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Three autosomal recessive disorders turn up again and again in Class 12 biology: sickle-cell anaemia, thalassemia and phenylketonuria (PKU). Marks are lost on detail: which codon changes, which chromosome carries which gene, and probability questions that hinge on "unaffected sibling". This post covers the molecular cause of each disorder and then works through the carrier calculations step by step.
What does the syllabus include?
The NEET (UG) 2026 syllabus lists, under heredity and variation, "Mendelian disorders in humans-Thalassemia", along with sex-linked inheritance (haemophilia and colour blindness) and chromosomal disorders. The CBSE Biology curriculum for 2026-27 uses the same wording. NCERT's Class 12 Chapter 4, Principles of Inheritance and Variation (2026-27 reprint), section 4.8.2, goes further and describes colour blindness, haemophilia, sickle-cell anaemia, phenylketonuria and thalassemia. The syllabus line names only thalassemia, but the NCERT section treats all three autosomal disorders together, so learn them as a set. For sex determination, pedigree symbols and Down's, Turner's and Klinefelter's syndromes, see our post on chromosomal disorders and pedigrees.
What is a Mendelian disorder?
NCERT defines Mendelian disorders as those "mainly determined by alteration or mutation in the single gene", passed on in the same way as Mendel's factors. Chromosomal disorders, by contrast, come from missing, extra or rearranged chromosomes. All three disorders in this post are autosomal recessive: the gene is on an autosome, and only people with two mutant copies show the disease. People with one copy are carriers.
What goes wrong in sickle-cell anaemia?
The gene has two alleles, HbA and HbS. Only HbS HbS individuals show the disease; HbA HbS carriers "appear apparently unaffected", in NCERT's words.
The change is tiny. A single base substitution at the sixth codon of the beta-globin gene, GAG to GUG, puts valine (Val) in place of glutamic acid (Glu) at the sixth position of the beta-globin chain. Under low oxygen, the mutant haemoglobin polymerises and the red cell changes from a biconcave disc to an elongated sickle shape.
The idea that one protein change could cause a disease is old. Pauling, Itano, Singer and Wells (1949) titled their paper "Sickle Cell Anemia, a Molecular Disease", and described red cells that change from the normal biconcave disk to crescent and other shapes when oxygen pressure is lowered.
How is thalassemia different?
Thalassemia is also autosomal recessive, but the problem is quantity, not quality. Mutation or deletion reduces the rate at which one globin chain is made.
α thalassemia: controlled by two closely linked genes, HBA1 and HBA2, on chromosome 16. With two copies of chromosome 16, a person has four α genes, and the disorder comes from mutation or deletion of one or more of them. The more genes affected, the less α globin is made.
β thalassemia: controlled by a single gene, HBB, on chromosome 11, and occurs from mutation of one or both copies.
NCERT sums up the contrast: thalassemia is a quantitative problem of making too few globin molecules, while sickle-cell anaemia is a qualitative problem of making a globin that works incorrectly.
Why does PKU damage the brain?
PKU is an inborn error of metabolism. The affected person lacks the enzyme that converts phenylalanine into tyrosine. Phenylalanine builds up and is converted into phenylpyruvic acid and related compounds, which accumulate in the brain and cause intellectual disability.
Because the damage comes from accumulation, early detection matters. In 1963, Guthrie and Susi described a simple method for screening large numbers of newborn babies for raised blood phenylalanine, reporting results for 682 infants.
Worked example 1: two carrier parents
Both parents are HbA HbS. What are the outcomes for each child?
Gametes from each parent: HbA or HbS, each with probability 1/2. Combining them gives HbA HbA with probability 1/4, HbA HbS with probability 1/2, and HbS HbS with probability 1/4.
So each child has a 1/4 chance of sickle-cell anaemia, 1/2 of being a carrier and 1/4 of having neither allele. The chance is the same for every pregnancy.
For three children, the chance that at least one is affected is 1 − (3/4)³ = 1 − 27/64 = 37/64.
Worked example 2: the 2/3 trap
A man has a brother with thalassemia. He and both his parents are unaffected. What is the probability that he is a carrier?
His parents must both be carriers, since they are unaffected but had an affected child. A tempting answer is 1/2. But we already know he is unaffected, so the "two mutant copies" outcome is ruled out. Of the remaining three equally likely combinations, two are carriers and one is not. The answer is 2/3.
Now suppose he marries a woman with the same family history. The chance their first child is affected is (2/3) × (2/3) × (1/4) = 4/36 = 1/9: both must be carriers, and then the child must get the mutant allele from each.
Worked example 3: carrier with non-carrier
A carrier marries someone with two normal alleles. No child can be affected, because every child gets at least one normal allele from the non-carrier parent. Half the children, on average, are carriers. This is why NCERT stresses that such disorders show up when both partners are carriers.
Which mistakes cost the most marks?
Calling thalassemia X-linked. It is autosomal; haemophilia and colour blindness are the X-linked ones.
Mixing up the chromosomes. α genes on chromosome 16; β gene (HBB) on chromosome 11.
Writing the mutation backwards. GAG becomes GUG; glutamic acid becomes valine.
Answering 1/2 when the question says "unaffected sibling". Use 2/3.
Saying PKU patients lack phenylalanine. They cannot convert it, so it accumulates.
A practice routine
Day 1: Write a three-column table from memory: disorder, gene or chromosome, molecular defect. Check against NCERT section 4.8.2.
Day 2: Ten probability questions, mixing "carrier parents", "unaffected sibling" and "at least one of n children".
Day 3: Draw one pedigree for an autosomal recessive trait and one for an X-linked trait, and explain the difference aloud.
If the link between a codon and an amino acid is still shaky, the Fermi Concept Library covers every concept in biology with its own lesson and its place in the map. Our post on solving genetics problems has more practice on crosses. If your family has a history of any of these conditions, a doctor or genetic counsellor is the right person to talk to.
Try this now: both parents are carriers of PKU. What is the chance that their two children are both unaffected?
