हिंदी
Tamil Nadu Board of Secondary EducationHSC Science Class 12
पाठ्यपुस्तक उत्तर१०९५२
अवधारणा स्पष्टीकरण१९९
पाठ्यक्रम

Pedigree Analysis

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Estimated time: 13 minutes
CBSE: Class 12
CISCE: Class 12

Definition: Pedigree analysis

The study of inheritance of a particular trait in humans by collecting information from family history and representing it through a family tree using standard symbols is called pedigree analysis.

CBSE: Class 12
CISCE: Class 12

Introduction

Pedigree Analysis is the study of the inheritance of a specific trait, abnormality, or disease in a family by constructing and analysing a multi-generational family tree (called a pedigree chart) using a standardised set of symbols, in order to determine the mode of inheritance.

  • The family tree constructed is called a Pedigree Chart (or simply, a Pedigree).​
  • The individual from whom the pedigree study begins is called the Propositus (male) or Proposita (female), also known as the Proband.​
  • Pedigree analysis helps determine whether a trait is dominant or recessive, and whether it is autosomal or sex-linked

Why Pedigree Analysis?

Reason Explanation
No controlled crosses possible Ethical and social barriers prevent planned human matings​
Long generation time Humans take ~20–25 years per generation; controlled studies are impractical​
Small family size Limited offspring make statistical analysis difficult​
Medical application Helps predict risk of genetic disorders in future generations​
CBSE: Class 12
CISCE: Class 12

Standard Pedigree Symbols

CBSE: Class 12
CISCE: Class 12

Pedigree Analysis & Construction

1. Drawing a Pedigree Chart

  • Step 1 (Data Collection): Identify affected, unaffected, and carrier individuals across generations.
  • Step 2 (Layout): Assign each generation a row, labelled with Roman numerals (I, II, III...) from top to bottom.
  • Step 3 (Symbols & Connections): Use standard shapes (Squares = Males; Circles = Females). Connect mating pairs with a horizontal line and offspring with a vertical line below.
  • Step 4 (Shading): Fully shade affected individuals. Use half-shading for known carriers.
  • Step 5 (Analysis): Evaluate the layout to deduce the mode of inheritance.

2. The 3-Step Shortcut Strategy: How to Analyse a Pedigree?

Step 1: The Dominance Test (Generation Skipping)

  • Recessive: Two unaffected parents have an affected child, or the trait skips generations.
  • Dominant: Every affected child has at least one affected parent; the trait appears in every generation without skipping.

Step 2: The Gender Bias Test (Autosomal vs. Sex-Linked)

  • Autosomal: Males and females are affected in roughly equal proportions.
  • X-Linked Recessive: Males are predominantly affected. An affected mother will pass the trait to all of her sons.
  • X-Linked Dominant: An affected father passes the trait to all of his daughters.
  • Y-Linked: Trait passes exclusively from father to all sons.

Step 3: The Genotype Cross-Check

  • Assign tentative genotypes (e.g., AA, Aa, aa) to all individuals based on your hypothesis.
  • If the genotypes are internally consistent throughout the entire chart, your hypothesis is correct. If a contradiction arises, eliminate that option and test the next one.
CBSE: Class 12
CISCE: Class 12

Key Points: Pedigree Analysis

  • Pedigree Analysis is the study of inheritance patterns of traits across several generations of a family; the chart representing this is called a family tree (pedigree).
  • Since controlled crosses (like in pea plants) cannot be done in humans, family history analysis is used as an alternative to study inheritance.
  • Pedigree analysis is a powerful tool in human genetics used to trace the inheritance of a specific trait, abnormality or disease.
  • Standard symbols used - Square = normal male, Circle = normal female, Diamond = sex unspecified; filled/shaded shapes = affected individuals.
  • A horizontal line between two symbols = mating; a double horizontal line = consanguineous mating (mating between relatives); children are shown below parents in order of birth from left to right.

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Related QuestionsVIEW ALL [4]

The pedigree chart given below shows a particular trait which is absent in parents but present in the next generatoin irrespective of sexes. Draw your conclusion on the basis of the pedigree.

Study the Pedigree chart given below and answer the questions that follow:

Symbols used in the given Pedigree Chart are as follows:

Carrier male
Affected male
Normal male
Carrier female
Affected female
Normal female

(a) On the basis of the inheritance pattern exhibited in this pedigree chart, what conclusion can you draw about the pattern of inheritance?

(b) If the female is homozygous for the affected trait in this pedigree chart, then what percentage of her sons will be affected?

(c) Give the genotype of offspring 1, 2, 3 and 4 in III generations.

OR

(c) In this type of inheritance pattern, out of male and female children which one has less probability of receiving the trait from the parents? Give a reason.

Suggest how pedigree analysis can be useful?

A normal visioned woman, whose father is colour blind, marries a normal visioned man. What would be probability of her sons and daughters to be colour blind? Explain with the help of a pedigree chart.

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