Principles of Genetics - Chromosomal Alterations
Chapter Overview
- Focus on chromosomal alterations in genetics.
Key Updates
- Class Schedule:
- Lecture on Chapter 10.
- Homework due Sunday.
- Office hours scheduled for Tuesday and Wednesday.
Basics of Chromosomes
Chromosome Composition:
- Comprised of DNA and proteins (histones).
- Structure includes nucleosomes and base pairs: Guanine, Cytosine, Adenine, Thymine.
Chromosome Number and Shape:
- Varies by species, with closely related species having similar chromosome counts.
- Chromosome pairs differ in size, shape, and genetic content.
Chromosome Count in Different Species (Diploid Numbers)
| Species | Diploid Chromosome Number (2n) |
|---|---|
| Carp | 104 |
| Cat | 38 |
| Chicken | 78 |
| Chimpanzee | 48 |
| Cow | 60 |
| Dog | 78 |
| Frog | 26 |
| Fruit Fly | 8 |
| Horse | 64 |
| Human | 46 |
| Mouse | 40 |
| Rat | 42 |
| Rhesus Monkey | 42 |
Chromosomal Organization in Nucleus
- Distribution: Chromosomes are not randomly located; they occupy distinct regions called chromosome territories.
- Movement: They are active within territories, moving during transcription and replication.
Karyotypes
- Definition: Visual display of chromosomes to identify number and structural abnormalities.
- Identification: 1-22 for autosomal homologs; sex chromosomes identified separately.
Cytogenetics and Molecular Techniques
- Techniques include:
- Karyotyping
- Fluorescent in Situ Hybridization (FISH)
- Chromosome banding
Chromosome Shape and Classification
Centromeres: Divide chromosomes into arms:
- p arm: Short arm
- q arm: Long arm
Types of Chromosomes:
- Metacentric: Centromere in the middle.
- Submetacentric: Centromere slightly off-center.
- Acrocentric: Centromere near one end.
- Telocentric: Centromere at the tip.
Nondisjunction
- Definition: Failure of chromosomes to separate properly during cell division.
- Consequence: Alters the chromosome number leading to aneuploidy.
- Aneuploidy Types:
- Euploid: Complete sets of chromosomes (e.g., n, 2n).
- Aneuploid: Abnormal number not euploid.
Implications of Nondisjunction in Gametes
- Can produce aneuploid gametes, leading to trisomic (2n+1) or monosomic (2n-1) offspring.
Aneuploidy in Humans
- High sensitivity to gene dosage changes; most aneuploids do not survive.
- Only autosomal trisomies (13, 18, 21) and specific sex-chromosome aneuploidies are viable.
Specific Syndromes and Frequencies
| Syndrome | Frequency at Birth | Characteristics |
|---|---|---|
| Trisomy 13 | 1 in 15,000 | Major organ abnormalities, early death |
| Trisomy 18 | 1 in 8,000 | Severe development issues, early death |
| Trisomy 21 (Down Syndrome) | 1 in 1,500 | Cognitive disabilities, characteristic facial features |
| Klinefelter Syndrome | 1 in 1,000 | Issues with fertility in males |
| Turner Syndrome | 1 in 5,000 | Short stature, infertility in females |
Age Factor in Down Syndrome Risk
- Maternal age correlates with risk:
- Higher age increases risk of trisomy 21 occurrences.
Down Syndrome Critical Region (DSCR)
- Genes on chromosome 21 linked to cognitive disabilities; highlights importance of gene dosage in development.