What Is Aneuploidy and Non-disjunction? (NEET Genetics)
Biology · Principles of Inheritance and Variation · NEET
Aneuploidy means an individual has one extra or one missing chromosome (like 47 or 45 instead of 46). It is caused by non-disjunction, which is the failure of chromosomes or chromatids to separate properly during cell division. Memory hook: NON-disjunction = "NO separation" causes the wrong chromosome count.
Non-disjunction is the failure of chromosomes to separate in meiosis. It makes one gamete with an extra chromosome (n+1) and one missing a chromosome (n-1). After fertilisation with a normal gamete these give aneuploid zygotes: trisomy (2n+1, e.g. Down's) or monosomy (2n-1, e.g. Turner's).
Your doubts, answered
What exactly is aneuploidy in simple words?
Aneuploidy is when a cell has one or a few EXTRA or MISSING chromosomes, not a full set. A normal human has 46 chromosomes (2n). If a person has 47 (2n+1) or 45 (2n-1), that is aneuploidy. NCERT defines it as the gain or loss of a chromosome(s). It is important for NEET because Down's, Turner's and Klinefelter's syndromes are all examples.
What is non-disjunction and why does it happen?
Non-disjunction means chromosomes FAIL to separate during cell division. In meiosis, homologous chromosomes (Meiosis I) or sister chromatids (Meiosis II) are supposed to move to opposite poles. If they do not separate, one gamete gets an extra chromosome (n+1) and the other gamete is missing one (n-1). After fertilisation with a normal gamete, this gives an aneuploid zygote (2n+1 or 2n-1).
What is the difference between aneuploidy and polyploidy?
Aneuploidy = gain or loss of ONE or a FEW chromosomes (example 2n+1 = 47). Polyploidy = gain of one or more WHOLE SETS of chromosomes (example 3n or 4n). Polyploidy is caused by failure of cytokinesis (no cell-plate forms), while aneuploidy is caused by non-disjunction. NEET loves this trap, so remember: aneuploidy = single chromosome, polyploidy = whole set.
What is the difference between trisomy and monosomy?
Trisomy is when a chromosome has 3 copies instead of 2 (2n+1). Example: Down's syndrome has three copies of chromosome 21. Monosomy is when a chromosome pair has only 1 copy instead of 2 (2n-1). Example: Turner's syndrome has only one X (45, X0). Both are types of aneuploidy.
How does non-disjunction lead to Down's syndrome?
During egg or sperm formation, chromosome 21 sometimes fails to separate (non-disjunction). This makes a gamete with two copies of chromosome 21. When it joins a normal gamete carrying one copy, the child gets three copies of chromosome 21 (trisomy 21), which is Down's syndrome. This is why NEET calls Down's syndrome an autosomal primary non-disjunction disorder.
Is aneuploidy a gene mutation or a chromosomal disorder?
Aneuploidy is a CHROMOSOMAL disorder, not a single-gene (Mendelian) disorder. It involves a change in the NUMBER of chromosomes, not a change in the DNA sequence of one gene. Sickle-cell anaemia and phenylketonuria are single-gene disorders; Down's, Turner's and Klinefelter's are chromosomal (aneuploidy) disorders.
⚠️ The NEET trap ✗ Aneuploidy means the cell gains a whole extra set of chromosomes (like 3n). ✓ Aneuploidy is the gain or loss of ONE or a FEW single chromosomes (2n+1 or 2n-1). Gaining a whole set is POLYPLOIDY, caused by failure of cytokinesis, not non-disjunction. 🧠 Aneuploidy = ONE chromosome off. Polyploidy = whole SET added. In the 2016 telophase/no-cell-plate PYQ the answer was Polyploidy, NOT aneuploidy.
Real NEET questions
NEET 2017
A disease caused by an autosomal primary non-disjunction is
A · Down's syndrome ✓
B · Klinefelter's syndrome
C · Turner's syndrome
D · Sickle cell anemia
Solution: Primary non-disjunction is the failure of chromosomes to separate during meiosis, producing aneuploidy. Down's syndrome comes from non-disjunction of an AUTOSOME (chromosome 21), giving trisomy 21. Klinefelter's and Turner's involve SEX-chromosome non-disjunction, and sickle-cell anaemia is a single-gene point mutation, not non-disjunction. So the autosomal primary non-disjunction disease is Down's syndrome.
NEET 2016
A cell at telophase is seen with no cell-plate formation, so it contains more chromosomes than other dividing cells. This would result in:
A · Aneuploidy
B · Polyploidy ✓
C · Somaclonal variation
D · Polyteny
Solution: No cell-plate means cytokinesis failed, so the duplicated WHOLE chromosome set stays in one cell, giving an extra complete set. Gain of a whole set is POLYPLOIDY, not aneuploidy. Aneuploidy is gain/loss of single chromosomes caused by non-disjunction. This is the classic trap that separates aneuploidy from polyploidy.
NEET 2016
Pick out the correct statements: (i) Haemophilia is a sex-linked recessive disease (ii) Down's syndrome is due to aneuploidy (iii) Phenylketonuria is an autosomal recessive gene disorder (iv) Sickle cell anaemia is an autosomal recessive gene disorder
A · (i) and (iv) are correct
B · (ii) and (iv) are correct
C · (i), (iii) and (iv) are correct
D · (i), (ii) and (iii) are correct ✓
Solution: Haemophilia is X-linked recessive (i true). Down's syndrome is caused by aneuploidy = trisomy 21 (ii true). Phenylketonuria is autosomal recessive (iii true). Statement (iv) is also biologically true, but per the official key the intended answer is (i), (ii) and (iii). The key point for this concept: Down's syndrome IS an aneuploidy disorder.
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the chromosome number in a normal human cell?
A normal human body cell has 46 chromosomes, written as 2n = 46 (23 pairs). A gamete (egg or sperm) has 23, written as n = 23.
Give one example of aneuploidy in humans.
Down's syndrome (trisomy 21, 47 chromosomes) is the most common example. Turner's syndrome (45, X0) and Klinefelter's syndrome (47, XXY) are also aneuploidy disorders.
Can non-disjunction happen in both meiosis and mitosis?
Yes. It usually happens in meiosis (during gamete formation), affecting all cells of the offspring. It can also happen in mitosis after fertilisation, giving mosaic aneuploidy where only some body cells are affected.
Is aneuploidy always harmful?
In humans, aneuploidy usually causes serious effects because chromosome balance is disturbed. NCERT states such situations lead to very serious consequences in the individual.
