Concept understanding — Applications of Reasoning -- Blood Relation
Blood Relations — The Logic of Family Trees
You already know what a family is. The trick in blood relation problems is that you are given a set of statements about who is related to whom, and you must figure out a specific relationship — or answer a question like "How is A related to B?" — without ever seeing the people.
The entire subject rests on one skill: building a family tree in your head (or on paper) from verbal clues.
The Intuition
Imagine you hear: "Ravi is the son of Sunita. Sunita is the sister of Vikram." You instantly know Ravi is Vikram's nephew. How? Because you mentally connected the two facts. That's all a blood relation problem ever does — it gives you a chain of such statements, sometimes with a twist (in-laws, multiple generations), and asks you to walk that chain.
The difficulty comes from two things:
The statements are not always in chronological or generational order.
The same person may be referred to by different descriptions ("my mother's brother" vs "my uncle").
The Precise Method
Every blood relation problem reduces to constructing a family tree using these rules:
PersonrelationAnother Person
The standard relations are:
Relation
Meaning
A is the father of B
A is male, B is child
A is the mother of B
A is female, B is child
A is the brother of B
Same parents, A is male
A is the sister of B
Same parents, A is female
A is the husband of B
Married, A is male
A is the wife of B
Married, A is female
A is the son of B
A is male, B is parent
A is the daughter of B
A is female, B is parent
From these, you derive everything else:
Uncle: brother of a parent
Aunt: sister of a parent
Nephew: son of a sibling
Niece: daughter of a sibling
Cousin: child of a parent's sibling
Grandfather/Grandmother: parent of a parent
Sibling-in-law: sibling of a spouse, or spouse of a sibling
The Step-by-Step Process
Step 1: Identify all distinct persons mentioned. Give each a symbol (A, B, C, ... or use the names given).
Step 2: Start with the most concrete statement. Usually a statement like "X is the father of Y" gives you a clear parent-child link. Draw it.
Step 3: Add each new statement to your tree. Use these conventions:
Horizontal line — for marriage
Vertical line | for parent-child
Same level = same generation
Step 4: Mark gender. Use (M) and (F) or a symbol convention (square for male, circle for female).
Step 5: When stuck, work backwards. If the question asks "How is P related to Q?" and you have statements about P and about Q, try to connect them through a common person.
Tip
Always draw the tree. Never try to hold more than 3-4 relations in your head. A quick sketch takes 10 seconds and saves you from errors.
A Worked Example
Problem: A is the brother of B. C is the daughter of A. D is the sister of C. How is D related to B?
Step 1: Persons: A, B, C, D.
Step 2: "A is the brother of B" → A and B are siblings. A is male. B's gender unknown.
Step 3: "C is the daughter of A" → A is father of C. C is female.
Step 4: "D is the sister of C" → C and D are siblings, both children of A. D is female.
Tree:
A (M) — (spouse, not named)
|
┌────┴────┐
C (F) D (F) …
Work backward from "my son's uncle" to identify the man as Akshay's father.
[!FORMULA] Family-relation rules: a father's brother is that child's (paternal) uncle, so "my son's uncle" is my own brother; and two brothers share the same father.
"My son's uncle" refers to my son's paternal uncle — that is, my own brother (since a child's father's brother is the child's uncle).
So "his son" (the son of the man being pointed at) = Akshay's brother.