Deletions and Duplications

A deletion (deficiency) is the loss of a chromosomal segment, ranging from a few base pairs to many megabases spanning dozens of genes. A duplication is the presence of an extra copy of a segment, which may sit adjacent to the original (tandem duplication), be inserted elsewhere on the same chromosome, or reside on a different chromosome entirely. Both are classified as structural chromosomal mutations, distinct from point mutations (which change sequence within a gene) and from aneuploidies (which change whole-chromosome number). Deletions and duplications change gene dosage: the number of functional copies of each gene in the affected region, without necessarily altering the DNA sequence of the genes that remain. The genetic consequences follow directly from this dosage change. An organism heterozygous for a deletion has only one copy of every gene in the deleted region — it is hemizygous there, exactly as a male is hemizygous for X-linked genes. Any recessive allele on the homologous, non-deleted chromosome is now unopposed and is expressed, a phenomenon called pseudodominance. This is the single most diagnostic genetic signature of a deletion: a cross that "should" give a recessive phenotype only in 1/4 of offspring instead gives it in half, because the deletion-bearing chromosome contributes nothing to mask the recessive allele. Duplications produce the mirror-image dosage problem — an extra copy raises the amount of gene product, which for dosage-sensitive genes (those whose phenotype depends on precise stoichiometry, often transcription factors, ion channels, or components of multiprotein complexes) is itself pathogenic. Both classes of variant are visible cytologically. Homozygous deletions and duplications are frequently lethal or severely deleterious — most of the informative genetics is done with heterozygotes, where the deleted or duplicated chromosome is paired at meiosis with a structurally normal homolog. Because the two homologs differ in length or content over the affected region, the chromosome cannot align gene-for-gene along its entire length. The homolog compensates by looping out the unpaired material, producing a visible deletion (compensation) loop or duplication loop in the synaptonemal complex — first exploited cytologically in the giant polytene chromosomes of Drosophila salivary glands, whose banding pattern is fine enough to see individual missing or duplicated bands under the light microscope.

How deletions and duplications arise. The dominant mechanism is non-allelic homologous recombination (NAHR), also called unequal crossing-over. Chromosomes carry many repeated sequences — transposable elements, segmental duplications, tandem gene clusters. During meiotic prophase I, homologous chromosomes (or, on rare occasions, sister chromatids) can misalign so that non-corresponding copies of a repeat pair up instead of the true allelic copies. If crossing-over then occurs within this misaligned pairing, one recombinant chromatid gains the segment between the repeats (a duplication) and the reciprocal chromatid loses it (a deletion). Because this requires only that repeats be similar enough to mispair, chromosomal regions rich in repeats (segmental duplications, Alu elements, low-copy repeats) are recombination "hotspots" for recurrent deletions and duplications — this is why particular human microdeletion/microduplication syndromes recur at the same genomic coordinates in unrelated patients. A second route is chromosome breakage. Ionizing radiation, certain chemicals, or replication stress can cause double-strand breaks; if repair machinery (non-homologous end joining or microhomology-mediated end joining) rejoins broken ends incorrectly, or if a broken fragment lacking a centromere is lost during cell division, the result is a terminal or interstitial deletion. Breakage-fusion-bridge cycles, in which a chromosome lacking a telomere fuses with a sister, forms a dicentric bridge, and breaks again at anaphase, can generate deletions and duplications repeatedly across cell generations — a process important in cancer genome instability.

How the phenotype follows. Consider a heterozygote carrying one normal chromosome and one chromosome with an interstitial deletion spanning genes A–*B–C*. At synapsis, the normal homolog cannot find a partner for A, B, C along the deleted chromosome, so it buckles outward into a single-stranded loop while the flanking regions pair normally. If the normal homolog carries a recessive loss-of-function allele of B (call it b), the organism has only the b allele functioning in that region — the deleted chromosome contributes nothing — so the recessive phenotype is expressed despite the organism being nominally "heterozygous." This pseudodominance is the operational test for a deletion.

Dosage sensitivity. Not all genes respond the same way to copy-number change. Many genes tolerate having only one functional copy (haplosufficient) with no discernible phenotype — this is why heterozygous deletions of most single genes are silent carriers. Other genes are haploinsufficient: one functional copy does not make enough product to sustain a normal phenotype, so losing one copy — even with the other copy fully normal — causes disease. Symmetrically, some genes are dosage-sensitive to gain: an extra copy from a duplication disrupts stoichiometric balance in a pathway or complex, producing disease from triplosensitivity. Genes encoding parts of large stoichiometric assemblies (ribosomes, transcription factor complexes, myelin components) are disproportionately dosage-sensitive in both directions, which is why segmental duplications and deletions of the same chromosomal region can each cause distinct clinical syndromes.

