Blood Relations – Study Notes for J&K Police SI (Reasoning Ability)
Aim – To grasp every nuance of consanguineous (blood) relationships, the terminology used in reasoning‑based exams, the logic for deducing relationships, and the underlying mathematical ideas (degree of kinship, coefficient of relatedness). These notes are written for a competitive exam style (IIT‑JEE/Police‑SI), so they include derivations, tricky pitfalls and a hard practice problem.
1. Core Definitions
| Term | Formal Definition | Example |
|---|---|---|
| Consanguine (blood) relation | Relatives sharing a common ancestor by blood (no marriage or adoption). | A son and his mother. |
| Affinal relation | Relatives by marriage only (no blood). | A son‑in‑law and his mother‑in‑law. |
| Ancestral line | The vertical chain of parents to a common ancestor. | Father → Grandfather → Great‑Grandfather. |
| Collateral line | The horizontal chain of siblings or their descendants. | Aunt → Cousin. |
| Degree of relationship | Number of generations between two persons and their common ancestor. | |
| Coefficient of relatedness (r) | Probability that a randomly selected allele from one person is identical by descent (IBD) to the same allele in another person. |
Key Point – In reasoning questions the relative terms are usually enough; the mathematical coefficient is rarely required unless the question explicitly asks for it.
2. Classification of Blood Relationships
| Category | Relationship | Example |
|---|---|---|
| Immediate | Parent, Child, Sibling (full or half) | Father, Daughter, Brother |
| Extended | Grandparent, Grandchild, Grand‑sibling (niece/nephew) | Grandfather, Granddaughter |
| Collateral | Aunt/Uncle (paternal/maternal), Cousin (first/second/third…) | Paternal aunt, First cousin |
| Descendant of Collateral | Niece/Nephew, Great‑Aunt, Great‑Uncle | Daughter of your sibling |
| Other | Great‑Grandparent, Great‑Grandchild | Great‑grandfather, Great‑granddaughter |
2.1. Full vs Half Relations
| Relationship | Full | Half |
|---|---|---|
| Sibling | Same mother and father | Same mother or same father |
| Cousin | First cousin via full parentage | Half‑first cousin via one side |
| Aunt/Uncle | Full sibling of parent | Half‑sibling of parent |
| Niece/Nephew | Child of sibling | Child of half‑sibling |
Tricky Point: “Half‑cousin” is not typically used in reasoning tests; instead they say “half‑first cousin” or “cousin through one parent”.
3. Degree of Relationship & Coefficient of Relatedness
3.1. Derivation
- Number of edges in the tree connecting two persons = n
- Edge = parent‑to‑child link.
- Coefficient of relatedness
[ r = \frac{1}{2^{n}} ]- Proof: Each generation halves the gene pool.
| Relationship | n | r |
|---|---|---|
| Parent–Child | 1 | 1/2 |
| Sibling | 2 | 1/4 |
| Grandparent–Grandchild | 2 | 1/4 |
| First Cousin | 4 | 1/16 |
| Great‑Grandparent–Great‑Grandchild | 4 | 1/16 |
| Second Cousin | 6 | 1/64 |
Exam Tip: When the question asks “who is more closely related to X?”, compute n or compare r values.
3.2. Important Formulas
| Scenario | Formula | Example |
|---|---|---|
| Coefficient of relationship (r) | ( r = \frac{1}{2^n} ) | 1st cousin: ( 1/2^4 = 1/16 ) |
| Degree of consanguinity | ( \text{Degree} = n ) | Parent–child = 1, Siblings = 2 |
| Blood relation to a common ancestor | ( \text{Generations} = \text{rank of common ancestor} ) | If common ancestor is great‑grandfather → 3 generations |
4. Commonly Used Terminology in Reasoning Tests
| Term | How to Interpret |
|---|---|
| Paternal | Relates to the father’s side. |
| Maternal | Relates to the mother’s side. |
| First cousin | Children of your parents’ siblings. |
| Second cousin | Children of your parents’ first cousins. |
| Half‑uncle | Half‑sibling of a parent. |
| Step‑relation | Not a blood relation. |
| Sister‑in‑law | Wife of brother or sister’s husband. |
| Brother‑in‑law | Husband of sister or brother’s wife. |
Tricky Point: “Sister‑in‑law” can refer to two people: (a) wife of your brother, (b) sister of your spouse. Use context to choose.