Q.If a father and son are both defective in red-green colour vision, is it likely that the son inherited the trait from his father? Comment.

Pedigree Analysis: A Family Tree for Traits

Imagine you're looking at a family photograph album. You notice that your grandfather, your uncle, and your cousin all have the same unusual hairline or the same colour blindness. You start wondering: Is this just a coincidence, or is it something that runs in the family? How does this pattern pass from one generation to the next?

That curiosity is exactly where pedigree analysis begins.

What is a Pedigree?

A pedigree is simply a family tree that records the presence or absence of a particular trait — like a disease, a physical feature, or a genetic condition — across several generations. Instead of writing names and stories, biologists use a standard set of symbols to map out who has the trait and who doesn't, and how they are related.

The NCERT textbook defines it as a method to study the inheritance of a trait in several generations of a family. It is a tool, not a formula. You don't calculate anything; you observe patterns.

Why Do We Need It?

Humans cannot be bred in a laboratory like fruit flies or pea plants. You cannot ask two cousins to have children just to see if a disease is inherited. So pedigree analysis becomes the only ethical, practical way to study how traits pass through human families.

It helps in:

• Predicting the likelihood that a child might inherit a genetic disorder
• Tracing whether a trait is dominant or recessive
• Identifying carriers — people who carry a gene for a disease but do not show it themselves
• Offering genetic counselling to families with a history of inherited conditions

The Basic Symbols (What You Need to Know)

In a pedigree chart:

• Males are represented by squares
• Females are represented by circles
• A horizontal line connecting a square and a circle shows a marriage (mating)
• A vertical line descending from that couple leads to their children
• Shaded (filled) symbols indicate individuals who show the trait
• Unshaded (empty) symbols indicate individuals who do not show the trait
• A half-shaded symbol often indicates a carrier (someone who has the gene but does not express the trait)

How to Read a Pedigree: The Core Idea

You are essentially looking for a pattern. Ask yourself:

1. Does the trait appear in every generation? If yes, it might be dominant — because a dominant trait only needs one copy of the gene to show up, so it rarely skips a generation.
2. Does the trait skip a generation? If yes, it might be recessive — because a recessive trait needs two copies (one from each parent), and carriers can pass it on without showing it themselves.
3. Does the trait affect mostly males? If yes, it might be sex-linked (carried on the X chromosome). For example, colour blindness and haemophilia are much more common in males because they have only one X chromosome.

A Simple Example (Without Numbers)

Suppose you see a pedigree where:

• A grandfather has a condition (shaded square)
• His daughter does not have it (empty circle)
• But his grandson (the daughter's son) has it (shaded square)

What does this tell you? The trait skipped a generation (from grandfather to grandson, missing the daughter). That suggests it is recessive. Also, it appears only in males in this case, so it might be X-linked recessive — the daughter is a carrier, and she passed the gene to her son. …