Bridges (1917) and the discovery of pseudodominance via Bar. Calvin Bridges, working in T. H. Morgan's fly lab, studied the X-linked Bar eye mutation, which reduces the normally round Drosophila eye to a narrow bar shape. Bridges found stocks in which a recessive allele at a nearby locus began appearing in flies that should have been heterozygous — behaving, in his words, as though "uncovered." Cytological examination (aided later by salivary gland chromosome squashes, pioneered by Painter and exploited extensively by Bridges himself in the 1930s) showed that some Bar chromosomes actually carried a small physical deletion removing the neighboring gene entirely, and separately he documented that Bar itself often arose as a tandem duplication of a chromosomal band (region 16A on the X chromosome), which further duplicated (Ultrabar) to worsen the phenotype in exact proportion to the number of extra copies — an early, precise demonstration of a dosage effect: eye facet number fell in a graded, roughly stepwise manner with one, two, or three copies of the duplicated region. This was one of the first direct demonstrations that a phenotype could depend on gene dose rather than on any altered gene sequence.

Bridges' deletion mapping in Drosophila. Bridges and later Dobzhansky formalized the use of deletions to map genes. The logic: if a strain heterozygous for a deletion and a recessive allele shows the recessive phenotype (pseudodominance), the deleted region must include that gene's locus. By assembling a panel of Drosophila strains bearing different, partially overlapping X-chromosome deletions (each precisely visible as a missing band or set of bands in polytene chromosome squashes), and testing each deletion against a battery of recessive markers, researchers could localize each recessive gene to the smallest chromosomal interval common to all deletions that uncovered it — and absent from all deletions that did not. This method, requiring no molecular biology at all, placed dozens of Drosophila genes onto physical chromosome maps with band-level resolution decades before DNA sequencing existed, and its logic (deletion-mapping / deficiency-mapping) remained a standard tool in bacterial and phage genetics (e.g., Benzer's fine-structure mapping of the rII locus in bacteriophage T4) and in early human gene mapping using deletion syndromes.

McClintock and the discovery of the ring chromosome / breakage-fusion-bridge cycle. Barbara McClintock's studies of broken maize chromosomes in the 1930s–40s showed directly, cytologically, that a chromosome lacking a telomere after breakage will fuse with a sister chromatid, form a dicentric bridge at anaphase, and break again — repeatedly generating new deletions and duplications across successive cell divisions. This established chromosome breakage and faulty rejoining, not just meiotic misalignment, as a mechanistic source of structural variants, and it is the same breakage-fusion-bridge instability now recognized as a driver of copy-number changes in tumor genomes.

Human cytogenetics: Lejeune and cri-du-chat (1963). Jérôme Lejeune, who three years earlier had shown that Down syndrome is caused by trisomy 21, next showed that a distinctive human syndrome — characterized by a high-pitched, cat-like infant cry, intellectual disability, and characteristic facial features — was associated not with an extra chromosome but with a partial deletion of the short arm of chromosome 5 (5p-). This was among the first demonstrations that loss of a chromosomal segment, rather than loss or gain of a whole chromosome, produces a distinct, reproducible human clinical syndrome, directly extending Bridges' deletion logic from flies to humans and founding clinical deletion cytogenetics.

Example 1 — pseudodominance reveals a hidden deletion in Drosophila. A researcher crosses a wild-type female, heterozygous for a recessive autosomal allele cn (cinnabar eye color; genotype cn⁺/cn), to a male from a mutagenized stock whose visible phenotype is wild-type but whose chromosome structure is unknown. Ordinary Mendelian expectation for cn⁺/cn × cn⁺/cn is 3/4 wild-type : 1/4 cinnabar among F1. Instead, among 400 F1 offspring, the researcher observes:

Roughly a 1:1 ratio, not 3:1. This is the signature of pseudodominance: half the offspring inheriting the mutant father's chromosome received a chromosome that is deleted for the cn⁺ region rather than carrying a normal cn⁺ allele. Any offspring that also inherited the mother's cn allele has no functional cn⁺ anywhere in the genome (hemizygous), so cn is expressed — explaining why cinnabar appears in about half the progeny instead of a quarter. Cytological examination of the father's polytene chromosomes would be expected to show a missing band at the cn locus, confirming the deletion directly. Note the diagnostic feature: the ratio shifts from 3:1 toward 1:1, not because dominance relationships changed, but because one "allele" is physically absent.

