---
title: "Genes and DNA"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 65
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna
---

# Chapter 65 — Genes and DNA

The [chromosomes](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) carry the determinants — but a thread is not yet a text. What, along a [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes), *is* one determinant? And what is the thread made of, that a [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) thousandths of a millimetre wide can hold instructions for a whole human being ([Exercise 63.12](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#exo-g9-heredity-and-traits-12))? Two names answer, and they are the most famous in modern [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living): the [gene](#def-g9-genes-and-dna-gene), and [DNA](#def-g9-genes-and-dna-dna).

## 65.1 Genes: the determinants, located

**Definition 65.1 (Gene and alleles).**

A *gene* is one determinant made precise: a *segment of a [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes)* carrying the instructions for one [trait](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits)’s piece of the build — one gene for the blood-group machinery, others for eye-color pigments, thousands upon thousands along the 46 threads, each at its fixed address. A gene can exist in several versions — *alleles*: the blood-group gene’s A, B and O versions, the eye-color gene’s brown and blue. Same address, different spelling.

**Proposition 65.2 (Two copies, and what showing means).**

[Chromosomes](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) come in pairs ([Proposition 64.2](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#prop-g9-chromosomes-counts)) — so every [gene](#def-g9-genes-and-dna-gene) comes in *two copies*, one per partner, one from each parent. The copies may be the same [allele](#def-g9-genes-and-dna-gene) or different; when different, one may be *expressed* and the other carried silent — and the family album’s mysteries resolve into mechanism:

- carriage-without-showing ( [Example 63.5](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#ex-g9-heredity-and-traits-skip) ): the silent [allele](#def-g9-genes-and-dna-gene) rides the second partner;
- the group-O child of A- and B-parents ( [Problem 63.1](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#pb-g9-heredity-and-traits-1) ): each parent carried O silent, and the halving dealt both O’s together;
- the blue-eyed child of brown-eyed parents: two silent blues, meeting — one chance in four, when both parents carry one each: the halving deals each parent’s blue with one chance in two, and the two deals are independent.

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

**Example 65.3 (Blood groups, fully mechanized).**

One [gene](#def-g9-genes-and-dna-gene), three [alleles](#def-g9-genes-and-dna-gene), two copies per person: A with A or O shows group A; B with B or O shows B; A with B shows AB (both expressed — [alleles](#def-g9-genes-and-dna-gene) are not always silencable); O with O shows O. The donation register’s whole pattern — including every “surprise” — is the arithmetic of two-copy deals.

## 65.2 DNA: what the text is written in

**Definition 65.4 (DNA).**

Chemically, a [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) is one immensely long, coiled molecule of *DNA* (with packing proteins): a double strand — the famous twisted ladder — whose rungs are pairs of four chemical letters. *The order of the letters along the strand is the information*: a [gene](#def-g9-genes-and-dna-gene) is a stretch of that sequence, as a sentence is a stretch of a page. Four letters, three billion of them in one human deck — enough text for a build’s whole instruction set, coiled into every [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts).

**Example 65.5 (Seeing it yourself).**

[DNA](#def-g9-genes-and-dna-dna) is kitchen-extractable: mash a banana or an onion, add salty water and a little washing-up liquid ([membranes](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) undone — [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)’s envelopes are its target), strain, and layer cold spirit on top: at the boundary rises a tangle of whitish threads, spoolable on a toothpick. Those threads are [DNA](#def-g9-genes-and-dna-dna) — the actual molecule of this chapter, in actual centimetres.

![The kitchen extraction’s payoff: the molecule of heredity, spooled on a stick.](https://one-course.com/images/onecourse/chapters/biology-1/g9-genes-and-dna/img-f9f8e0635367.jpg)

*The kitchen extraction’s payoff: the molecule of [heredity](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-heredity), spooled on a stick.*

**Proposition 65.6 (What DNA’s structure explains).**

The double strand is not decoration — each property answers an old question:

