---
title: "Sexual Reproduction"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 54
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction
---

# Chapter 54 — Sexual Reproduction

Frog song in May, cherry blossom in April, the stag’s autumn bellow, a sunflower turning its crowded face to the bees: across the living world, enormous [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) is spent on one project — the next generation. Earlier years collected the pieces ([Chapter 23](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#ch-g4-animal-reproduction), [Chapter 31](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#ch-g5-flower-to-fruit)); this year assembles them into one theory, and asks the sharper question: why *two* parents at all?

![Enormous energy for one project: the male’s inflated vocal sac, convening the pond for the gametes’ meeting.](https://one-course.com/images/onecourse/chapters/biology-1/g8-sexual-reproduction/img-2e4ca9da8d36.jpg)

*Enormous [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) for one project: the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)’s inflated vocal sac, convening the pond for the [gametes](#def-g8-sexual-reproduction-gametes)’ meeting.*

## 54.1 One mechanism across the living world

**Definition 54.1 (Gametes).**

The reproductive [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of [Definition 23.1](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) bear the general name *gametes*: the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) gamete ([sperm cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) in [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), the [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) grain’s [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) in flowering [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)) and the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) gamete (the [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), in ovary or ovule). Gametes are single [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ([Chapter 29](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ch-g5-first-look-at-cells)), and each is half a beginning: alone, a gamete dies; joined, two make a life.

**Proposition 54.2 (The invariant core).**

Wherever [sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) runs — [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) or fir tree, snail or human — the core is invariant:

1. two parents produce [gametes](#def-g8-sexual-reproduction-gametes) , [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) and [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) ;
2. *[fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps)* : one [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [gamete](#def-g8-sexual-reproduction-gametes) joins one [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [gamete](#def-g8-sexual-reproduction-gametes) , forming a single new [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — the *[egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) fertilized* , or *zygote* ;
3. the [zygote](#prop-g8-sexual-reproduction-core) divides and divides ( [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) ), building a new individual that carries something from *both* parents ( [Remark 23.3](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#rem-g4-animal-reproduction-mix) ).

Around this core, everything else is variation: where the [gametes](#def-g8-sexual-reproduction-gametes) meet, how many, who cares for the young.

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

**Example 54.3 (The variations, sorted).**

The old collection files neatly under the core. External against internal [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) ([Proposition 23.6](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-where)): the meeting place. [Oviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi) against [viviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi) ([Definition 23.4](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi)): the [zygote](#prop-g8-sexual-reproduction-core)’s lodging. Codfish millions against elephant singletons ([Proposition 23.7](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-strategies)): the numbers ledger. [Pollination](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) by bee or wind ([Definition 31.3](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps)): the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [gamete](#def-g8-sexual-reproduction-gametes)’s travel service. One mechanism, a catalogue of settings.

**Remark 54.4 (Plants run the same core).**

Set [Chapter 31](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#ch-g5-flower-to-fruit) beside [Chapter 23](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#ch-g4-animal-reproduction) and the match is exact: [stamens](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) and [pistil](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) produce the [gametes](#def-g8-sexual-reproduction-gametes), the [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) tube is internal [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps)’s corridor, the [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) lodges the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)’s [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) with its packed lunch ([Definition 9.1](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed)). Only [vegetative reproduction](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-vegetative) ([Definition 24.5](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-vegetative)) stands apart — one parent, no [gametes](#def-g8-sexual-reproduction-gametes), exact copies: the contrast that makes the question of the next section sharp.

## 54.2 Why two parents?

**Proposition 54.5 (Sexual reproduction shuffles).**

The runner’s strawberry copies ([Remark 24.9](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#rem-g4-plants-without-seeds-copy)) show what one parent yields: sameness. Two parents yield the opposite. Because every [zygote](#prop-g8-sexual-reproduction-core) combines one [gamete](#def-g8-sexual-reproduction-gametes) from each parent — and, as a later chapter will show, no two [gametes](#def-g8-sexual-reproduction-gametes) even of one parent carry quite the same inheritance — [sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) deals every offspring a *new combination*. Siblings differ; no two [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) of one pod match; the variety within every [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) ([Remark 30.4](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#rem-g5-biodiversity-scales)) is largely this shuffle’s work.

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

**Example 54.6 (The orchard’s two bets).**

The orchard of [Remark 24.9](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#rem-g4-plants-without-seeds-copy) plays both games knowingly. Grafts and [cuttings](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#met-g4-plants-without-seeds-cutting): one parent, exact copies of the excellent tree — and one shared weakness, so a blight that fells one can fell all. Pips: two parents, every [seedling](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) a fresh combination — most unremarkable, some worse, a rare one better than either parent, and no two alike for a blight to sweep. Copying banks the present; shuffling explores the future.

