How One Changed Letter in DNA Can Cause Sickle-Cell Anaemia
Classify mutations as spontaneous or induced and understand point mutations, then study Mendelian disorders such as haemophilia, sickle-cell anaemia, phenylketonuria and thalassaemia, and chromosomal disorders such as Down, Klinefelter and Turner syndromes.
What happens when genetic instructions change?
DNA is copied with remarkable accuracy, but mistakes and damage still happen. Most changes are harmless, some are useful, and a few cause disorders that run in families or arise when a whole chromosome is gained or lost.
This lesson covers the types of mutation, Mendelian genetic disorders and chromosomal disorders in humans.
This lesson covers the types of mutation, Mendelian genetic disorders and chromosomal disorders in humans.
What are the types of mutation, and what is a point mutation?
A mutation is a sudden, heritable change in DNA; it is spontaneous when it arises naturally and induced when caused by a mutagen, and mutations range from point mutations that change a single base pair to changes in large chromosome segments or whole chromosomes.
By origin:
- Spontaneous — arise naturally, for example from errors in DNA replication
- Induced — caused by mutagens such as UV radiation, X-rays and certain chemicals
By extent:
- Gene or point mutations — change one or a few base pairs within a gene
- Chromosomal aberrations — deletion, duplication, inversion or translocation of chromosome segments, often seen in cancer cells
- Numerical changes — aneuploidy, the gain or loss of chromosomes, and polyploidy, an increase in whole chromosome sets, common in plants
Point mutations:
- Substitution — one base is replaced by another, as in sickle-cell anaemia
- Frameshift — inserting or deleting a base shifts the reading frame, changing every codon after that point
Worked example. In the sickle-cell allele, the codon GAG becomes GUG, so glutamic acid at the sixth position of the beta-globin chain is replaced by valine.
An everyday example. Doctors advise shade and sunscreen in strong sunlight because UV radiation is a mutagen that can damage the DNA of skin cells.
The substance. Only mutations in reproductive cells pass to offspring — a mutation in a skin cell may cause cancer in that person but is not inherited.
By origin:
- Spontaneous — arise naturally, for example from errors in DNA replication
- Induced — caused by mutagens such as UV radiation, X-rays and certain chemicals
By extent:
- Gene or point mutations — change one or a few base pairs within a gene
- Chromosomal aberrations — deletion, duplication, inversion or translocation of chromosome segments, often seen in cancer cells
- Numerical changes — aneuploidy, the gain or loss of chromosomes, and polyploidy, an increase in whole chromosome sets, common in plants
Point mutations:
- Substitution — one base is replaced by another, as in sickle-cell anaemia
- Frameshift — inserting or deleting a base shifts the reading frame, changing every codon after that point
Worked example. In the sickle-cell allele, the codon GAG becomes GUG, so glutamic acid at the sixth position of the beta-globin chain is replaced by valine.
An everyday example. Doctors advise shade and sunscreen in strong sunlight because UV radiation is a mutagen that can damage the DNA of skin cells.
The substance. Only mutations in reproductive cells pass to offspring — a mutation in a skin cell may cause cancer in that person but is not inherited.
What are the Mendelian genetic disorders in humans?
Mendelian disorders are caused by a mutation in a single gene and are inherited according to Mendel's principles, as autosomal dominant, autosomal recessive or sex-linked traits.
Haemophilia (X-linked recessive):
- A protein in the blood-clotting chain is affected, so a small cut can bleed for a long time
- Affects males far more often; heterozygous females are carriers
Colour blindness (X-linked recessive). A defect in the red or green cone cells of the eye makes red and green hard to tell apart.
Sickle-cell anaemia (autosomal recessive):
- Occurs when both alleles are ; carriers, , are usually healthy but can pass the allele on
- Under low oxygen, the mutant haemoglobin forms long chains that bend red cells into a sickle shape
Phenylketonuria (autosomal recessive):
- The enzyme that converts phenylalanine into tyrosine is missing
- Phenylalanine and its products build up and damage the brain, causing intellectual disability
Thalassaemia (autosomal recessive):
- Too little of one globin chain of haemoglobin is made, causing severe anaemia
- Alpha thalassaemia affects alpha chains, controlled by genes on chromosome 16; beta thalassaemia affects beta chains, controlled by a gene on chromosome 11
Myotonic dystrophy is an example of an autosomal dominant disorder.
