---
title: "Chromosomes"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 64
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/64-chromosomes
---

# Chapter 64 — Chromosomes

[Remark 63.7](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#rem-g9-heredity-and-traits-question) cornered the determinants 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). Catch a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) at the right moment and the [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) shows its hand: its contents wind up into countable, photographable bodies — the [chromosomes](#def-g9-chromosomes-chromosomes). Counting them, in body [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) and in [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes), turns out to explain almost everything the family album demanded: the halves, the pairs, the shuffle, and the flip that decides whether a baby is a boy or a girl.

## 64.1 The nucleus, caught in the act

**Definition 64.1 (Chromosomes).**

A *chromosome* is a threadlike body 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), visible (well stained, at division time) as the [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts)’s contents coil up for transport. Between divisions the same threads run loose and invisible — working state; at division they condense — moving state. Photographed and sorted, a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s chromosomes make its *karyotype*: the full deck, laid out.

**Proposition 64.2 (The human counts).**

The [karyotypes](#def-g9-chromosomes-chromosomes) deliver three numbers that carry the whole chapter:

1. every human body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) holds *46* [chromosomes](#def-g9-chromosomes-chromosomes) — [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) , [liver](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) , [neuron](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-neuron) alike ( [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 divisions copy the deck faithfully to every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) );
2. laid out, the 46 sort into *23 pairs* , the two partners of a pair matching in size and banding;
3. the *[gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes)* break the rule: [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) and [sperm cell](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-male) carry *23* — one [chromosome](#def-g9-chromosomes-chromosomes) from each pair, no pair complete.

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

**Example 64.3 (The arithmetic of fertilization).**

The counts snap together like a proof. [Gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) at 23 each; [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) ([Proposition 54.2](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core)) adds them: the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) holds $23 + 23 = 46$ — the pairs restored, one partner of each pair from the mother, one from the father. Every body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of the child then carries both parents’ contribution — which is exactly what [Remark 23.3](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#rem-g4-animal-reproduction-mix) observed from outside, foal and blaze, a chapter-generation ago.

**Remark 64.4 (Why gametes must halve).**

Suppose they did not: 46-chromosome [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) would make 92-chromosome children, 184-chromosome grandchildren — the deck doubling every generation. The [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes)’ special halving division — each receiving *one partner from each pair* — is what keeps the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’ count steady at 46, generation after generation. And note the bonus: *which* partner of each pair a [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) receives is settled independently, pair by pair — $2^{23}$ possible decks from one parent, the shuffle of [Proposition 54.5](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-shuffle) counted at last.

## 64.2 The chromosomes carry the determinants

**Proposition 64.5 (Evidence that the deck matters).**

That the [chromosomes](#def-g9-chromosomes-chromosomes) *are* the determinants’ vehicles is not assumption but evidence:

- they behave exactly as [Proposition 63.6](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#prop-g9-heredity-and-traits-conclusions) requires: present in every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , halved into [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) , restored at [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) , selectable partner-by-partner;
- [karyotype](#def-g9-chromosomes-chromosomes) accidents prove the cargo: a deck with one [chromosome](#def-g9-chromosomes-chromosomes) extra or missing changes the whole body’s development — an extra copy of the small pair 21, for instance, produces the recognizable set of [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) called Down syndrome. One thread miscounted, many [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) shifted: the threads carry trait-instructions;
- and one pair announces its cargo in every [karyotype](#def-g9-chromosomes-chromosomes) : the *sex [chromosomes](#def-g9-chromosomes-chromosomes)* .

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

**Definition 64.6 (X, Y, and the coin flip).**

Pair 23 comes in two versions: two matching *X* [chromosomes](#def-g9-chromosomes-chromosomes) — the [karyotype](#def-g9-chromosomes-chromosomes) of a girl; or one X and a small *Y* — a boy. The halving then runs the flip: every [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) carries an X; a [sperm cell](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-male) carries X or Y, half and half. [Fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) by an X-sperm makes XX, by a Y-sperm XY — one chance in two ([Chapter 63](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#ch-g9-heredity-and-traits)’s language of chance, finally with its mechanism), decided by which swimmer arrives, at the instant of [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps).

![The 23rd pair’s coin flip: the egg’s X meets an X- or a Y-sperm — and the karyotype is decided with the meeting.](https://one-course.com/images/onecourse/chapters/biology-1/g9-chromosomes/fig-5af3e6856116.svg)

*The 23rd pair’s coin flip: the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)’s X meets an X- or a Y-sperm — and the [karyotype](#def-g9-chromosomes-chromosomes) is decided with the meeting.*

**Example 64.7 (Old questions, short answers).**

The deck settles historic scores. Whether a child is a boy or a girl owes nothing to either parent’s wishes, diets or seasons — it is the flip, run in [Definition 64.6](#def-g9-chromosomes-xy)’s mechanism (and, for the record of old injustices: the deciding difference rides in the *sperm*). Equal halves also answer the family album: mother and father contribute one full half-deck each — 23 and 23 — and [heredity](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-heredity)’s arithmetic honors both lines alike.

