Chromosomal abnormality Study Guide

📖 Core Concepts Chromosomal abnormality – missing, extra, or irregular DNA segment; can be numerical (wrong chromosome number) or structural (altered chromosome pieces). Euploidy – cells have the correct complete set of chromosome sets (e.g., diploid). Aneuploidy – abnormal chromosome number (e.g., monosomy = missing one chromosome, trisomy = extra one). Balanced rearrangement – chromosome pieces are shuffled but total genetic material is unchanged → usually phenotypically normal. Unbalanced rearrangement – net gain or loss of genetic material → often causes disease. Constitutional (germline) abnormality – present in every cell because it arose during gamete formation or early embryogenesis. Acquired abnormality – arises later in life; not inherited. Karyotype – visual map of all chromosomes used to spot numerical or large structural changes. 📌 Must Remember Nondisjunction = most common cause of aneuploidy (failure of homologues or sister chromatids to separate). Common trisomies: Trisomy 21 → Down syndrome. Monosomy X (45,X) = Turner syndrome. Robertsonian translocation – joins long arms of two acrocentric chromosomes (13, 14, 15, 21, 22) after short‑arm loss. Inversion – segment flips orientation; reciprocal translocation exchanges segments between two chromosomes. Ring chromosome – ends break and fuse, forming a circle (may lose material). Isochromosome – two copies of the same arm, one centromere. Maternal age ↑ → higher risk of meiotic nondisjunction. Detection hierarchy: Karyotype → FISH → Spectral karyotype → Chromosomal microarray (higher resolution). 🔄 Key Processes Meiotic nondisjunction Replicated homologues fail to separate → one gamete gets two copies, the other gets none → fertilization yields trisomy or monosomy. Formation of a Robertsonian translocation Break at short arms of two acrocentric chromosomes → loss of short arms → long arms fuse → creates a single chromosome. Balanced vs unbalanced outcome Balanced: break‑rejoin preserves total DNA → carrier often phenotypically normal. Unbalanced: extra or missing segment → dosage imbalance → disease. Karyotype analysis workflow Harvest metaphase cells → stain → photograph → arrange chromosomes by size/centromere → compare to reference. 🔍 Key Comparisons Nondisjunction vs Anaphase lag Nondisjunction: whole chromosome fails to separate → full‑chromosome aneuploidy. Anaphase lag: chromosome lags behind spindle → may be lost → partial or full monosomy. Balanced translocation vs Reciprocal translocation Balanced translocation: often a single exchange that does not alter gene dosage. Reciprocal translocation: two-way exchange; still balanced if no net loss/gain. Robertsonian vs Other translocations Robertsonian: only involves long arms of acrocentric chromosomes, short arms lost. Other translocations: can involve any chromosome arms, may be balanced or unbalanced. ⚠️ Common Misunderstandings “All chromosomal abnormalities cause disease.” – Balanced rearrangements usually have no phenotype. “Trisomy always results from paternal errors.” – Most human trisomies arise from maternal meiotic nondisjunction, especially with advanced maternal age. “A normal karyotype rules out any genetic problem.” – Sub‑microscopic copy‑number variations require microarray or sequencing. 🧠 Mental Models / Intuition “Dosage = phenotype” – Think of each chromosome as a “gene dosage package.” Adding or losing a whole package (full chromosome) usually produces a clear clinical picture; swapping packages without changing total count (balanced) often leaves the organism unchanged. “Chromosome traffic jam” – During meiosis, imagine chromosomes as cars on a highway; if two cars (homologs) fail to exit at the right interchange (nondisjunction), the downstream “city” (zygote) ends up with an extra or missing passenger. 🚩 Exceptions & Edge Cases Mosaicism – Some cells carry the abnormality, others are normal → phenotype can be milder or variable. Partial trisomies/monosomies – Only a chromosome segment is duplicated or deleted; clinical severity depends on the genes involved. Gonosomal mosaicism – Abnormality present in both somatic and germ cells → can be transmitted even if the parent appears normal. 📍 When to Use Which Suspected large‑scale aneuploidy (e.g., Down syndrome) → start with karyotype (quick, whole‑genome view). Need to detect sub‑microscopic copy‑number changes → chromosomal microarray (higher resolution). Identify a specific translocation or probe for a known gene → FISH or spectral karyotyping. Prenatal diagnosis – Amniocentesis → karyotype ± microarray; CVS for earlier sampling. Pre‑implantation genetic diagnosis – biopsy blastocyst → targeted FISH or microarray. 👀 Patterns to Recognize Extra chromosome + characteristic phenotype → think trisomy (e.g., 21 → Down). Single X chromosome in phenotypic female → Turner syndrome. Balanced carrier with recurrent miscarriages → suspect a Robertsonian or reciprocal translocation. Multiple independent chromosomal gains/losses in a tumor → hallmark of chromosomal instability in cancer. 🗂️ Exam Traps “All translocations are unbalanced.” – Many (reciprocal, Robertsonian) can be balanced carriers. “Polyploidy is a common human condition.” – Polyploidy (triploid, tetraploid) is usually lethal; not a typical clinical diagnosis. “A normal karyotype excludes any genetic disease.” – Sub‑microscopic deletions/duplications require microarray; a normal karyotype alone is insufficient. “Mosaicism always results in severe disease.” – Phenotype depends on proportion of abnormal cells; many mosaics are mild or asymptomatic. --- Use this guide for a quick, high‑yield review before your genetics exam.