Genetics Study Guide: Chromosome Variation, Sex Determination, Extranuclear Inheritance, and DNA Replication

Sex Determination

Klinefelter Syndrome (47, XXY)

Klinefelter Syndrome is a chromosomal disorder affecting males, characterized by the presence of an extra X chromosome.

• Symptoms: Tall stature, sterile males, small testes, reduced facial/body hair, possible learning difficulties.

• Karyotype: 47,XXY (extra X chromosome in males).

• Cause: Nondisjunction during meiosis.

Turner Syndrome (45, X or X0)

Turner Syndrome is a chromosomal disorder affecting females, resulting from the absence of one X chromosome.

• Symptoms: Short stature, sterile females, underdeveloped ovaries, webbed neck, normal intelligence.

• Karyotype: 45,X (missing one X chromosome).

• Cause: Nondisjunction during meiosis.

Scientific Insight

Research has shown that the presence of the Y chromosome determines maleness, not the number of X chromosomes.

• Normal male: XY; absence of Y = female development.

Other Abnormalities

• 47,XXX: Phenotypically female; may have learning difficulties.

• 47,XYY: Taller males; sometimes mild learning or behavioral issues.

SRY / TDF Genes

The SRY (Sex-determining Region on Y) gene encodes the TDF (Testis Determining Factor), which triggers male development.

• Female with SRY: Develops male characteristics.

• Male without SRY: Develops as female (XY female).

Dosage Compensation

Dosage compensation balances gene expression between XX females and XY males.

• X inactivation: One X chromosome in females is randomly inactivated early in development.

Barr Bodies

Barr bodies are inactive, condensed X chromosomes found in the nuclei of female cells.

• Definition: Inactive, condensed X chromosome in female nuclei.

• Location: Nucleus of somatic cells.

• Count: Number of X chromosomes minus one.

Karyotype	Barr Bodies
XX	1
X0	0
XXX	2

Lyon Hypothesis

The Lyon Hypothesis states that one X chromosome in females is randomly inactivated early in development, resulting in mosaicism (different cells express different X alleles).

Variation in Chromosome Number and Structure

Key Terms

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy, monosomy).

• Monosomy (2n-1): Missing one chromosome.

• Trisomy (2n+1): One extra chromosome.

• Euploidy: Exact multiples of haploid number (e.g., 3n, 4n).

Effects

• Plants: Often tolerate monosomy/trisomy better.

• Animals: Usually lethal or causes disorders.

Examples

• Cri-du-Chat (5p-): Partial monosomy; cat-like cry, developmental delay.

• Down Syndrome: Trisomy 21.

• Edwards Syndrome: Trisomy 18.

• Patau Syndrome: Trisomy 13.

Polyploidy

• Triploid (3n), Tetraploid (4n), etc.

• Plants: Common, can increase size and hardiness.

• Animals: Rare and usually lethal.

Types of Polyploidy

• Autopolyploidy: Multiple chromosome sets from one species.

• Allopolyploidy: Combined sets from different species.

Formation

• Natural: Errors in meiosis or fertilization.

• Artificial: Colchicine prevents spindle formation, causing chromosome doubling.

Commercial Value

• Autopolyploids: Larger fruits/flowers.

• Allopolyploids: Hybrid vigor and fertility restoration (e.g., wheat).

Structural Chromosome Mutations

• Deletion: Loss of part of chromosome.

  • Terminal: End lost.

  • Intercalary: Internal loss.

• Duplication: Extra copies; gene redundancy, new traits.

• Inversion: Segment reversed.

  • Paracentric: Does not include centromere.

  • Pericentric: Includes centromere.

  • Acentric: No centromere; lost in division.

  • Dicentric: Two centromeres; chromosome breaks.

• Translocation: Exchange between chromosomes.

  • Reciprocal: Two-way swap.

  • Nonreciprocal: One-way transfer.

Familial Down Syndrome

Familial Down Syndrome is caused by Robertsonian translocation (fusion between chromosomes 21 and 14).

Extranuclear Inheritance

Organelle Heredity

Traits inherited through mitochondria or chloroplasts are examples of maternal inheritance.

• mtDNA: Contains genes for respiration and protein synthesis.

• Needs nuclear genes for full mitochondrial function.

Maternal Effect

The mother's genotype determines the offspring's phenotype due to cytoplasmic factors in the egg that direct early development.

• Example: Snail shell coiling is determined by maternal genotype.

DNA Characteristics and Replication

DNA Characteristics

DNA stores, replicates, expresses, and mutates genetic information.

Key Experiments

• Griffith: Transformation in bacteria (R → S).

• Avery: DNA is the transforming principle.

• Hershey-Chase: DNA (not protein) carries genetic info in phages.

Nucleotides

• Components: Phosphate group + deoxyribose + nitrogenous base.

• Purines: Adenine (A), Guanine (G).

• Pyrimidines: Cytosine (C), Thymine (T).

• Bond: Phosphodiester linkage (5'→3').

• Ribose vs Deoxyribose: Ribose has -OH on 2' carbon; deoxy lacks it.

Watson–Crick Model

• Right-handed double helix.

• Antiparallel strands: 5'→3', 3'→5'.

• A-T: 2 H-bonds, G-C: 3 H-bonds.

• 10 bases per turn = 34 Å, 20 Å diameter.

• Major & minor grooves present.

Chargaff’s Rule

• %A = %T and %G = %C

• $A + G = T + C$

Gel Electrophoresis

Separates DNA by size (smaller moves farther) and charge (DNA is negatively charged).

DNA Replication

Models

• Conservative: Old molecule conserved.

• Semiconservative: Each strand serves as a template (correct model).

• Dispersive: Fragments mix old and new DNA.

Meselson-Stahl Experiment

Used isotopes of nitrogen (15N/14N) to confirm semiconservative replication.

Mechanism

• 5' → 3' synthesis.

• Leading strand: Continuous.

• Lagging strand: Discontinuous (Okazaki fragments).

Key Proteins

Protein	Function
Helicase	Unwinds DNA helix
SSBP	Stabilizes single strands
Gyrase (Topoisomerase)	Relieves supercoiling
Primase	Synthesizes RNA primers
DNA Pol III	Major replication enzyme
DNA Pol I	Removes primers, fills gaps
DNA Ligase	Joins Okazaki fragments

Telomerase

Extends chromosome ends to prevent loss of DNA after replication.

Notes Section

Use this space to add diagrams (e.g., karyotypes, DNA structure, replication fork) or mnemonics for key terms.

Additional info: Some explanations and examples have been expanded for clarity and completeness.