**Method 64.8 (Reading a karyotype).**

Handed a [karyotype](#def-g9-chromosomes-chromosomes) photograph:

1. count: 46 expected in a body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , 23 in a [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) ;
2. pair: partners matched by size and banding, 22 ordinary pairs plus the 23rd;
3. read the 23rd: XX or XY;
4. check the counts: any pair with one or three members explains, by [Proposition 64.5](#prop-g9-chromosomes-evidence) , a shifted development.

## 64.3 Exercises

**Exercise 64.1 ★.**

What is a [chromosome](#def-g9-chromosomes-chromosomes), and when is it visible?

**Solution of Exercise 64.1.**

A threadlike body of the [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) — its contents coiled for transport. Visible, well stained, when the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) divides; loose and invisible between divisions.

**Exercise 64.2 ★.**

Give the three human counts, and where each holds.

**Solution of Exercise 64.2.**

46 in every body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell); sorted as 23 pairs; 23 — one partner per pair — in the [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes).

**Exercise 64.3 ★.**

Run the [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) arithmetic, and say what it restores.

**Solution of Exercise 64.3.**

$23 + 23 = 46$: [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) restores the pairs, one partner of each from the mother, one from the father — both parents in every body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of the child.

**Exercise 64.4 ★.**

Why must [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) halve the deck? Show the runaway otherwise.

**Solution of Exercise 64.4.**

Unhalved [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) would double the deck each generation: 46-gametes make 92-children, then 184-grandchildren. The halving keeps the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) steady at 46.

**Exercise 64.5 ★.**

What are the two versions of pair 23, and which parent’s [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) decides between them?

**Solution of Exercise 64.5.**

XX (a girl’s [karyotype](#def-g9-chromosomes-chromosomes)) and XY (a boy’s). The father’s: [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) all carry X, sperm carry X or Y half and half — the arriving sperm decides.

**Exercise 64.6 ★.**

What does an extra copy of [chromosome](#def-g9-chromosomes-chromosomes) 21 produce, and what does that prove about the threads?

**Solution of Exercise 64.6.**

The recognizable trait-set called Down syndrome. It proves the threads carry trait-instructions: one thread’s count changed, many [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) shift.

**Exercise 64.7 ★★.**

Where exactly does [Proposition 54.5](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-shuffle)’s shuffle happen, in this chapter’s terms — and how large is one parent’s shuffle space?

**Solution of Exercise 64.7.**

In the halving division: which partner of each pair a [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) receives is settled independently, pair by pair — $2^{23}$ possible half-decks per parent, before [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) multiplies two such spaces together.

**Exercise 64.8 ★★.**

Match each requirement of [Proposition 63.6](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#prop-g9-heredity-and-traits-conclusions) to the [chromosome](#def-g9-chromosomes-chromosomes) behavior that satisfies it.

**Solution of Exercise 64.8.**

Determinants, not [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits): threads carry instructions — compact, cell-borne. Carried unshown: both partners ride in every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) whatever is displayed. Selections differing child to child: the halving deals one partner per pair, independently each time.

**Exercise 64.9 ★★.**

“The chance of a boy is one in two at every [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle), whatever came before.” Defend with the mechanism — why does a run of three girls change nothing?

**Solution of Exercise 64.9.**

Each [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) is a fresh flip: the sperm [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) is half X, half Y at every attempt, and no past [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) reaches into the race. Three girls are three fair flips — the fourth is as fair.

**Exercise 64.10 ★★.**

Run [Method 64.8](#met-g9-chromosomes-read) on: (a) a 46,XX body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell); (b) a 23,Y [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes); (c) a body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) with 47, the extra in pair 21.

**Solution of Exercise 64.10.**

(a) A body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of a girl or woman: full deck, XX. (b) A [sperm cell](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-male) carrying the Y: a [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes), half-deck — its [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) would make a boy. (c) A body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) with the pair-21 extra: 47 threads — the [karyotype](#def-g9-chromosomes-chromosomes) of Down syndrome, XX: a girl’s.

**Exercise 64.11 ★★.**

History blamed queens for kingdoms’ daughters. Correct the record with [Example 64.7](#ex-g9-chromosomes-answers), [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) by [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes).

**Solution of Exercise 64.11.**

Every [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) the queens supplied carried an X — their contribution to pair 23 admits no alternative. The X-or-Y variety rides in the kings’ [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) alone: if the record must assign the flip’s coin, it sat in the royal sperm — and in any case the flip is chance, blamable on no one.

**Exercise 64.12 ★★★.**

The body’s every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) carries the full 46 — yet a [neuron](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-neuron) and 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) differ utterly ([Remark 45.6](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#rem-g6-cell-unit-of-life-twoways)). Formulate the puzzle this sets, and the direction of its resolution (the next chapter’s business: what, on a [chromosome](#def-g9-chromosomes-chromosomes), is *read*).

