Unit 5: Important Concepts in Genetics II

1. Types of Chromosomal Aberrations

Chromosomal aberrations are structural or numerical alterations in chromosomes that deviate from the normal diploid genome. These alterations can lead to severe biological consequences, including phenotypic abnormalities, hereditary disorders, and evolutionary changes.

A. Structural Chromosomal Aberrations

Structural aberrations occur when chromosome breaks are followed by loss, gain, or rearrangement of genomic segments.

1. Deletion (Deficiency):

The loss of a chromosomal segment resulting in reduced total genetic content.

• Terminal Deletion: Involves a single double-stranded break resulting in the loss of an end segment.
• Interstitial Deletion: Involves two breaks within a chromosome arm, the loss of the interior segment, and the rejoining of the outer ends.

Definition: Deletion is a structural aberration in which a segment of genetic material is lost from a chromosome.

Example: Cri-du-chat syndrome in humans arises from a terminal or interstitial deletion in the short arm of chromosome 5 (5p-).

2. Duplication:

The presence of an additional chromosomal segment resulting in repeated genetic sequences.

• Tandem Duplication: Repeated segment is positioned immediately adjacent to the original in the same gene orientation.
• Reverse Duplication: Repeated segment is adjacent but oriented in reverse order.
• Displaced Duplication: Repeated segment is located elsewhere on the same chromosome or on a non-homologous chromosome.

3. Inversion:

A structural alteration in which a chromosome segment breaks, rotates by 180 degrees, and reinserts into its original position. The overall genetic content remains constant.

• Paracentric Inversion: The inverted segment does not include the centromere (confined to a single arm).
• Pericentric Inversion: The inverted segment includes the centromere (spans both arms).

4. Translocation:

The transfer of a chromosome segment to a non-homologous chromosome.

• Simple Translocation: Unidirectional transfer of a segment to another chromosome.
• Reciprocal Translocation: Mutual exchange of chromosomal segments between two non-homologous chromosomes.
• Robertsonian Translocation: Fusion of two acrocentric chromosomes near their centromeric regions, producing a single large metacentric chromosome.

B. Numerical Chromosomal Aberrations

Numerical aberrations involve changes in the total number of individual chromosomes or complete chromosomal sets relative to the normal diploid state (2n).

1. Aneuploidy:

The loss or gain of one or a few individual chromosomes from a normal set, usually caused by non-disjunction during meiotic division.

• Hypoploidy (Loss of chromosomes):
  • Monosomy (2n - 1): Loss of a single chromosome (e.g., Turner Syndrome: 45, X0).
  • Nullisomy (2n - 2): Loss of an entire pair of homologous chromosomes.
• Hyperploidy (Gain of chromosomes):
  • Trisomy (2n + 1): Addition of one extra chromosome (e.g., Down Syndrome: Trisomy 21).
  • Tetrasomy (2n + 2): Addition of a pair of homologous chromosomes.

2. Polyploidy (Euploidy):

The presence of entire additional sets of chromosomes beyond the diploid state (e.g., 3n, 4n, 6n).

• Autopolyploidy: Multiplication of chromosome sets originating from a single species.
• Allopolyploidy: Combination and doubling of chromosome sets derived from two distinct species through hybridization.

Structural Aberrations Comparison

Exam Note: Inversion and Translocation are balanced structural changes (no net loss or gain of genetic material), whereas Deletion and Duplication are unbalanced structural changes.

2. Giant Chromosomes (Polytene and Lampbrush): Structure and Functions

Giant chromosomes are specialized, significantly enlarged chromosomes found in specific cells of certain organisms, characterized by extraordinarily high levels of metabolic and transcriptional activity.

A. Polytene Chromosomes

Polytene chromosomes were discovered by E.G. Balbiani in 1881 in the salivary gland cells of Chironomus larvae and are prominent in Dipteran insects like Drosophila melanogaster.

Structure:

• Endomitosis (Endoreplication): Formed through repeated cycles of DNA replication without intervening nuclear or cell division. Thousands of identical DNA strands remain aligned side-by-side.
• Bands and Interbands: Exhibit a distinct pattern of dark bands and light interbands under a light microscope.
  • Bands (Chromomeres): Densely packed regions rich in DNA and histone proteins.
  • Interbands: Less dense regions with lower DNA concentration and higher non-histone protein levels.
• Chromosome Puffs (Balbiani Rings): Swollen, uncoiled regions of bands representing active transcription sites where DNA unwinds to synthesize RNA.

Definition: Polytene chromosomes are multi-stranded giant somatic chromosomes produced by repeated DNA replication without nuclear division (endomitosis), displaying banded organization and transcriptional puffs.

Functions:

• Drive rapid synthesis of large quantities of specific proteins and enzymes required in larval salivary tissues.
• Provide a model to visually analyze temporal and hormone-induced gene activation patterns (e.g., ecdysone-stimulated puffing).

