Biology · Principles of Inheritance and Variation · NEET
A Mendelian disorder is caused by a mutation (change) in a SINGLE gene. The chromosome number stays normal (46 in humans). Examples: haemophilia, sickle cell anaemia, phenylketonuria, thalassemia, colour blindness. A chromosomal disorder is caused by the absence, excess, or wrong arrangement of a WHOLE chromosome or a set of chromosomes. The gene may be fine, but the chromosome count is abnormal. Examples: Down's syndrome, Turner's syndrome, Klinefelter's syndrome. Simple rule: Mendelian = gene problem; Chromosomal = chromosome number problem.
Down's syndrome is a CHROMOSOMAL disorder, not Mendelian. It happens because the person has an EXTRA copy of chromosome 21 (three copies instead of two). This is called trisomy of 21. Because a whole extra chromosome is present, it is chromosomal. It is NOT caused by a single gene mutation, so it is not Mendelian. NEET tests this a lot, so remember: Down's = extra chromosome 21 = chromosomal.
Sickle cell anaemia is caused by a single change in the beta-globin gene. Just one base pair changes, so the amino acid glutamic acid is replaced by valine at the 6th position of the beta-globin chain. Only ONE gene is affected and the chromosome number stays normal (46). Because it is a single-gene mutation, it is a MENDELIAN disorder (autosomal recessive). It is not chromosomal because no whole chromosome is added or lost.
No. This confuses many students. Haemophilia is a MENDELIAN disorder. It is caused by a mutation in a single gene that happens to sit on the X chromosome, so we call it 'sex-linked (X-linked) recessive'. Being on a sex chromosome does NOT make it chromosomal. 'Chromosomal disorder' only means the NUMBER or structure of whole chromosomes is abnormal. In haemophilia the chromosome count is normal (46); only one gene is faulty.
Aneuploidy means the gain or loss of one or a few chromosomes (not a full extra set). It happens when chromatids fail to separate properly during cell division (called non-disjunction). Down's syndrome (gain of one chromosome 21) and Turner's syndrome (loss of one X chromosome) are aneuploidy. This is different from polyploidy, which is a gain of a WHOLE extra set of chromosomes and is mostly seen in plants. Aneuploidy is the main cause of chromosomal disorders in humans.
Look at the CAUSE described in the question. If it says single gene, point mutation, base change, autosomal recessive/dominant, or X-linked recessive, it is MENDELIAN (e.g. sickle cell, haemophilia, phenylketonuria, thalassemia, colour blindness). If it says trisomy, extra chromosome, missing chromosome, non-disjunction, aneuploidy, or a karyotype like 47,XXY or 45,X0, it is CHROMOSOMAL (e.g. Down's, Klinefelter's, Turner's). The keyword tells you the answer.
A disease caused by an autosomal primary non-disjunction is
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
Statement I: Down's syndrome is caused by the absence of one of the X-chromosomes. Statement II: Turner's syndrome is caused by the presence of an additional copy of the chromosomes. Choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Mendelian disorders: haemophilia and sickle cell anaemia (also phenylketonuria, thalassemia, colour blindness). Chromosomal disorders: Down's syndrome and Turner's syndrome (also Klinefelter's syndrome).
Mendelian disorders follow inheritance patterns and can be passed to offspring, and you can trace them in a family using pedigree analysis. They may be dominant or recessive, and some are sex-linked like haemophilia. Chromosomal disorders like Down's usually arise from non-disjunction during gamete formation rather than simple gene inheritance.
Thalassemia is a MENDELIAN disorder. It is caused by a mutation in the genes controlling globin chain synthesis, leading to less production of globin molecules (a quantitative defect). Only genes are affected, not whole chromosomes.
Aneuploidy is the gain or loss of one or a few chromosomes (from failure of chromatids to separate), like Down's or Turner's. Polyploidy is the gain of a whole extra SET of chromosomes (from failure of cytokinesis) and is mostly seen in plants.
A normal human cell has 46 chromosomes, which is 23 pairs. Of these, 22 pairs are autosomes and 1 pair are sex chromosomes. In Mendelian disorders this number stays 46; in chromosomal disorders this number changes (like 47 in Down's or 45 in Turner's).