**Solution of Exercise 64.12.**

The puzzle: one deck, many [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) kinds — if every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) holds the same 46 threads of instructions, why is a [neuron](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-neuron) not 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)? Direction of resolution: [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) must differ in which instructions they *read*, not which they hold — the deck is a library, and specialization is a borrowing policy. What the readable units on a [chromosome](#def-g9-chromosomes-chromosomes) are is the next chapter’s question.

## 64.4 Problem: The Cytogenetics Bench

**Problem 64.1.**

Weekend problem — four karyotypes on one light table

A hospital cytogenetics lab’s teaching table: [karyotype](#def-g9-chromosomes-chromosomes) A — 46 [chromosomes](#def-g9-chromosomes-chromosomes), 23rd pair XY; B — 46, 23rd pair XX; C — 23 [chromosomes](#def-g9-chromosomes-chromosomes), one X among them; D — 47, with three copies at pair 21, 23rd pair XX.

**Part I — The readings.**

1. Read A, B and C by [Method 64.8](#met-g9-chromosomes-read) : whose [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) could each be?
2. C’s count is not an error. What kind of [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) is it, and what history of division does its 23 record?
3. Read D: count, pair, and the development it announces.
4. Which parent’s [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) decided A’s 23rd pair, and what were the odds at that [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) ?

**Part II — The arithmetic questions.**

5. A trainee asks why body [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of one person all agree with [karyotype](#def-g9-chromosomes-chromosomes) A. Answer with the copying divisions of [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) .
6. Another asks how two 46-cell parents made C’s 23. Name the special division and its rule.
7. Combine C with an X-bearing [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) , and with its own lab’s Y-bearing neighbor — wait: C *is* Y-free. State what C’s X guarantees about any child it helps make.
8. Compute the possible decks one parent can deal: state the pair-by-pair independence and the $2^{23}$ conclusion — then add [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) ’s pairing of two such deals.

**Part III — The counseling room.**

9. Parents of a child with [karyotype](#def-g9-chromosomes-chromosomes) D ask “what did we do wrong?” Give the mechanism’s honest answer about halving accidents — and what the child’s 47 threads do and do not change about the love in the room. (Answer as the counselor, with the [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) straight.)
10. A couple with three daughters asks the odds of a son next. Answer with the flip, and correct the run-of-luck instinct.
11. A visitor claims a diet can choose a baby’s sex. Audit the claim against [Definition 64.6](#def-g9-chromosomes-xy) : where is the decision made, and what could a diet reach?
12. Close the bench with one sentence: what the four photographs together prove about where [heredity](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-heredity) rides.

**Solution of Problem 64.1.**

**1.** A: a body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of a boy or man — 46, XY. B: a body [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of a girl or woman — 46, XX. C: a [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) — 23, one partner per pair; its X makes it either an [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) or an X-sperm.

**2.** A [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes). Its 23 records the special halving division: from a 46-cell, one partner of each pair taken, no pair complete.

**3.** 47 threads, the extra at pair 21, 23rd pair XX: the [karyotype](#def-g9-chromosomes-chromosomes) of a girl with Down syndrome — one miscounted thread, a shifted development.

**4.** The father’s — his sperm’s X-or-Y. Odds at the meeting: one in two, each way.

**5.** Because the body grew from one [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) by copying divisions: each division duplicates the deck and deals a full copy to each daughter [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — 46 faithful copies from [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) to [neuron](https://one-course.com/books/biology/1/en/chapter/60-nervous-communication#def-g8-nervous-communication-neuron).

**6.** The [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes)’ halving division, with its rule: one partner from each of the 23 pairs — a complete half-deck, no pair doubled, none dropped.

**7.** That the child receives an X from it: whatever partner arrives from the other side, C’s contribution to pair 23 is fixed — an [X-gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) writes X into the child’s 23rd pair unconditionally.

**8.** Each of the 23 pairs contributes either partner, independently: $2 \times 2 \times \dots$, 23 times — $2^{23}$ half-decks, several million, from one parent. [Fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) pairs one such deal from each parent: the two spaces multiply, and the shuffle’s arithmetic explains why siblings never repeat.

**9.** Nothing was done wrong: the extra 21 arises in a halving accident — a pair’s partners failing to separate — a mischance of the mechanism, not of the parents’ acts or worth. The 47th thread shifts the child’s development; it changes nothing about the child being wholly their child — the counseling truth and the cytogenetic truth agree.

**10.** One in two, exactly as at every [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle): the sperm [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) is half X, half Y at each [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), and past flips leave no trace in it. The run feels meaningful; the mechanism keeps no memory.

**11.** The decision is made at [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), by which sperm arrives — inside the father’s contribution, in the race’s last millimetres. A diet reaches neither the sperm [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population)’s half-and-half manufacture nor the race’s outcome: the claim has no link to block or bias — [Method 59.8](https://one-course.com/books/biology/1/en/chapter/59-contraception#met-g8-contraception-evaluate)’s question 1, failed again.

**12.** [Heredity](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-heredity) rides on countable threads 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): halved into [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes), restored at [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), trait-shifting when miscounted, sex-deciding at pair 23 — four photographs, one vehicle.