---
title: "Unique Individuals"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 66
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/66-unique-individuals
---

# Chapter 66 — Unique Individuals

Seven billion people, and no two alike — not in a family, not in a schoolyard, not in all of history, identical twins half-excepted. Three chapters have now assembled the whole machinery of that fact. This short capstone chapter runs the argument once, end to end, and banks its consequences: for the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’ variety, for how we read “genetic” headlines, and for what uniqueness does and does not mean.

## 66.1 The uniqueness theorem

**Proposition 66.1 (Why no two people match).**

Assemble the three mechanisms:

1. *the shuffle of the halving* : each parent’s [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) carry one partner per pair, dealt independently — $2^{23}$ half-decks per parent ( [Remark 64.4](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#rem-g9-chromosomes-whyhalve) ), and in truth far more, since paired partners also *swap segments* during the halving division, cutting the deck below the level of whole [chromosomes](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) ;
2. *the lottery of [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps)* : which sperm of the millions ( [Example 57.2](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#ex-g8-reproductive-systems-sperm) ) meets which month’s [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) multiplies the two spaces together;
3. *[mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains)’s trickle* : every generation adds a few fresh spellings of its own ( [Proposition 65.6](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) ).

The combination space dwarfs the number of humans who have ever lived: every [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) deals a deck that has never been dealt before, and never will be again.

**Proof.** *Admitted at this level.* ∎

**Example 66.2 (The twins, and the exception’s limits).**

Identical twins share one deal ([Exercise 58.10](https://one-course.com/books/biology/1/en/chapter/58-from-egg-cell-to-birth#exo-g8-from-egg-to-birth-10)) — and even they diverge: separate lives write different acquired [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) ([Proposition 63.4](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#prop-g9-heredity-and-traits-sorting)), different [synapse](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-synapse) histories ([Example 60.6](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#ex-g8-nervous-communication-juggler)), even a few different late [mutations](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) in each body’s own [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell). The one crack in uniqueness seals itself: after the deal, living differentiates.

**Remark 66.3 (Unique, and mostly alike).**

Hold both truths: any two humans’ [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) texts agree over $99$ percent — the shared build of [Proposition 29.2](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle)’s one [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) — and the disagreements, scattered through three billion letters, still suffice for strict uniqueness. “All different” and “all kin” are the same fact at two magnifications ([Method 65.8](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#met-g9-genes-and-dna-levels)); every use ever made of the first truth against the second was bad [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) before it was bad ethics.

## 66.2 Uniqueness at work

**Example 66.4 (DNA identification).**

Uniqueness is checkable, and courts check it: enough of a person’s variable positions, read from a trace of [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), match one individual (identical twins aside) out of humanity. The same comparison, run at family magnification, resolves parentage — the child’s every variable position must be dealt from its two parents’ decks. The album’s logic ([Chapter 63](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#ch-g9-heredity-and-traits)) has become an instrument.

**Example 66.5 (Transfusion and transplantation).**

Medicine negotiates uniqueness daily. [Blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) groups ([Example 65.3](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#ex-g9-genes-and-dna-bloodgroups)) are the easy case — four broad classes, matchable from the register. Organ transplants meet the hard case: the immune system ([Chapter 70](https://one-course.com/books/biology/1/en/chapter/70-the-immune-defenses#ch-g9-immune-defenses) explains the machinery) reads finer-grained self-markers, unique enough that donors are searched for by family first — the closer the [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship), the nearer the deal.

**Proposition 66.6 (Variety is the species’ reserve, banked).**

[Remark 54.7](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#rem-g8-sexual-reproduction-reserve) promised the shuffle’s point; the mechanisms now cash it. A [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) of unique individuals holds, scattered among them, [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) for every occasion — the resistance some carry to this disease, the tolerance others carry to that stress ([Example 54.6](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#ex-g8-sexual-reproduction-bets)’s orchard, at [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) scale). When [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) turn, the checks of [Proposition 55.6](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#prop-g8-reproduction-and-environment-limits) fall unevenly across that variety — and what that unevenness does, generation after generation, is precisely the subject the next two chapters open: the changing of [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species).

**Proof.** *Admitted at this level.* ∎

**Method 66.7 (Auditing a “genetic” claim).**

For any headline about [genes](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) and people:

1. level check ( [Method 65.8](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#met-g9-genes-and-dna-levels) ): is the claim about a [gene](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) , a deck, or a family pattern?
2. weave check ( [Proposition 63.4](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#prop-g9-heredity-and-traits-sorting) ): does it respect the inherited-range-lived-in weave, or pretend one [gene](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) equals one destiny?
3. variety check: does it treat a [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) ’s spread as the reserve it is, or as deviations from a “normal” deck that does not exist?
4. twin check: would it survive the identical twins who share the deal and differ anyway?

