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The second half of the inheritance chapter moves from pea plants to people. Questions here ask who determines a baby's sex, why a disorder skips a generation, or how many chromosomes someone with a syndrome has. They are quick to answer if you can track one chromosome through a family, and easy to get wrong if you rely on memory alone. This post covers sex determination, pedigree reading, the named disorders, a worked problem and a practice routine.
What does the NEET syllabus include?
Under heredity and variation, the NEET (UG) 2026 syllabus lists sex determination in humans, birds and honey bee; sex-linked inheritance, with haemophilia and colour blindness; Mendelian disorders, with thalassemia; and chromosomal disorders: Down's syndrome, Turner's and Klinefelter's syndromes. NCERT Class 12 Chapter 4, Principles of Inheritance and Variation covers these, and adds pedigree analysis and sickle-cell anaemia and phenylketonuria as Mendelian disorders.
How is sex determined in different organisms?
NCERT describes four systems:
XO type (grasshopper): females have two X chromosomes; males have one X and no partner. Males make two kinds of sperm, with or without an X.
XY type (humans, Drosophila): females are XX and males XY. Of 23 pairs in humans, 22 are autosomes. Half the sperm carry X and half carry Y; every ovum carries X.
ZW type (birds): females are ZW and males ZZ, so here it is the female that makes two kinds of gametes.
Haplodiploid (honey bee): fertilised eggs become females (queens or workers) with 32 chromosomes; unfertilised eggs become males (drones) with 16.
XO and XY are male heterogamety; ZW is female heterogamety. In humans the sperm decides the sex of the child, and NCERT is direct that blaming women for having daughters is based on a false notion.
How do you read a pedigree?
A pedigree is a family tree showing who has a trait across generations. Squares are males, circles are females, shaded symbols are affected individuals. NCERT's Figure 4.13 shows the standard symbols. Three quick tests:
Two unaffected parents have an affected child? The trait is recessive.
An affected child always has at least one affected parent, in every generation? Suspect dominant.
Mostly males affected, passed from unaffected mothers to sons? Suspect X-linked recessive, as with haemophilia.
Which disorders should you know?
Colour blindness: X-linked recessive. NCERT gives it as about 8 per cent of males and about 0.4 per cent of females.
Haemophilia: X-linked recessive. A carrier mother can pass it to sons. NCERT notes that Queen Victoria was a carrier.
Sickle-cell anaemia: autosomal recessive. A single base change (GAG to GUG) puts valine in place of glutamic acid at the sixth position of the beta-globin chain.
Phenylketonuria: autosomal recessive. Missing the enzyme that converts phenylalanine to tyrosine.
Thalassemia: autosomal recessive. Too little of one globin chain: a quantitative problem, whereas sickle-cell is qualitative.
Down's syndrome: an extra chromosome 21 (trisomy 21), 47 chromosomes. First described by Langdon Down in 1866.
Klinefelter's syndrome: 47, XXY. Jacobs and Strong reported in Nature in 1959 a case with a likely XXY sex-chromosome make-up.
Turner's syndrome: 45, X0. Ford and colleagues described a sex-chromosome anomaly in a case of Turner's syndrome in The Lancet the same year.
NCERT explains aneuploidy as gain or loss of a chromosome from failed segregation of chromatids, and polyploidy as a whole extra set of chromosomes from failed cytokinesis, which is more common in plants.
Worked example: a colour blindness cross
Question. A woman with normal vision whose father was colour blind marries a man with normal vision. What is the chance that (a) a son is colour blind, (b) a daughter is colour blind, and (c) their first child is a colour-blind boy?
Find her genotype. Her father was XᶜY, and a daughter always gets her father's X. So she is XᴺXᶜ, a carrier.
His genotype. Normal man: XᴺY.
Cross. Her gametes: Xᴺ or Xᶜ. His: Xᴺ or Y. Children: XᴺXᴺ, XᴺXᶜ (daughters, normal vision) and XᴺY, XᶜY (sons, one normal, one colour blind).
Answers. (a) Among sons, 1 in 2 is colour blind. (b) Among daughters, 0, since every daughter gets her father's Xᴺ. (c) Among all children, 1 in 4 is a colour-blind boy.
Notice that (a) and (c) differ only in the wording: "a son" fixes the sex first, "the first child" does not. That word choice is where most marks are lost.
Common mistakes and how to avoid them
Counting chromosomes wrong. Down's and Klinefelter's give 47; Turner's gives 45.
Mixing up heterogamety. In birds the female is heterogametic, so the egg decides the chick's sex.
Saying honey bee drones have a father. Drones come from unfertilised eggs. NCERT notes they have no father, cannot have sons, but have a grandfather and can have grandsons.
Assuming dominance from frequency. A common trait can be recessive. Use the pedigree tests, not how many people are affected.
A practice routine
Write the four sex-determination systems from memory, with one example and which sex is heterogametic.
Do five X-linked crosses a day for a week: carrier mother with normal father, normal mother with affected father, and carrier mother with affected father. Draw every cross.
Draw three pedigrees of your own, one autosomal dominant, one autosomal recessive, one X-linked recessive, then swap with a friend and identify each other's.
Revise Mendelian ratios first with our post on genetics problems for NEET, and link the sickle-cell mutation to the molecular basis of inheritance.
When a cross keeps coming out wrong and you cannot see why, the Fermi Problem Library has a step-by-step solver that explains the reasoning, so you can find the exact line where your working went off.
Today, solve the colour blindness example again with the father colour blind instead of normal, and check that half the daughters are now affected.
