NCERT grounding
The NCERT Class 12 chapter Principles of Inheritance and Variation introduces pedigree analysis as the opening idea of its section on genetic disorders. The text observes that the practice of analysing inheritance patterns in humans began after the rediscovery of Mendel's work, and states the central constraint plainly: the controlled crosses that can be performed on a pea plant or some other organism are not possible in human beings. Studying the family history of a particular trait therefore provides the alternative.
NCERT defines the technique directly — an analysis of traits across several generations of a family is called the pedigree analysis, in which the inheritance of a particular trait is represented in a family tree over generations. The chapter further notes that pedigree study provides a strong tool in human genetics, used to trace the inheritance of a specific trait, abnormality, or disease, and that by pedigree analysis one can easily understand whether the trait in question is dominant or recessive, and whether it is linked to a sex chromosome.
"In human genetics, pedigree study provides a strong tool, which is utilised to trace the inheritance of a specific trait, abnormality or disease." — NCERT Class 12 Biology, Chapter 4
The NIOS supplement reinforces the same logic from the disorder side, describing how X-linked recessive traits such as haemophilia and colour blindness move from an affected father to his carrier daughters and then to her sons — the criss-cross pattern that a pedigree makes visible. Both sources together give us the two halves of this topic: the standardised drawing conventions, and the deduction rules that turn a drawing into a diagnosis.
Reading a pedigree: symbols and inheritance rules
A pedigree is not a casual sketch. It is a standardised diagram, and reading it for NEET means doing two things in sequence — first decoding the symbols correctly, then applying a fixed set of logical rules to deduce the mode of inheritance. This section walks through both, in the order an examiner expects you to think.
The standard pedigree symbols
Every pedigree is built from a small, fixed vocabulary of symbols. NCERT presents these standard symbols in Figure 4.13 of the chapter, and a NEET question can be set on any one of them. The two foundational shapes encode sex: a square represents a male and a circle represents a female. A diamond is used when the sex is unknown or unspecified. Shading then encodes the trait — a filled (shaded) symbol is an affected individual, while an unfilled symbol is unaffected.
The connecting lines carry just as much information. A horizontal mating line joins two partners; when that line is doubled, it indicates a consanguineous mating — a marriage between relatives. A vertical line drops from the mating line to the children, and a horizontal sibship line connects all the offspring of one couple, drawn left to right usually in birth order. A small dot inside a symbol, or a half-filled symbol, marks a carrier — a heterozygous individual who does not show the trait but can transmit it. An arrow points to the proband (also called the propositus or index case), the person through whom the family came to study.
Figure 1. The standard symbol vocabulary of a pedigree: shapes encode sex, shading encodes the trait, and the connecting lines encode matings and offspring relationships.
Labelling convention: generations are numbered with Roman numerals (I, II, III...) from oldest at the top, and individuals within a generation with Arabic numerals (1, 2, 3...) from left to right — so "II-3" names a specific person.
Shapes & shading
Square = male; circle = female; diamond = unknown sex.
Filled symbol = affected; unfilled = unaffected.
Connecting lines
Single horizontal line = mating; double line = consanguineous mating.
Vertical + sibship line = children of a couple.
Special markers
Dot inside a symbol = carrier (heterozygous, unaffected).
Arrow = proband / propositus, the index case.
Deducing the mode of inheritance
Once the chart is decoded, the real work begins: deciding which of four standard patterns the trait follows — autosomal dominant, autosomal recessive, X-linked recessive, or X-linked dominant. Each pattern leaves a fingerprint in the chart. The strategy is to scan the pedigree for three diagnostic questions and let the answers narrow the field.
Three questions to interrogate any pedigree
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Step 1
Dominant or recessive?
If the trait skips generations, suspect recessive. If it appears in every generation, suspect dominant.
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Step 2
Autosomal or X-linked?
Roughly equal sexes affected points to autosomal. A strong male excess points to X-linked.
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Step 3
Confirm with a key cross
Test the candidate pattern against an affected-parent-to-child link that would rule it out.
Autosomal dominant. A dominant allele is expressed even in a single (heterozygous) copy, so an affected individual will almost always have an affected parent. The fingerprint: the trait appears in every generation and does not skip, every affected child has at least one affected parent, and males and females are affected in roughly equal numbers. Two unaffected parents cannot produce an affected child. NCERT cites myotonic dystrophy as the representative autosomal dominant disorder in its pedigree figure.
Autosomal recessive. A recessive allele is masked in the heterozygote, so the trait can lie hidden in unaffected carriers for generations. The fingerprint: the trait frequently skips generations, an affected child can be born to two unaffected (carrier) parents, and the appearance of the trait is often associated with consanguineous matings, because related partners are more likely to share the same rare recessive allele. Again, both sexes are affected about equally. NCERT names sickle-cell anaemia as its autosomal recessive example.
