Distinguish chromosomal mutations (deletion, duplication, inversion, translocation)

This is WHY chromosomal mutations cause syndromes (Down syndrome, Cri-du-chat) while point mutations usually affect single proteins.

The Four Major Types

1. Deletion: Missing Chromosomal Segments

HOW it happens:

• Two breaks in the same chromosome
• DNA repair machinery fails to retrieve the middle fragment
• Broken ends ligate together

WHAT happens to genes:

• Hemizygosity: If the deletion is heterozygous, you now have only ONE copy of those genes (not two)
• Even recessive alleles on the normal homolog are now expressed
• Complete loss of gene function if both copies deleted

Why this step? The deleted region contains genes critical for brain development. With only ONE copy instead of two, haploinsufficiency occurs—not enough gene product for normal function.

Molecular detail: The CTND2 gene (catenin delta-2, involved in neural development) is in the critical region. Losing one copy drops production below the threshold needed for normal cortical development.

2. Duplication: Extra Copies of Segments

HOW it happens:

• Unequal crossing over during meiosis (misalignment of homologous chromosomes)
• DNA replication errors
• Breakage-fusion-bridge cycles

WHAT happens to genes:

• Gene dosage imbalance: Three copies instead of two (or four vs. two if both homologs affected)
• Overexpression of gene products
• Can be stepping stone to evolution (duplicated genes free to mutate)

Many cellular processes are dosage-sensitive—they require precise stoichiometric ratios of proteins. If gene A makes 100 units and normally binds gene B making 100 units, but now gene A makes 150 units, you get:

• 100 functional A-B complexes
• 50 free A proteins (may form toxic aggregates or compete with other pathways)

$\text{Functional complex} = \min(\text{protein A}, \text{protein B})$

This is why trisomies (whole chromosome duplications) are often lethal—hundreds of genes out of balance simultaneously.

Why this step? PMP22 codes for a myelin protein. THREE copies →1.5× normal protein → myelin sheath instability. The excess PMP22 overwhelms the cellular machinery for myelin assembly.

Derivation of dosage effect:

• Normal 2 PMP22 alleles → 100% myelin stability
• CMT1A: 3 PMP22 alleles → 150% protein → myelin aggregates form → demyelination
• Note: DELETION of PMP22 causes a different disease (HNPP)—proving dosage sensitivity

3. Inversion: Flipped Segments

TWO types based on centromere inclusion:

Paracentric inversion: Does NOT include the centromere (both breaks in one arm)

• Para = "beside"
• Meiotic problems: dicentric (two centromeres) and acentric (no centromere) products

Pericentric inversion: DOES include the centromere (breaks in both arms)

• Peri = "around"
• Meiotic problems: duplications and deletions in crossover products

HOW it causes problems: During meiosis, the inverted chromosome must form a loop to pair with its normal homolog. If crossing over occurs within the loop:

For paracentric:

1. Crossover in the loop
2. Products include:
   • One normal chromosome
   • One inverted chromosome dicentric bridge (stretched between poles → breaks randomly)
   • One acentric fragment (lost, no centromere to attach to spindle)

For pericentric:

1. Crossover in the loop
2. Products have duplications of one arm, deletions of the other
3. Result: unbalanced gametes → miscarriage or malformations

Clinical significance:

• Usually benign (carriers are phenotypically normal)
• Small increased risk of miscarriage if crossover occurs
• Genetic counseling recommended for carriers

Why this matters: Shows inversions are not always pathogenic—depends on size, gene content, and crossover frequency.

