Biology · Principles of Inheritance and Variation · NEET
Pedigree analysis is the study of how one trait passes down through many generations of a family, drawn as a family tree. We cannot do test crosses in humans, so this family history is used instead. Memory hook: Square = male, Circle = female, Filled = affected, Empty = normal.
Standard NCERT pedigree symbols: squares are males, circles are females, filled shapes are affected. A single line is normal mating and a double line is consanguineous mating (relatives). A vertical line drops to the children.
Your doubts, answered
What is pedigree analysis in simple words?
It is a chart, like a family tree, that shows how ONE trait (like a disease) passes from parents to children across many generations. NCERT calls it 'analysis of traits in several generations of a family.' You look at the chart and figure out if the trait is dominant, recessive, or sex-linked. For NEET, remember it replaces the test cross, because you cannot force matings in humans.
What does a square and a circle mean in a pedigree?
A square always means a MALE. A circle always means a FEMALE. This never changes. A diamond is used when the sex is unknown. Tip: think 'circle = female' because both words are round.
What does a filled (shaded) shape mean?
A filled or shaded shape means that person is AFFECTED (has the trait or disorder). An empty (unfilled) shape means the person is NORMAL (unaffected). So a filled circle = an affected female, an empty square = a normal male.
What is the difference between a single line and a double line in a pedigree?
A SINGLE horizontal line joining a square and circle means a normal marriage or mating. A DOUBLE horizontal line means a consanguineous mating, which is mating between close relatives (like cousins). This exact point was asked in NEET 2023, so learn it well.
Why do we use pedigree analysis in humans instead of a test cross?
Because we cannot do controlled crosses in humans. In pea plants Mendel could choose which plants to cross, but you cannot tell people who to marry or have children with. So scientists study the family history of a trait that already exists. This is the main reason NCERT gives for pedigree analysis.
What is the symbol for parents joined to their children?
A vertical line drops down from the horizontal mating line to the children. The children (brothers and sisters, called a sibship) are drawn on one horizontal line below. This vertical-to-horizontal drop is the offspring or sibship line.
⚠️ The NEET trap ✗ A double horizontal line between a square and a circle means twins or many children. ✓ A double horizontal line means a consanguineous mating (mating between relatives). A single line is a normal mating. 🧠 Double line = double relatives = consanguineous. NEET 2023 tested exactly this.
Real NEET questions
NEET 2023 Phase 1
Which of the following symbols represents mating between relatives in human pedigree analysis?
A · A square (male) and a circle (female) joined by a single horizontal marriage/mating line.
B · A square (male) and a circle (female) joined by a double horizontal line (consanguineous mating line). ✓
C · A square (male) and a circle (female) joined by a mating line with a vertical line descending to offspring (sibship).
D · A filled square, a filled circle and a filled diamond (affected individuals/unknown sex symbols) on a sibship line.
Solution: In standard pedigree notation a SINGLE horizontal line between a square and a circle is a normal mating. A DOUBLE horizontal line means a consanguineous mating, that is mating between relatives. Option C shows offspring (sibship) and option D shows affected individuals, so neither is about relatives. Correct answer is B.
NEET 2022
Which of the following occurs due to the presence of an autosome linked dominant trait?
A · Sickle cell anaemia
B · Myotonic dystrophy ✓
C · Haemophilia
D · Thalassemia
Solution: Pedigree analysis of an autosomal DOMINANT trait uses the NCERT example Myotonic dystrophy (Figure 4.14a). Sickle cell anaemia and thalassemia are autosomal RECESSIVE, and haemophilia is X-linked recessive. So the only autosome-linked dominant trait here is Myotonic dystrophy. Correct answer is B.
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It helps trace how a genetic trait or disorder is inherited in a family. Doctors use it to predict the chance that a future child will have a disorder, and to find out if a trait is dominant, recessive, or sex-linked. This is called genetic counselling.
Is a diamond used in a pedigree?
Yes. A diamond shape is used when the sex of a person is unknown or not stated. Squares are male and circles are female.
How do you know if a trait is dominant or recessive from a pedigree?
If the trait appears in every generation and affected people usually have an affected parent, it is often dominant. If the trait skips generations and can appear in children of two normal parents, it is often recessive.
What is the difference between an empty and a filled symbol?
An empty (unshaded) symbol means the person is normal or unaffected. A filled (shaded) symbol means the person is affected by the trait or disorder being studied.