## 66.3 Exercises

**Exercise 66.1 ★.**

Name the three mechanisms of the uniqueness theorem.

**Solution of Exercise 66.1.**

The halving’s shuffle (with segment-swapping), the [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) lottery, and [mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains)’s generational trickle.

**Exercise 66.2 ★.**

What does segment-swapping during the halving add to the shuffle?

**Solution of Exercise 66.2.**

It cuts the deck below whole [chromosomes](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes): partners exchange segments before separating, so even one [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) hands on a new mosaic — the $2^{23}$ count is only the floor.

**Exercise 66.3 ★.**

State both halves of [Remark 66.3](#rem-g9-unique-individuals-alike) — the percentage and the uniqueness — in one sentence.

**Solution of Exercise 66.3.**

Any two humans agree in over $99$ percent of their [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) text, and the scattered disagreements still make every individual strictly unique — [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) and difference in one molecule.

**Exercise 66.4 ★.**

How do identical twins end up distinguishable after all?

**Solution of Exercise 66.4.**

Their lives diverge after the shared deal: acquired [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits), trained [synapses](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-synapse), and each body’s own late [mutations](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) write distinguishable records on identical starts.

**Exercise 66.5 ★.**

What two questions does [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) comparison answer for a court?

**Solution of Exercise 66.5.**

Whose [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) a trace is (identification), and who a child’s parents are (every variable position must be dealt from the two parental decks).

**Exercise 66.6 ★.**

Why are transplant donors searched for by family first?

**Solution of Exercise 66.6.**

Because the immune system reads fine-grained self-markers, and the closer the [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship), the nearer the marker deal — family decks share the most.

**Exercise 66.7 ★★.**

Multiply the argument: combine the two parents’ half-deck spaces and the [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) lottery into the “never dealt before” conclusion.

**Solution of Exercise 66.7.**

Each parent offers millions of possible half-decks; a child is one pairing of one maternal deal with one paternal deal, chosen by which [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) met which — the two spaces multiply, and the product so exceeds all humans ever born that a repeated deal has effectively never occurred and never will.

**Exercise 66.8 ★★.**

Cash [Proposition 66.6](#prop-g9-unique-individuals-reserve) on the vineyard of [Exercise 54.10](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#exo-g8-sexual-reproduction-10): what did the clones lack that a seed-grown [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) holds?

**Solution of Exercise 66.8.**

A reserve: the clones were one deck repeated — one constitution, one shared weakness — while a seed-grown [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population)’s unique decks would have scattered resistances among them, some vines standing where the pest found the rest to its taste.

**Exercise 66.9 ★★.**

Run [Method 66.7](#met-g9-unique-individuals-claims) on: “a [gene](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) for school success discovered”.

**Solution of Exercise 66.9.**

Level: the claim jumps from a [gene](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) to a family-level life outcome — magnifications skipped. Weave: school success is an inherited range lived in schools, homes and languages — one [gene](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) equals no destiny. Variety: it grades a spread against a phantom “normal”. Twins: identical decks routinely produce different school stories. The headline fails all four checks.

**Exercise 66.10 ★★.**

Why is “the normal human deck” a phrase without a referent? Answer with the reserve and the theorem.

**Solution of Exercise 66.10.**

Because the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) *is* its variety: the reserve scatters [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) across unique individuals, and no deck is the standard from which others deviate — “normal” names an average nobody carries, not a text anyone was meant to match.

**Exercise 66.11 ★★.**

A parentage test clears a doubted father wrongly accused by [Exercise 63.11](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#exo-g9-heredity-and-traits-11)’s neighbor. Explain what the test checks, position by position.

**Solution of Exercise 66.11.**

At each variable position, the child’s two spellings must come one from the mother’s pair, one from the father’s. The test checks position after position: the blue-eyed child’s every spelling is dealable from those two decks — the skip explained the [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), the positions certify the parentage.

**Exercise 66.12 ★★★.**

“All different, all kin, and both by the same mechanism.” Write the capstone paragraph: shuffle, [mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) and shared code, each placed.

**Solution of Exercise 66.12.**

For example: one shuffle deals every human a never-repeated selection from the parental decks, and [mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) salts each generation with spellings never seen at all — hence all different. Yet every deck is written in the same four letters, read by the same code, and agrees with any other human deck in over $99$ letters of each hundred — hence all kin. The mechanisms are not rivals but one system: common text, endlessly re-dealt — [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) supplying the deck, the shuffle guaranteeing no hand is ever repeated.