X-linked recessive. Here the gene sits on the X chromosome and the allele is recessive. A male, having only one X, is affected by a single copy — he is said to be hemizygous — whereas a female needs the allele on both X chromosomes to be affected. The fingerprint is therefore a striking male excess. The trait shows criss-cross inheritance: an affected father transmits the allele to all his daughters, who become unaffected carriers, and each carrier daughter passes it to half her sons. An affected son almost always traces back to a carrier mother. NCERT notes this for haemophilia — an X-linked recessive trait shows transmission from carrier female to male progeny — and famously cites the pedigree of Queen Victoria's haemophilic descendants.
Autosomal recessive
≈ 1 : 1
Affected males : affected females
- Skips generations; carriers hide the allele
- Both sexes affected in equal numbers
- Affected child can come from two unaffected parents
- Consanguineous matings raise the chance
- NCERT example: sickle-cell anaemia
X-linked recessive
Male >> Female
Strong male excess among affected
- Criss-cross inheritance through carrier females
- Affected father → all daughters become carriers
- Affected son → carrier (or affected) mother
- Affected female is rare; needs both X alleles defective
- NCERT example: haemophilia, colour blindness
X-linked dominant. This fourth pattern is less commonly drawn in NEET papers but completes the framework. The gene is on the X chromosome and the allele is dominant, so it is expressed in any individual carrying even one copy. The trait appears in every generation, like an autosomal dominant trait, but with a tell-tale asymmetry at one type of mating: an affected father transmits the trait to all his daughters and none of his sons, because a father gives his X only to daughters and his Y to sons. An affected heterozygous mother passes it to roughly half her children of either sex. Affected females tend to outnumber affected males, the mirror image of the X-linked recessive bias.
Figure 2. A worked pedigree. The affected proband III-2 is born to two unaffected carrier parents (II-2 and II-3), the trait skips generation II, and both parents are themselves children of an affected grandparent — the signature of an autosomal recessive trait.
One subtlety that examiners exploit: a pedigree rarely proves a single pattern beyond doubt — it makes some patterns possible and rules others out. The disciplined approach is elimination. If an affected child is born to two unaffected parents, the trait cannot be dominant; it must be recessive. If an affected father has an affected son, the trait cannot be X-linked recessive carried purely on the X (a father gives his son only a Y). Each link you read either kills a hypothesis or survives it, and the surviving hypothesis is your answer.
Why the male excess is so visible
NCERT records that red-green colour blindness, an X-linked recessive trait, occurs in about 8 per cent of males but only about 0.4 per cent of females — a twenty-fold gap that makes the male bias in such pedigrees unmistakable.
Worked examples
In a pedigree, two phenotypically normal parents produce a daughter who shows the trait. Both sexes are affected in the wider family in roughly equal numbers, and the affected daughter's parents are first cousins. What is the most probable mode of inheritance?
An affected child from two unaffected parents immediately rules out a dominant pattern — a dominant trait cannot hide. So the trait is recessive. Both sexes being affected equally rules out an X-linked bias and points to an autosomal location; an X-linked recessive trait would show a male excess and an affected daughter would be rare. The double mating line (consanguinity) strongly supports a rare recessive allele shared by related parents. Conclusion: autosomal recessive inheritance, the pattern NCERT illustrates with sickle-cell anaemia.
A pedigree shows an affected man married to an unaffected woman. All of their daughters are affected and all of their sons are unaffected. The trait appears in every generation. Which mode of inheritance fits?
The trait appearing in every generation suggests a dominant pattern. The decisive clue is the mating outcome: an affected father transmitting the trait to all daughters and none of his sons. A father passes his X chromosome only to daughters and his Y only to sons; if the dominant disease allele sits on the X, every daughter receives it and is affected, while every son receives the Y and is spared. This is the signature of X-linked dominant inheritance. An autosomal dominant trait would not split so cleanly by sex.
A pedigree records a haemophilic trait. Affected individuals are almost all male. An affected man's daughters are clinically normal, yet several of those daughters' sons are affected. Identify the inheritance pattern and explain the pathway of the allele.
A strong male excess and the absence of affected females point to an X-linked recessive trait — exactly what NCERT states for haemophilia. The affected man passes his single defective X to all his daughters, making each an unaffected carrier (her second, normal X masks the allele). Each carrier daughter then transmits the defective X to about half her sons, who, being hemizygous, are affected. The disease therefore appears to leap from grandfather to grandson through an unaffected daughter — the classic criss-cross inheritance pattern.
In a pedigree, an arrow points to one circle in the third generation, and that circle carries a central dot. Generations are labelled I, II and III with Roman numerals. State what the arrow and the dot signify, and what III tells you about that individual.
The arrow identifies the proband (propositus or index case) — the individual through whom this family first came to genetic study; the pedigree investigation begins from this person. The circle tells you the proband is female. The central dot marks her as a carrier: heterozygous for the trait, clinically unaffected because the normal dominant allele masks the recessive one, but able to transmit the recessive allele to her offspring. The Roman numeral III places her in the third (youngest shown) generation of the family tree.
Common confusion & NEET traps
Pedigree questions punish two reflexes: rushing the symbol decode, and over-claiming what a chart proves. The callouts below isolate the errors that most often cost a mark.