- *copying* ( [Proposition 29.7](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-growth) ’s faithful divisions): the two strands are letter-by-letter complements, so pulled apart, each rebuilds its partner — one ladder becomes two identical ladders, one per daughter [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ;
- *[alleles](#def-g9-genes-and-dna-gene)* : versions of a [gene](#def-g9-genes-and-dna-gene) are small spelling differences in the sequence — same address, changed letters;
- *mutation* : copying is near-perfect, not perfect — a rare miscopied letter creates a *new* [allele](#def-g9-genes-and-dna-gene) . Mostly neutral, sometimes harmful, occasionally useful: [mutation](#prop-g9-genes-and-dna-explains) is where fresh spellings, and ultimately all [alleles](#def-g9-genes-and-dna-gene) , come from — the raw novelty [Chapter 68](https://one-course.com/books/biology/1/en/chapter/68-how-species-change#ch-g9-how-species-change) will need;
- *reading* : a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) uses a [gene](#def-g9-genes-and-dna-gene) by reading its sequence and building the corresponding working molecule — and each [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) kind reads its own selection of the library: [Exercise 64.12](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#exo-g9-chromosomes-12) ’s puzzle, resolved as promised.

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

**Remark 65.7 (One code, all of life).**

The deepest fact comes last: *every* [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) writes its [genes](#def-g9-genes-and-dna-gene) in the same [DNA](#def-g9-genes-and-dna-dna) letters and reads them by the same code — oak, [yeast](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread), whale, the [microbes](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-fermentation) of [Chapter 43](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ch-g6-producing-our-food), you. This is the [unity of life](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle) ([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)) at its foundation, and the [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) tree’s ([Remark 44.8](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#rem-g6-classification-kinship-implies)) strongest evidence yet: a shared language argues a shared origin. It is also why genetic engineering works at all — a [gene](#def-g9-genes-and-dna-gene) moved between [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) is still readable, as a sentence photocopied into another book.

**Method 65.8 (The three-level reading).**

Any [heredity](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-heredity) question can now be read at three levels — practice switching:

1. *family level* : who shows, who carries, who transmits ( [Chapter 63](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#ch-g9-heredity-and-traits) );
2. *[chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) level* : which threads, halved and restored, carry the deal ( [Chapter 64](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#ch-g9-chromosomes) );
3. *molecule level* : which [gene](#def-g9-genes-and-dna-gene) , which [alleles](#def-g9-genes-and-dna-gene) , what spelling — and, for a new [trait](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) , what [mutation](#prop-g9-genes-and-dna-explains) .

One story, three magnifications; an answer is complete when it runs at all three.

## 65.3 Exercises

**Exercise 65.1 ★.**

Define [gene](#def-g9-genes-and-dna-gene) and [allele](#def-g9-genes-and-dna-gene), with the blood-group example.

**Solution of Exercise 65.1.**

A [gene](#def-g9-genes-and-dna-gene) is a segment of a [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) carrying one [trait](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits)’s instructions, at a fixed address; [alleles](#def-g9-genes-and-dna-gene) are its versions — the blood-group [gene](#def-g9-genes-and-dna-gene)’s A, B and O.

**Exercise 65.2 ★.**

Why does every [gene](#def-g9-genes-and-dna-gene) come in two copies, and where do they sit?

**Solution of Exercise 65.2.**

Because [chromosomes](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) come in pairs: one copy of each [gene](#def-g9-genes-and-dna-gene) rides each partner — one from the mother, one from the father, at the same address.

**Exercise 65.3 ★.**

What is [DNA](#def-g9-genes-and-dna-dna)’s information, physically? How many letters, of how many kinds, in a human deck?

**Solution of Exercise 65.3.**

The order of the chemical letters along the strand. Four kinds of letter, about three billion of them in one human deck.

**Exercise 65.4 ★.**

How does the double strand make faithful copying possible?

**Solution of Exercise 65.4.**

The strands are letter-by-letter complements: pulled apart, each rebuilds its partner, and one ladder becomes two identical ladders — a full copy per daughter [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**Exercise 65.5 ★.**

What is a [mutation](#prop-g9-genes-and-dna-explains), and what are its three possible characters?

**Solution of Exercise 65.5.**

A rare copying error — changed letters creating a new [allele](#def-g9-genes-and-dna-gene). Mostly neutral, sometimes harmful, occasionally useful.

**Exercise 65.6 ★.**

State [Remark 65.7](#rem-g9-genes-and-dna-universal)’s fact and its two consequences.

**Solution of Exercise 65.6.**

All [living things](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) write and read their [genes](#def-g9-genes-and-dna-gene) in the same [DNA](#def-g9-genes-and-dna-dna) letters and code. Consequences: the strongest evidence for common origin — and the workability of moving [genes](#def-g9-genes-and-dna-gene) between [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species), since the language matches everywhere.

**Exercise 65.7 ★★.**

Mechanize the blue-eyed child of brown-eyed parents: copies, deals, and the one-in-four.

