromosome
- Common and often lethal—many result in miscarriage
- If the embryo survives, it may develop a disorder with a characteristic syndrome
Down Syndrome (Trisomy 21)
- Caused by three copies of chromosome 21
- Affects ~1 in 830 U.S. births
- Symptoms: facial features, heart defects, developmental delay, higher risk of leukemia and Alzheimer’s
- Most are sterile and have shorter lifespans, though many live into adulthood
- Risk increases with maternal age (especially after age 35)
- Most cases result from nondisjunction in meiosis I
Aneuploidy of Sex Chromosomes
- Often less severe than autosomal aneuploidy
- XXY (Klinefelter syndrome): male, sterile, some female characteristics
- XYY: male, taller, normal fertility
- XXX: female, taller, some learning difficulties
- X0 (Turner syndrome): female, sterile, only known viable monosomy in humans
- Extra X chromosomes typically become inactive (Barr bodies)
Polyploidy
- More than two full sets of chromosomes (e.g., 3n, 4n)
- Arises from nondisjunction of entire sets or failure of zygote division
- Common in plants, rare in animals
- Examples: bananas (3n), wheat (6n), strawberries (8n)
- Polyploids are generally more normal than aneuploids due to balanced gene dosage
Alterations of Chromosome Structure
- Caused by errors in meiosis or mutagens
- Four major types:
- Deletion: gene segment is lost
- Duplication: gene segment is repeated
- Inversion: gene segment reversed within chromosome
- Translocation: segment moves to a nonhomologous chromosome
- Deletions and duplications usually have harmful effects
- Translocations and inversions may disrupt gene expression due to position effects
Human Disorders from Structural Changes
- Cri du chat syndrome: deletion on chromosome 5
- Features: small head, intellectual disability, distinctive cat-like cry
- Usually fatal in infancy or early childhood
- Chronic Myelogenous Leukemia (CML): caused by Philadelphia chromosome, a reciprocal translocation between chromosomes 9 and 22
- Produces a fused gene → uncontrolled cell division
- Demonstrates how translocation can lead to cancer
In a Nutshell
Errors in chromosome number (aneuploidy) or structure (deletions, translocations, etc.) can cause serious genetic disorders. Some, like Down syndrome, result from extra chromosomes; others, like cancer, arise from disrupted gene regulation due to structural changes. These chromosomal abnormalities highlight the importance of correct meiotic division and genomic stability.