Chromosomes, Genetic Disorders and Inheritance Flashcards

Chromosomes

  • Structures made of DNA and proteins carrying genetic information.
  • Located in the nucleus of cells.
  • Humans have 46 chromosomes arranged in 23 pairs.

Chromosome Structure

  • Key parts:
    • Centromere (center)
    • Telomeres (ends)
    • Chromatids (arms)

Somatic Cells vs. Gametes

  • Somatic Cells: 46 chromosomes (23 pairs) - Diploid.
  • Gametes (Egg/Sperm): 23 chromosomes (1 set) - Haploid.

Autosomes vs. Sex Chromosomes

  • Autosomes: First 22 pairs - control body traits.
  • Sex Chromosomes: 23rd pair - XX = Female, XY = Male.

Karyotype

  • Photograph/diagram of an individual's chromosomes arranged in pairs.
  • Used to detect chromosomal abnormalities.

Chromosomal Abnormalities

  • Numerical abnormalities (Aneuploidy, Polyploidy).
  • Structural abnormalities (Deletions, Duplications, Inversions, Translocations, Fragile sites).

Aneuploidy

  • Abnormal number of chromosomes.
    • Trisomy (3 copies): Down Syndrome (Trisomy 21), Patau Syndrome (Trisomy 13), Edwards Syndrome (Trisomy 18).
    • Monosomy (1 copy).

Polyploidy

  • More than two complete sets of chromosomes.
    • Triploidy (3 sets), Tetraploidy (4 sets) - Usually fatal.

Nondisjunction

  • Failure of chromosome pairs to separate during meiosis/mitosis.

Structural Chromosome Abnormalities

  • Deletion: Part of a chromosome is missing (e.g., Cri du Chat syndrome).
  • Duplication: Part of a chromosome is present in more than one copy.
  • Inversion: Part of a chromosome is flipped and reattached.
  • Translocation: Part of one chromosome breaks off and attaches to another.
  • Fragile Site: Regions prone to gaps/breaks (e.g., Fragile X syndrome).

Causes of Chromosomal Abnormalities

  • Environmental factors (radiation, chemicals, viruses).
  • Errors in cell division (maternal age >35).

Genetic Counseling and Testing

  • Karyotyping: Analyzes fetus's chromosomes.
  • Amniocentesis: Amniotic fluid test.
  • Chorionic villus sampling (CVS): Tissue from placenta.

Genetic Terms

  • Gene: DNA segment coding for a trait.
  • Allele: Variant of a gene.
  • Genotype: Combination of alleles.
  • Phenotype: Observable traits.
  • Homozygous: Identical alleles (e.g., BB or bb).
  • Heterozygous: Different alleles (e.g., Aa).
  • Dominant: Allele expresses its trait.
  • Recessive: Allele with hidden effect unless paired with identical allele.

Pedigree

  • Diagram to study inheritance patterns across family generations.

Types of Inheritance

  • Autosomal dominant: One mutated allele needed (50% recurrence risk).
  • Autosomal recessive: Two mutated alleles needed (25% risk if both parents are carriers).
  • X-Linked recessive: Genes on the X chromosome.

Proband

  • First individual diagnosed in a pedigree.

Penetrance and Expressivity

  • Penetrance: Percentage expressing the trait (incomplete = no phenotype).
  • Expressivity: Variations in symptoms among individuals with the same genotype.

X-Linked Inheritance

  • Gene mutation on the X chromosome.
  • Males more affected.
  • Examples: hemophilia, Duchenne muscular dystrophy.

X-Inactivation

  • One X chromosome is randomly inactivated in females.
  • Prevents too much X gene expression.
  • Inactivated X becomes a Barr body.

Key Takeaways

  • Chromosomes carry DNA.
  • Errors in number or structure cause genetic disorders.
  • Traits are inherited through dominant, recessive, or X-linked patterns.
  • Pedigrees and genetic testing help diagnose conditions.
  • X-inactivation balances gene dosage in females.