1. Big Idea — What a deletion is

• A deletion = a segment of DNA is missing from a chromosome.
• It is a type of chromosomal abnormality.
• Tumors frequently show deletions that remove tumor suppressor genes (these normally act as brakes on cell growth).

2. Why deletions cause cancer — the “two-hit” rule

Tumor suppressor genes require both alleles to work.

Cancer develops when both copies are lost:

• Hit 1: a point mutation damages one allele
• Hit 2: a deletion removes the second allele

This produces:

• biallelic inactivation (both alleles lost)
• → functional loss of the tumor suppressor gene
• → cells grow without control → cancer progression

Memory hook:

• One snip, one clip
• snip = mutation
• clip = deletion

3. Key named examples & loci (“addresses”)

RB gene

• Chromosome locus: 13q14
• 13 = chromosome number
• q = long arm
• 14 = band
• Classic in retinoblastoma

TP53 gene

• Chromosome locus: 17p
• 17 = chromosome number
• p = short arm
• Loss of p53 → central to many cancers
• p53 = “guardian of the genome”

4. Quick analogy

Think of two brakes on a bicycle:

• First brake cracks → mutation damages first allele
• Second brake is removed → deletion of second allele
• Now the bike can’t stop → unchecked cell proliferation

5. Compact exam-style phrasing

• Deletions = loss of chromosomal material, common in tumors.
• Often remove tumor suppressor genes, requiring two hits (mutation + deletion) for complete functional loss.
• Named loci to know:
• RB – 13q14
• TP53 – 17p
• p53 = most important tumor suppressor.

6. Optional but helpful term

Loss of heterozygosity (LOH)

• After a deletion, the remaining normal allele is lost.
• The cell is no longer heterozygous (good + bad copy).
• Now it has only the abnormal genotype → functional loss of tumor suppressor activity.

Summary Table — All Concepts Integrated

If you want next:

• MCQs based only on this note
• flashcards
• white-text answer tables
• map of all mutation types (deletion, amplification, translocation, point mutation)