## 66.4 Problem: The Cold-Case Seminar

**Problem 66.1.**

Weekend problem — one hair, three questions, thirty years

A (fictional) cold-case seminar: a thirty-year-old unsolved burglary, one preserved hair with its [root](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), and modern comparison methods. The seminar walks students through what [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) identification can and cannot say.

**Part I — What the hair holds.**

1. Why must the hair’s *[root](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts)* be preserved for a full reading ( [Definition 45.1](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) knows where [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) lives)?
2. The lab reads a set of highly variable positions. Why variable ones — what would the 99-percent-shared positions establish?
3. The profile matches a suspect at every position. State what the match asserts, and its one systematic exception ( [Example 66.2](#ex-g9-unique-individuals-twins) ).
4. The defense notes the suspect has an identical twin abroad. What must the court now do, and why?

**Part II — The family questions.**

5. An earlier, partial profile had pointed to “a close relative of the K family”. Explain how a *partial* match reads as [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) ( [Proposition 66.1](#prop-g9-unique-individuals-theorem) ’s dealing, run backward).
6. A juror asks whether the profile reveals the suspect’s [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) — looks, temperament, “criminal [genes](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) ”. Run [Method 66.7](#met-g9-unique-individuals-claims) on the question, check by check.
7. Another juror asks whether the match could be “once in this city” rather than once in humanity. What sets the number of positions read?
8. The seminar leader warns: “the [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) says whose [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ; the case must still say how they got there.” Distinguish the two claims.

**Part III — The seminar’s close.**

9. Summarize the instrument in one sentence: what two chapters’ mechanisms ( [Chapter 64](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#ch-g9-chromosomes) , [Chapter 65](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#ch-g9-genes-and-dna) ) make a profile unique and a family readable.
10. Why does the same instrument work on a thirty-year-old hair? (What property of [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) — [Proposition 65.6](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) ’s copying aside — is being relied on: stability.)
11. The seminar ends on ethics: list two uses of uniqueness this chapter shows serving people, and the historical misuse [Remark 66.3](#rem-g9-unique-individuals-alike) warns against.
12. Close with the theorem restated for the courtroom: what has never happened twice, and why.

**Solution of Problem 66.1.**

**1.** Because [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna) lives in the [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts), and the hair’s shaft is dead keratin — the [root](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts)’s [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) carry the nuclei a full reading needs.

**2.** Variable positions are where individuals differ — the identifying scatter. The shared positions would establish only that the hair is human: kinship-level truth, useless for naming one person.

**3.** That the hair’s [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) and the suspect’s share one deal — one individual, out of all humans ever, with the single exception of an identical twin, who shares the deal by origin.

**4.** Rule the twin in or out by other evidence — whereabouts, witnesses, the case’s “how it got there”: the profile genuinely cannot separate the pair, so the court must.

**5.** A relative shares, by descent, a large fraction of the variable spellings — more than a stranger, fewer than a match: partial overlap graded by [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) is the dealing run backward up the tree.

**6.** Level: a profile is deck-level data — the variable positions read are chosen precisely for saying nothing about [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits). Weave: temperament and conduct are ranges lived in lives. Variety: “criminal [genes](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene)” grades people against a phantom normal. Twins: shared decks, different lives. The profile names [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), not character.

**7.** The number of independent variable positions: each added position divides the matching [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) further — enough positions, and “once in the city” becomes once in humanity. The lab reads as many as the standard demands.

**8.** The [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) asserts whose [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) the hair grew from. The case must still establish placement — when, how, innocently or not, the hair arrived: matching is not narrative, and courts convict on narratives proven.

**9.** The halving-and-fertilization deal makes each deck unrepeatable, and [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna)’s spelling differences make the deal readable — [chromosomes](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) shuffle it, the molecule records it.

**10.** On [DNA](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-dna)’s chemical stability: the text neither fades nor rewrites in a drawer — the same double strand, intact after thirty years, reads as it was dealt.

**11.** Serving: identification that clears the innocent and resolves parentage; transplant matching that finds the nearest deal. The warned misuse: reading the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’ variety as ranks — difference pressed into hierarchy, the bad [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) behind old injustices.

**12.** No two [fertilizations](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) have ever dealt the same deck: the parental half-deck spaces multiplied by the [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) lottery exceed all humans ever conceived — so a full-profile match names one individual, and the court may treat the hair as signed.