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.