Primary & Middle School Biology · Grades 1–9
65Genes and DNA
The chromosomes carry the determinants — but a thread is not yet a text. What, along a chromosome, is one determinant? And what is the thread made of, that a nucleus thousandths of a millimetre wide can hold instructions for a whole human being (Exercise 63.12)? Two names answer, and they are the most famous in modern biology: the gene, and 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 carrying the instructions for one trait’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 come in pairs (Proposition 64.2) — so every gene comes in two copies, one per partner, one from each parent. The copies may be the same allele 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): the silent allele rides the second partner;
- the group-O child of A- and B-parents (Problem 63.1): 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, three alleles, 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 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 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 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.
Example 65.5 (Seeing it yourself)
DNA is kitchen-extractable: mash a banana or an onion, add salty water and a little washing-up liquid (membranes undone — Definition 45.1’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 — the actual molecule of this chapter, in actual centimetres.
Proposition 65.6 (What DNA’s structure explains)
The double strand is not decoration — each property answers an old question:
- copying (Proposition 29.7’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;
- alleles: versions of a 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. Mostly neutral, sometimes harmful, occasionally useful: mutation is where fresh spellings, and ultimately all alleles, come from — the raw novelty Chapter 68 will need;
- reading: a cell uses a gene by reading its sequence and building the corresponding working molecule — and each cell kind reads its own selection of the library: Exercise 64.12’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 writes its genes in the same DNA letters and reads them by the same code — oak, yeast, whale, the microbes of Chapter 43, you. This is the unity of life (Proposition 29.2) at its foundation, and the kinship tree’s (Remark 44.8) strongest evidence yet: a shared language argues a shared origin. It is also why genetic engineering works at all — a gene moved between species is still readable, as a sentence photocopied into another book.
Method 65.8 (The three-level reading)
Any heredity question can now be read at three levels — practice switching:
- family level: who shows, who carries, who transmits (Chapter 63);
- chromosome level: which threads, halved and restored, carry the deal (Chapter 64);
- molecule level: which gene, which alleles, what spelling — and, for a new trait, what mutation.
One story, three magnifications; an answer is complete when it runs at all three.
65.3 Exercises
Exercise 65.1 ★
Define gene and allele, with the blood-group example.
Solution
Solution of Exercise 65.1.
A gene is a segment of a chromosome carrying one trait’s instructions, at a fixed address; alleles are its versions — the blood-group gene’s A, B and O.
Exercise 65.2 ★
Why does every gene come in two copies, and where do they sit?
Solution
Solution of Exercise 65.2.
Because chromosomes come in pairs: one copy of each gene rides each partner — one from the mother, one from the father, at the same address.
Exercise 65.3 ★
What is DNA’s information, physically? How many letters, of how many kinds, in a human deck?
Solution
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
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.
Exercise 65.5 ★
What is a mutation, and what are its three possible characters?
Solution
Solution of Exercise 65.5.
A rare copying error — changed letters creating a new allele. Mostly neutral, sometimes harmful, occasionally useful.
Exercise 65.6 ★
State Remark 65.7’s fact and its two consequences.
Solution
Solution of Exercise 65.6.
All living things write and read their genes in the same DNA letters and code. Consequences: the strongest evidence for common origin — and the workability of moving genes between 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
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: list the allele pairs behind each of the four groups.
Solution
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 in this chapter’s terms: what does a neuron read that a skin cell does not?
Exercise 65.10 ★★
Describe the kitchen extraction and what each kitchen ingredient does.
Solution
Solution of Exercise 65.10.
Mash the fruit (cells opened); salty water and washing-up liquid (membranes — cell’s and nucleus’s envelopes — dissolved, DNA freed and kept dissolved); strain (debris out); cold spirit layered on top (DNA undissolves at the boundary): whitish spoolable threads — the molecule itself.
Exercise 65.11 ★★
Run Method 65.8’s three levels on the chapel chin of Problem 63.1.
Solution
Solution of Exercise 65.11.
Family level: the chin recurs along reproduction’s lines, carried unshown through one generation. Chromosome level: its determinant rides one thread, halved into gametes and restored at fertilization, dealt to some children and not others. Molecule level: it is an allele — a spelling of some face-shaping gene — copied faithfully down the century since the mutation that first wrote it.
Exercise 65.12 ★★★
“Mutation writes, the shuffle deals, expression shows.” Assign each verb its mechanism and chapter — and state which of the three creates genuinely new spellings.
Solution
Solution of Exercise 65.12.
Mutation writes: copying errors create new spellings — molecule level, this chapter. The shuffle deals: halving and fertilization hand each child a fresh combination — chromosome level, Chapter 64 (and Proposition 54.5). Expression shows: of the two copies dealt, what is displayed and what rides silent — this chapter’s two-copy rule. Only mutation creates genuinely new spellings; the other two recombine and reveal what mutation 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 whose working allele, P, builds a skin-and-hair pigment; long ago, a copying error created the broken allele 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, sun-fragile eyes (the condition exists across many species).
Part I — Molecule level.
- Name p’s origin, by Proposition 65.6 — what physically happened, once, in some ancestor’s germ line?
- Why does P-with-p look ordinary? Which vocabulary word describes p’s ride?
- Why does p-with-p change the build? What is missing, and from which chapter’s kind of machinery (Definition 65.1)?
- The condition appears across many species. What does Remark 65.7 predict about their pigment genes’ spellings?
Part II — Chromosome and family levels.
- 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.
- Their p-with-p daughter is born to two ordinary-looking parents. Which album mystery (Example 63.5) has just run at full mechanism?
- The daughter’s own children with a P-with-P partner: what do they all carry, and what do none of them show?
- Trace the allele’s vehicles for one transmission: p’s ride from grandmother to grandchild, thread and gamete by thread and gamete.
Part III — The three-level write-up.
- Write the case at the three levels of Method 65.8, one sentence each.
- 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?
- Sun protection matters more for the daughter than for her carrier parents. Connect trait, allele pair and environment in one sentence (Proposition 63.4’s weave).
- Close with the photocopier image made exact: what copies, what mis-copied once, and why the error is now copied faithfully.
Solution
Solution of Problem 65.1.
1. A mutation: a copying error in the pigment gene, once, in some ancestor’s gamete 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’s product — is simply never built. The trait is the absence of a working spelling.
4. That their pigment genes are recognizably the same gene — similar sequences in the same code — kin-copies of one ancestral text, broken independently in each 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 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 of a pair; her gamete’s halving dealt that partner into the egg or sperm that made the carrier parent; the parent’s halving dealt it again into the gamete that made the grandchild — thread to gamete to zygote, twice over.
9. Family: two ordinary-looking parents, a one-in-four child, carriage proven by the birth. Chromosome: one pair’s partners carrying P and p, halved and recombined. Molecule: one gene, one working and one misspelled allele, 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 (no pigment) is inherited — p-with-p — but its consequence is lived in an environment full of sun: the missing pigment was the body’s sunscreen, so the environment 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 division and generation. What mis-copied once: a few letters of the pigment gene, in one ancient gamete. Why the error persists: the photocopier is faithful — it copies whatever is there, misspellings with the same fidelity as sense.