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)

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

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.