An everyday example. Thalassaemia screening camps for students and couples in several Indian states help carriers learn their status before planning a family.
The substance. Two healthy carrier parents have a one in four chance of an affected child with an autosomal recessive disorder in each pregnancy.
Haemophilia (X-linked recessive):
- A protein in the blood-clotting chain is affected, so a small cut can bleed for a long time
- Affects males far more often; heterozygous females are carriers
Colour blindness (X-linked recessive). A defect in the red or green cone cells of the eye makes red and green hard to tell apart.
Sickle-cell anaemia (autosomal recessive):
- Occurs when both alleles are ; carriers, , are usually healthy but can pass the allele on
- Under low oxygen, the mutant haemoglobin forms long chains that bend red cells into a sickle shape
Phenylketonuria (autosomal recessive):
- The enzyme that converts phenylalanine into tyrosine is missing
- Phenylalanine and its products build up and damage the brain, causing intellectual disability
Thalassaemia (autosomal recessive):
- Too little of one globin chain of haemoglobin is made, causing severe anaemia
- Alpha thalassaemia affects alpha chains, controlled by genes on chromosome 16; beta thalassaemia affects beta chains, controlled by a gene on chromosome 11
Myotonic dystrophy is an example of an autosomal dominant disorder.
An everyday example. Thalassaemia screening camps for students and couples in several Indian states help carriers learn their status before planning a family.
The substance. Two healthy carrier parents have a one in four chance of an affected child with an autosomal recessive disorder in each pregnancy.
What are chromosomal disorders, and how do Down, Klinefelter and Turner syndromes arise?
Chromosomal disorders are caused by an absence, excess or abnormal arrangement of one or more chromosomes, often because chromatids fail to separate during cell division.
Causes:
- Aneuploidy — gain or loss of a chromosome through non-disjunction during anaphase
- Polyploidy — failure of cytokinesis after chromosome duplication, increasing whole sets
Down syndrome (trisomy 21):
- An extra copy of chromosome 21, giving 47 chromosomes
- Short stature, a small round head, a furrowed tongue, a partly open mouth, a broad palm with a characteristic crease, and delayed physical and mental development
Klinefelter syndrome (47, XXY):
- An extra X chromosome in a male
- Overall masculine development with some feminine features, such as breast development, and sterility
Turner syndrome (45, X0):
- One X chromosome missing in a female
- Sterility with underdeveloped ovaries, short stature and a lack of secondary sexual characters
Diagnosis. A karyotype, a picture of a person's chromosomes arranged in pairs, reveals extra or missing chromosomes, and prenatal tests such as amniocentesis can detect them before birth.
An everyday example. Prenatal screening at government hospitals can flag a higher chance of Down syndrome, which is then confirmed by chromosome testing.
The substance. Down syndrome is not usually inherited from a parent with the condition — it mostly arises afresh from non-disjunction when an egg or sperm forms.
Causes:
- Aneuploidy — gain or loss of a chromosome through non-disjunction during anaphase
- Polyploidy — failure of cytokinesis after chromosome duplication, increasing whole sets
Down syndrome (trisomy 21):
- An extra copy of chromosome 21, giving 47 chromosomes
- Short stature, a small round head, a furrowed tongue, a partly open mouth, a broad palm with a characteristic crease, and delayed physical and mental development
Klinefelter syndrome (47, XXY):
- An extra X chromosome in a male
- Overall masculine development with some feminine features, such as breast development, and sterility
Turner syndrome (45, X0):
- One X chromosome missing in a female
- Sterility with underdeveloped ovaries, short stature and a lack of secondary sexual characters
Diagnosis. A karyotype, a picture of a person's chromosomes arranged in pairs, reveals extra or missing chromosomes, and prenatal tests such as amniocentesis can detect them before birth.
An everyday example. Prenatal screening at government hospitals can flag a higher chance of Down syndrome, which is then confirmed by chromosome testing.
The substance. Down syndrome is not usually inherited from a parent with the condition — it mostly arises afresh from non-disjunction when an egg or sperm forms.
Exam tip
What earns full marks on mutations and genetic disorders?