4. Translocation: Segments Swapped Between Chromosomes

TWO major types:

Reciprocal translocation: Two chromosomes exchange segments

• Both chromosomes break
• Segments swap positions
• No net gain/loss of genetic material in balanced carriers

Robertsonian translocation: Two acrocentric chromosomes fuse at centromeres

• Acrocentric = centromere near the end (chromosomes 13, 14, 15, 21, 22)
• Short arms lost (contain only ribosomal RNA genes—redundant)
• Two chromosomes become one

Balanced translocation carrier:

• Has all genetic material, just rearranged
• Usually phenotypically normal
• Problem: During meiosis, produces unbalanced gametes

Gamete possibilities after balanced reciprocal translocation: Let chromosomes be A, B (normal) and A-B', A'-B (translocated)

Possible gamete combinations:

1. A, B (normal) → 1/6 chance
2. A-B', A'-B (balanced translocation) → 1/6 chance
3. A, A'-B (unbalanced) → 1/6 chance
4. B, A-B' (unbalanced) → 1/6 chance
5. A, A-B' (unbalanced) → 1/6 chance
6. B, A'-B (unbalanced) → 1/6 chance

Only 2/6 gametes viable → explains high miscarriage rate in translocation carriers

Why it causes disease (Chronic Myeloid Leukemia):

Step 1: Normal ABL1 is a tyrosine kinase that phosphorylates proteins to control cell division

• Has regulatory domain that keeps it "off" when not needed

Step 2: Translocation breakpoint removes regulatory domain

• BCR-ABL1 fusion lacks the "off switch"

Step 3: Constitutive kinase activity

• Continuous proliferation signals
• Cell division without proper checkpoints
• Accumulation of white blood cells → leukemia

Step 4: Treatment insight

• Imatinib (Gleevec) specifically inhibits BCR-ABL1 kinase
• Turned CML from fatal to manageable chronic disease
• WHY? Because it's a single driver mutation

How it causes Down syndrome:

1. Parent has balanced rob(14;21) translocation

   • One chromosome with 14 + 21 material
   • Missing one normal14 and one normal 21
   • Total gene count still correct → phenotypically normal

2. During meiosis, gamete possibilities:

   • Normal 14 + normal 21 → viable
   • rob(14;21) + normal 21 → DOWN SYNDROME (three copies of chr21)
   • rob(14;21) only → monosomy 21 (lethal)
   • Normal 14 only → monosomy 21 (lethal)

Why this matters clinically:

• Recurrence risk MUCH higher than sporadic trisomy 21
• If mother carries translocation: ~10-15% recurrence
• If father carries: ~2-3% recurrence
• Requires karyotyping of parents after Down syndrome diagnosis

Comparison Table & Detection

| Type | Net DNA change | Gene order | Meiotic pairing | Clinical examples | |------|------------|-----------------|----------------| | Deletion | Loss | A-B-C-D-E → A-B-E | Deletion loop | Cri-du-chat, Wolf-Hirschhorn | | Duplication | Gain | A-B-C-D-E → A-B-C-C-D-E | Duplication loop | CMT1A, some autism cases | | Inversion | No change | A-B-C-D-E → A-D-C-B-E | Inversion loop | Chr 9 inversion (benign) | | Translocation | No change (balanced) | Chr1: A-B-C + Chr2: X-Y-Z → Chr1: A-B-Z + Chr2: X-Y-C | Quadrivalent | Philadelphia, rob(14;21) |

Detection methods:

• Karyotype (G-banding): See large changes (>5-10 Mb)
• FISH (fluorescence in situ hybridization): Target specific regions with fluorescent probes
• Microarray (aCGH): Detect copy number changes genome-wide
• Sequencing: Detect breakpoints at base-pair resolution

Why it's WRONG:

1. Position effects: Genes moved near heterochromatin may be silenced

   • Heterochromatin = tightly packed, transcriptionally inactive DNA
   • A gene moved from euchromatin to near heterochromatin loses expression

2. Breakpoint disruptions: The breaks might occur WITHIN genes

   • If a break occurs in an exon, that gene is now non-functional
   • Even intronic breaks can disrupt regulatory elements

3. Meiotic problems: Crossovers inversion loops → unbalanced gametes

   • High miscarriage rate in inversion carriers
   • Offspring with duplications/deletions

4. Gene fusion: Breakpoints can create fusion proteins

   • New protein with unpredictable function
   • Can be oncogenic if it affects growth control

The fix: Inversions can be benign IF:

• No genes broken at breakpoints
• No position effects on gene expression
• Inversion region small or rarely undergoes crossing over But they're NOT automatically harmless.