**Solution of Exercise 65.7.**

Each parent carries brown with blue — brown expressed, blue silent. Each deals blue with one chance in two; the deals are independent, so blue-with-blue lands with one chance in four — the blue-eyed child.

**Exercise 65.8 ★★.**

Work [Example 65.3](#ex-g9-genes-and-dna-bloodgroups): list the [allele](#def-g9-genes-and-dna-gene) pairs behind each of the four groups.

**Solution of Exercise 65.8.**

Group A: A with A, or A with O. Group B: B with B, or B with O. Group AB: A with B — both expressed. Group O: O with O.

**Exercise 65.9 ★★.**

Resolve [Exercise 64.12](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#exo-g9-chromosomes-12) in this chapter’s terms: what does a [neuron](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-neuron) read that a [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) does not?

**Solution of Exercise 65.9.**

Both hold the whole library — the same 46 threads. The [neuron](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-neuron) reads the nerve-machinery [genes](#def-g9-genes-and-dna-gene) and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)’s keratin chapters closed; the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) reads the reverse. Specialization is a borrowing policy, not a different library.

**Exercise 65.10 ★★.**

Describe the kitchen extraction and what each kitchen ingredient does.

**Solution of Exercise 65.10.**

Mash the [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) ([cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) opened); salty water and washing-up liquid ([membranes](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) — [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s and [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts)’s envelopes — [dissolved](https://one-course.com/books/biology/1/en/chapter/48-oxygen-in-the-water#def-g7-oxygen-in-the-water-dissolved), [DNA](#def-g9-genes-and-dna-dna) freed and kept [dissolved](https://one-course.com/books/biology/1/en/chapter/48-oxygen-in-the-water#def-g7-oxygen-in-the-water-dissolved)); strain (debris out); cold spirit layered on top ([DNA](#def-g9-genes-and-dna-dna) undissolves at the boundary): whitish spoolable threads — the molecule itself.

**Exercise 65.11 ★★.**

Run [Method 65.8](#met-g9-genes-and-dna-levels)’s three levels on the chapel chin of [Problem 63.1](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#pb-g9-heredity-and-traits-1).

**Solution of Exercise 65.11.**

Family level: the chin recurs along reproduction’s lines, carried unshown through one generation. [Chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) level: its determinant rides one thread, halved into [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) and restored at [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), dealt to some children and not others. Molecule level: it is an [allele](#def-g9-genes-and-dna-gene) — a spelling of some face-shaping [gene](#def-g9-genes-and-dna-gene) — copied faithfully down the century since the [mutation](#prop-g9-genes-and-dna-explains) that first wrote it.

**Exercise 65.12 ★★★.**

“[Mutation](#prop-g9-genes-and-dna-explains) writes, the shuffle deals, expression shows.” Assign each verb its mechanism and chapter — and state which of the three creates genuinely *new* spellings.

**Solution of Exercise 65.12.**

[Mutation](#prop-g9-genes-and-dna-explains) writes: copying errors create new spellings — molecule level, this chapter. The shuffle deals: halving and [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) hand each child a fresh combination — [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) level, [Chapter 64](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#ch-g9-chromosomes) (and [Proposition 54.5](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-shuffle)). Expression shows: of the two copies dealt, what is displayed and what rides silent — this chapter’s two-copy rule. Only [mutation](#prop-g9-genes-and-dna-explains) creates genuinely new spellings; the other two recombine and reveal what [mutation](#prop-g9-genes-and-dna-explains) once wrote.

## 65.4 Problem: The Broken Photocopier

**Problem 65.1.**

Weekend problem — one gene followed from spelling to family tree

Follow one (invented but typical) case: a pigment [gene](#def-g9-genes-and-dna-gene) whose working [allele](#def-g9-genes-and-dna-gene), P, builds a skin-and-hair pigment; long ago, a copying error created the broken [allele](#def-g9-genes-and-dna-gene) p — unreadably misspelled, building nothing. Carriers of P with p look ordinary; a child dealt p and p builds no pigment at all — very fair hair and [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), sun-fragile [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) (the condition exists across many [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)).