For every disorder, write three things: whether it is Mendelian or chromosomal, its mode of inheritance or chromosome change, and one key symptom.
- Point mutation: sickle-cell anaemia, GAG to GUG, glutamic acid to valine
- X-linked recessive: haemophilia and colour blindness
- Autosomal recessive: sickle-cell anaemia, phenylketonuria and thalassaemia
- Chromosomal: Down (trisomy 21), Klinefelter (XXY), Turner (X0)
The trap. Treating thalassaemia and sickle-cell anaemia as the same kind of defect. Thalassaemia makes too little globin, a quantitative defect; sickle-cell anaemia makes faulty globin, a qualitative defect.
- Point mutation: sickle-cell anaemia, GAG to GUG, glutamic acid to valine
- X-linked recessive: haemophilia and colour blindness
- Autosomal recessive: sickle-cell anaemia, phenylketonuria and thalassaemia
- Chromosomal: Down (trisomy 21), Klinefelter (XXY), Turner (X0)
The trap. Treating thalassaemia and sickle-cell anaemia as the same kind of defect. Thalassaemia makes too little globin, a quantitative defect; sickle-cell anaemia makes faulty globin, a qualitative defect.
Did you know
Why is the sickle-cell allele common in malaria-prone regions?
People who carry one sickle-cell allele are usually healthy, and they also resist severe malaria better than non-carriers, because the malaria parasite grows poorly in their red blood cells.
In regions where malaria has long been widespread, this advantage helped carriers survive and have children, so the allele stayed frequent — even though two copies cause a serious disease.
Parts of central India, Africa and the Middle East show this pattern, making the sickle-cell allele a classic example of natural selection balancing harm against benefit.
In regions where malaria has long been widespread, this advantage helped carriers survive and have children, so the allele stayed frequent — even though two copies cause a serious disease.
Parts of central India, Africa and the Middle East show this pattern, making the sickle-cell allele a classic example of natural selection balancing harm against benefit.
Exam relevance
How does NEET test mutations, Mendelian disorders and chromosomal disorders?
Principles of Inheritance and Variation is a recurring NEET chapter, and its disorders section is heavily fact-based.
What gets asked. The codon and amino acid change in sickle-cell anaemia, the modes of inheritance of haemophilia, colour blindness, phenylketonuria and thalassaemia, the chromosome makeup of Down, Klinefelter and Turner syndromes, and the difference between aneuploidy and polyploidy.
Question types. Mostly match-the-column and statement-based questions, with some pedigree-based questions on the mode of inheritance.
Why it matters later. Codons and point mutations return in Molecular Basis of Inheritance, and gene therapy in Biotechnology and its Applications.
The trap that costs marks. Writing Turner syndrome as XXY — Turner syndrome is 45, X0 in females, while Klinefelter syndrome is 47, XXY in males.
What gets asked. The codon and amino acid change in sickle-cell anaemia, the modes of inheritance of haemophilia, colour blindness, phenylketonuria and thalassaemia, the chromosome makeup of Down, Klinefelter and Turner syndromes, and the difference between aneuploidy and polyploidy.
Question types. Mostly match-the-column and statement-based questions, with some pedigree-based questions on the mode of inheritance.
Why it matters later. Codons and point mutations return in Molecular Basis of Inheritance, and gene therapy in Biotechnology and its Applications.
The trap that costs marks. Writing Turner syndrome as XXY — Turner syndrome is 45, X0 in females, while Klinefelter syndrome is 47, XXY in males.
Key takeaways
What must you be able to do from this lesson?
- Mutations: spontaneous and induced; point, chromosomal and numerical changes; substitution and frameshift
- Mendelian disorders: haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria and thalassaemia
- Chromosomal disorders: Down syndrome (trisomy 21), Klinefelter syndrome (XXY) and Turner syndrome (X0), caused mainly by non-disjunction
How can two healthy parents have a child with sickle-cell anaemia — and what is the chance in each pregnancy?
- Mendelian disorders: haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria and thalassaemia
- Chromosomal disorders: Down syndrome (trisomy 21), Klinefelter syndrome (XXY) and Turner syndrome (X0), caused mainly by non-disjunction
How can two healthy parents have a child with sickle-cell anaemia — and what is the chance in each pregnancy?