B. Lampbrush Chromosomes

Lampbrush chromosomes were discovered by Walther Flemming in 1882 in amphibian oocytes. They are the largest known chromosomes in eukaryotic organisms.

Structure:

• Stage of Occurrence: Observed during the prolonged diplotene stage of meiotic prophase I in growing oocytes of amphibians, birds, reptiles, and lower vertebrates.
• Main Axis: Composed of a central structural backbone containing two duplicated homologous chromosomes held together by chiasmata.
• Chromomeres: Condensed DNA regions positioned sequentially along the main axis.
• Lateral Loops: Paired loops that extend laterally from the chromomeres, giving the chromosome a bottle-brush appearance. Loops consist of active DNA templates surrounded by a thick matrix of newly synthesized RNA and proteins.

Definition: Lampbrush chromosomes are giant meiotic bivalents occurring during extended diplotene prophase I, characterized by condensed central chromomeres and transcriptionally active lateral DNA loops.

Functions:

• Support massive production of mRNA, pre-rRNA, and proteins necessary for yolk formation (vitellogenesis) and early embryonic development.
• Allow high-density transcription across extended loop structures during oocyte growth.

Comparative Summary: Polytene vs Lampbrush Chromosomes

3. Genetic Disorders and Pedigree Analysis

Genetic disorders are biological conditions caused by abnormalities in gene structure, gene dosage, or chromosome configuration.

A. Classification of Genetic Disorders

1. Mendelian Disorders (Single-Gene Mutations):

Disorders caused by mutations in a single gene that follow predictable Mendelian inheritance patterns.

• Autosomal Dominant: Expressed when a single mutant allele is present on an autosome.
  • Examples: Huntington's Disease, Achondroplasia.
• Autosomal Recessive: Requires two mutated alleles (homozygous condition) for phenotypic expression.
  • Examples: Cystic Fibrosis, Sickle Cell Anemia, Phenylketonuria (PKU), Thalassemia.
• X-Linked Recessive: Mutation on the X chromosome that is expressed predominantly in males (hemizygous).
  • Examples: Hemophilia, Red-Green Color Blindness, Duchenne Muscular Dystrophy.
• X-Linked Dominant: Expressed in both males and females with a single mutated X chromosome; affected males transmit the disorder to all daughters.
  • Example: Hypophosphatemic Rickets.

2. Chromosomal Disorders:

Disorders caused by abnormal numbers or structural alterations of full chromosomes.

• Down Syndrome (Trisomy 21): Caused by three copies of chromosome 21 (47, XX,+21 or 47, XY,+21). Features include intellectual disability, characteristic facial phenotype, and a single palmar crease.
• Klinefelter Syndrome (47, XXY): Presence of an additional X chromosome in males. Features include sterility, tall stature, sparse body hair, and gynecomastia.
• Turner Syndrome (45, X0): Complete or partial absence of one X chromosome in females. Features include sterility, short stature, webbed neck, and undeveloped secondary sexual characteristics.

B. Pedigree Analysis

Pedigree analysis is a graphical representation of family history across generations using standardized symbols to trace the inheritance of specific traits or genetic conditions.

Standard Pedigree Symbols:

• Square: Male
• Circle: Female
• Filled / Shaded Shape: Affected individual
• Unshaded Shape: Unaffected individual
• Half-Filled Shape / Centered Dot: Carrier (heterozygote for recessive trait)
• Horizontal Line between Shapes: Mating / Marriage
• Double Horizontal Line: Consanguineous mating (mating between close blood relatives)
• Roman Numerals (I, II, III): Generation levels
• Arabic Numerals (1, 2, 3): Individual identification within a generation

Systematic Step-by-Step Pedigree Analysis Rules

1. Determine Dominant vs. Recessive Mode:
   • Dominant Traits: Do not skip generations. Every affected individual must have at least one affected parent. Unaffected parents cannot produce affected children.
   • Recessive Traits: Frequently skip generations. Affected individuals can be born to unaffected heterozygous carrier parents.
2. Determine Autosomal vs. Sex-Linked Mode:
   • Autosomal Traits: Affect males and females with approximately equal frequency. Transmission occurs equally from either parent to sons or daughters.
   • X-Linked Recessive Traits: Affect males far more frequently than females. An affected male receives the allele from his mother. Affected mothers pass the condition to 100% of their sons. Father-to-son transmission never occurs.
   • X-Linked Dominant Traits: Affected males transmit the trait to 100% of their daughters and 0% of their sons.
   • Y-Linked (Holandric) Traits: Passed exclusively from father to son; females are never affected.

Pedigree Diagnostic Matrix

Common Exam Mistake: Incorrectly identifying a pedigree with father-to-son transmission as X-linked. Because fathers contribute a Y chromosome to their sons, direct father-to-son transmission excludes X-linked inheritance.