**Part I — Molecule level.**

1. Name p’s origin, by [Proposition 65.6](#prop-g9-genes-and-dna-explains) — what physically happened, once, in some ancestor’s germ line?
2. Why does P-with-p look ordinary? Which vocabulary word describes p’s ride?
3. Why does p-with-p change the build? What is missing, and from which chapter’s kind of machinery ( [Definition 65.1](#def-g9-genes-and-dna-gene) )?
4. The condition appears across many [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) . What does [Remark 65.7](#rem-g9-genes-and-dna-universal) predict about their pigment [genes](#def-g9-genes-and-dna-gene) ’ spellings?

**Part II — [Chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) and family levels.**

5. Two carrier parents (P with p each): list the four equally likely deals of one child, and the chance of the p-with-p build.
6. Their p-with-p daughter is born to two ordinary-looking parents. Which album mystery ( [Example 63.5](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#ex-g9-heredity-and-traits-skip) ) has just run at full mechanism?
7. The daughter’s own children with a P-with-P partner: what do they all carry, and what do none of them show?
8. Trace the [allele](#def-g9-genes-and-dna-gene) ’s vehicles for one transmission: p’s ride from grandmother to grandchild, thread and [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) by thread and [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) .

**Part III — The three-level write-up.**

9. Write the case at the three levels of [Method 65.8](#met-g9-genes-and-dna-levels) , one sentence each.
10. A classmate asks why the broken spelling was not simply “repaired away” long ago. Answer with the carrier’s ordinary look — where does p hide from any judgment on its effects?
11. Sun protection matters more for the daughter than for her carrier parents. Connect [trait](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) , [allele](#def-g9-genes-and-dna-gene) pair and [environment](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) in one sentence ( [Proposition 63.4](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#prop-g9-heredity-and-traits-sorting) ’s weave).
12. Close with the photocopier image made exact: what copies, what mis-copied once, and why the error is now copied faithfully.

**Solution of Problem 65.1.**

**1.** A [mutation](#prop-g9-genes-and-dna-explains): a copying error in the pigment [gene](#def-g9-genes-and-dna-gene), once, in some ancestor’s [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) line — letters changed, the instructions spoiled, and the misspelling copied faithfully ever since.

**2.** Because P’s working instructions build the pigment regardless of the silent partner: p rides *carried unshown* — the album’s word, now molecular.

**3.** With p on both partners, no working instructions exist at either address: the pigment machinery — the [gene](#def-g9-genes-and-dna-gene)’s product — is simply never built. The [trait](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) is the absence of a working spelling.

**4.** That their pigment [genes](#def-g9-genes-and-dna-gene) are recognizably the same [gene](#def-g9-genes-and-dna-gene) — similar sequences in the same code — kin-copies of one ancestral text, broken independently in each [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’ own history.

**5.** The four deals: P-with-P, P-with-p, p-with-P, p-with-p — equally likely. The unpigmented build: one chance in four.

**6.** The skip: two carriers displaying pigment transmitted what they did not show, and the hidden spellings met — [Example 63.5](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#ex-g9-heredity-and-traits-skip) run at full mechanism.

**7.** All carry one p — their mother’s whole contribution at that address is p. None show the condition: the partner’s P builds the pigment in every one.

**8.** Grandmother’s p rode one [chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes) of a pair; her [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes)’s halving dealt that partner into the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) or sperm that made the carrier parent; the parent’s halving dealt it again into the [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) that made the grandchild — thread to [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) to [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core), twice over.

**9.** Family: two ordinary-looking parents, a one-in-four child, carriage proven by the [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle). [Chromosome](https://one-course.com/books/biology/1/en/chapter/64-chromosomes#def-g9-chromosomes-chromosomes): one pair’s partners carrying P and p, halved and recombined. Molecule: one [gene](#def-g9-genes-and-dna-gene), one working and one misspelled [allele](#def-g9-genes-and-dna-gene), the spelling difference deciding everything.

**10.** From every judgment: in a carrier, p’s brokenness is invisible — the partner’s P covers it, so nothing about the carrier’s build “reports” the misspelling. What cannot be seen cannot be repaired away; hidden spellings persist indefinitely.

**11.** Her [trait](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) (no pigment) is inherited — p-with-p — but its *consequence* is lived in an [environment](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) full of sun: the missing pigment was the body’s sunscreen, so the [environment](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) writes on her more than on her shielded parents — inheritance setting the range, the world filling it in.

**12.** What copies: the double strand, each half rebuilding its partner at every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) division and generation. What mis-copied once: a few letters of the pigment [gene](#def-g9-genes-and-dna-gene), in one ancient [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes). Why the error persists: the photocopier is faithful — it copies whatever is there, misspellings with the same fidelity as sense.