Example 2 — deletion mapping localizes an unknown gene to a chromosome interval. A geneticist has four Drosophila X-chromosome deletion strains (Df1–Df4) with known, overlapping cytological extents across bands 10A–10F, and tests each against three uncharacterized recessive mutations (m1, m2, m3) by crossing deletion-heterozygous females to mutant males and scoring F1 males (hemizygous for the maternal X) for pseudodominant expression.

Reasoning band by band:

• m1 is uncovered only by deletions containing band 10A–10B (Df1, Df2) and not by Df3/Df4 (which lack 10A–10B) → m1 maps to 10A–10B.
• m2 is uncovered by Df2, Df3, and Df4, all of which share only band 10D in common → m2 maps to 10D.
• m3 is uncovered only by Df3, which uniquely includes bands 10E–10F beyond what Df2 and Df4 cover → m3 maps to 10E–10F.

This is exactly the logic Bridges and Dobzhansky used: the gene must lie within the intersection of all deletions that uncover the recessive phenotype, and outside any deletion that fails to uncover it. With enough overlapping deletions, resolution approaches single-band precision — comparable, in an era before sequencing, to a modern deletion panel used in bacterial or yeast fine-structure mapping.

Calvin Blackman Bridges (1889–1938) worked as an undergraduate assistant, then full research scientist, in Thomas Hunt Morgan's legendary "Fly Room" at Columbia University, alongside Alfred Sturtevant and Hermann Muller. Bridges was renowned for an almost obsessive precision at the microscope — he was the first to exploit the giant polytene chromosomes of Drosophila salivary glands for cytological mapping, matching visible chromosome bands to genetic loci with a rigor that transformed genetics from an abstract statistical science into one anchored in visible chromosome structure. His 1916 discovery of chromosomal nondisjunction had already proven the chromosome theory of heredity by showing exceptions to Mendelian ratios tracked exactly with visible chromosomal abnormalities; his subsequent work on Bar eye duplications and X-chromosome deletions extended that same logic — abnormal ratios correlate with abnormal chromosomes — to structural rearrangements, establishing deletion mapping as a rigorous genetic tool still conceptually identical to CNV (copy-number variant) mapping used in human genomics today.

Clinical cytogenetics now diagnoses dozens of human microdeletion and microduplication syndromes, many recurring at the same genomic coordinates because they arise by NAHR between flanking segmental duplications — DiGeorge syndrome (22q11.2 deletion), Williams syndrome (7q11.23 deletion), and Smith-Magenis / Potocki-Lupski syndromes (reciprocal 17p11.2 deletion and duplication of the same interval, respectively, producing distinct but related phenotypes). Charcot-Marie-Tooth disease type 1A, the most common inherited peripheral neuropathy, is caused by a duplication of PMP22; the reciprocal deletion of the same region causes a milder neuropathy (HNPP), a clean human demonstration that dosage in both directions is pathogenic for the same gene. Diagnostic laboratories detect these with array comparative genomic hybridization (array CGH) or FISH, methods that are the direct molecular descendants of Bridges' band-by-band cytology. In agriculture and plant breeding, controlled duplications and translocation stocks are used to map genes onto specific chromosome arms in crops, echoing classical Drosophila deletion mapping. In cancer biology, tumor genomes are riddled with somatic deletions (frequently inactivating tumor-suppressor genes such as TP53 or RB1 via loss of the remaining functional copy — "second hit" logic building on dosage) and duplications/amplifications (activating oncogenes such as ERBB2/HER2, whose amplification is directly targeted by trastuzumab therapy in breast cancer). In evolutionary biology, ancient gene duplications are the raw material for new gene function — the human globin gene cluster, with its tandem paralogs expressed at different developmental stages, is a textbook product of duplication followed by divergence, a process this article's mechanism directly explains and that foreshadows how still larger structural rearrangements — Inversions and Translocations — further reshape chromosome architecture and its genetic consequences.

Ask the tutor: How does deletion mapping logic extend to mapping genes using overlapping inversions or translocation breakpoints?