Why it's WRONG for reproductive health:

1. Unbalanced gametes: During meiosis, segregation patterns produce mostly unbalanced gametes

   • 2:1 segregation → duplications/deletions
   • Result: recurrent miscarriages, infertility

2. Adjacent-1 vs adjacent-2 vs alternate segregation: During meiosis I, the quadrivalent (four-chromosome structure) can segregate three ways:

   • Alternate (2:2 balanced): Both normal or both translocated chromosomes to same pole → OK
   • Adjacent-1: Homologs separate, but wrong combination → unbalanced
   • Adjacent-2: Sister centromeres separate → unbalanced

   Only alternate segregation (1/3 chance) produces balanced gametes

3. Position effects: Even if balanced, genes near breakpoints may have altered expression

The fix: Balanced translocation carriers need:

• Genetic counseling
• Prenatal diagnosis options
• Awareness that ~50-70% of conceptions may be lost to miscarriage

Deletion is like someone ripping out pages from the middle of book 5. Now you're missing instructions for building your left hand properly. Cri-du-chat syndrome happens when pages get ripped out of book 5.

Duplication is like when the photocopier messes up and prints pages 50-60 twice, so now they're stuck in the book twice. Your cells try to follow the instructions twice and get confused—like if a recipe said "add 1 cup sugar" but it was printed twice and you accidentally added 2 cups.

Inversion is like someone took pages100-120, flipped them upside down, and glued them back in backward. The instructions are all there, but they're in reverse order! It's like reading "bake at 350°F for 20 minutes" backward. Sometimes it causes problems (especially if you try to line it up with a normal copy to make kids), sometimes it doesn't.

Translocation is like taking Chapter 5 from Book 9 and swapping it with Chapter 3 from Book 22. Now Book 9 has Book 22's chapter and vice versa. The Philadelphia chromosome does this and the mixed-up chapter tells your white blood cells to divide constantly, causing leukemia. Scientists made a medicine (Gleevec) that specifically blocks the instruction from the mixed-up chapter!

The wild part? Even though the instructions are there, having them in the wrong order or wrong amount is enough to cause problems. Your cells are SUPER picky about following instructions exactly right.

Visual mnemonic: Draw chromosomes as BEADED STRINGS

• Deletion: string with beads missing
• Duplication: string with extra beads
• Inversion:ads rearranged backward
• Translocation: beads from two different strings swapped

Connections

• Point Mutations vs Chromosomal Mutations: Scale and impact differences
• Meiotic Nondisjunction: Another source of chromosomal abnormalities (whole chromosome errors)
• Karyotyping and FISH: Detection techniques for chromosomal mutations
• Position Effect Variegation: Why gene position matters for expression
• Gene Dosage Compensation: X-inactivation as mechanism to balance dosage
• Cancer Cytogenetics: Translocations as oncogenic drivers (BCR-ABL, MYC translocations)
• Evolutionary Role of Gene Duplication: How duplications create raw material for new genes
• Meiotic Recombination: Mechanism underlyingequal crossing over
• Prenatal Genetic Screening: Clinical applications of detecting chromosomal mutations
• Comparative Genomics: Using chromosomal rearrangements to trace evolutionary relationships

#flashcards/biology

What are the four major types of chromosomal mutations? :: Deletion (loss of segment), Duplication (extra copy of segment), Inversion (segment reversed), Translocation (segment moved to different chromosome)

Define chromosomal deletion and give an example :: Loss of a chromosomal segment containing genes; creates hemizygosity. Example: Cri-du-chat syndrome (deletion of 5p) causes intellectual disability and distinctive cry.

Explain the Philadelphia chromosome and its mechanism :: t(9;22) translocation that fuses BCR gene with ABL1 gene, creating constitutively active tyrosine kinase BCR-ABL1. Causes chronic myeloid leukemia by continuously signaling cell division. Treated with imatinib (Gleevec).

Define hemizygosity :: Having only ONE copy of a gene instead of two. Occurs with deletions or on the X chromosome in males. Means even recessive alleles are expressed because there's no second copy to mask them.

Concept Map

Hinglish (regional understanding)