**Remark 54.7 (Variety is the species’ reserve).**

Why the shuffle matters beyond the orchard: [Proposition 30.5](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#prop-g5-biodiversity-web) showed webs absorbing shocks through variety of [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species); within one [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species), variety of individuals plays the same part. When [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) turn — a new disease, a harsher winter — a varied [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) is likelier to hold someone suited to the new terms. What that hints at for the history of life, [Chapter 68](https://one-course.com/books/biology/1/en/chapter/68-how-species-change#ch-g9-how-species-change) will make explicit; note here only that [sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) is what keeps the reserve stocked.

## 54.3 Reading any species’ reproduction

**Method 54.8 (The reproduction dossier).**

For any [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species), compile:

1. [gametes](#def-g8-sexual-reproduction-gametes) : who produces which, where;
2. meeting: external or internal [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) , and the travel service (water, [mating](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-where) , pollinator, wind);
3. lodging: [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) laid or young carried; with what provisions;
4. numbers and care: the strategy line’s position ( [Proposition 23.7](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-strategies) );
5. season: when, and timed by what — next chapter’s question.

**Example 54.9 (Two dossiers side by side).**

The stickleback ([Exercise 23.10](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#exo-g4-animal-reproduction-10)): [gametes](#def-g8-sexual-reproduction-gametes) shed in water, external [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in a built nest, dozens, guarded by the father, spring. The cherry: [gametes](#def-g8-sexual-reproduction-gametes) in blossom, internal [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) down the [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) tube, [embryos](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) lodged in [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) inside [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform), thousands, provisioned then abandoned to the [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)’ delivery service ([Exercise 39.12](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#exo-g6-how-plants-spread-12)), timed to April. Same form, any [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species): the dossier is the theory in use.

## 54.4 Exercises

**Exercise 54.1 ★.**

Define [gamete](#def-g8-sexual-reproduction-gametes) and [zygote](#prop-g8-sexual-reproduction-core), and name the two [gametes](#def-g8-sexual-reproduction-gametes) for [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) and for flowering [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant).

**Solution of Exercise 54.1.**

A [gamete](#def-g8-sexual-reproduction-gametes) is a reproductive [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) or [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual); the [zygote](#prop-g8-sexual-reproduction-core) is the single new [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) their joining forms. [Animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal): [sperm cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) and [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual). Flowering [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant): the [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) grain’s [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) and the ovule’s [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual).

**Exercise 54.2 ★.**

State the invariant core in three steps.

**Solution of Exercise 54.2.**

Two parents produce [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) and [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [gametes](#def-g8-sexual-reproduction-gametes); [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) joins one of each into a [zygote](#prop-g8-sexual-reproduction-core); the [zygote](#prop-g8-sexual-reproduction-core) divides and divides, building a new individual carrying something from both parents.

**Exercise 54.3 ★.**

File under the core: external/internal [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), [oviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi)/[viviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi), millions/singletons — what does each pair vary?

**Solution of Exercise 54.3.**

The meeting place of the [gametes](#def-g8-sexual-reproduction-gametes); the lodging of the developing young; the numbers-and-care strategy. All settings around the same core.

**Exercise 54.4 ★.**

Match the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) parts to the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) roles: [stamen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts), [pistil](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts), [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) tube, [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed).

**Solution of Exercise 54.4.**

[Stamen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts): the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [gamete](#def-g8-sexual-reproduction-gametes) factory. [Pistil](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) (its [ovules](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#prop-g5-flower-to-fruit-roles)): the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) one. [Pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) tube: internal [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps)’s corridor. [Seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed): the lodged [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) with its provisions — the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)’s [oviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi) parcel.

**Exercise 54.5 ★.**

What does one-parent reproduction yield that two-parent never does — and the reverse?

**Solution of Exercise 54.5.**

One parent yields exact copies — sameness the shuffle can never give; two parents yield new combinations every time — variety that copying can never give.

**Exercise 54.6 ★.**

Why do siblings differ, by [Proposition 54.5](#prop-g8-sexual-reproduction-shuffle)?

**Solution of Exercise 54.6.**

Each sibling grew from a different pair of [gametes](#def-g8-sexual-reproduction-gametes), and no two [gametes](#def-g8-sexual-reproduction-gametes) carry quite the same inheritance: every [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) deals a fresh combination of the two parents’ contributions.

**Exercise 54.7 ★★.**

Explain the orchard’s two bets, and when each is the right one.

**Solution of Exercise 54.7.**

Copying (grafts, [cuttings](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#met-g4-plants-without-seeds-cutting)) banks a proven excellence — right when the present variety is exactly what is wanted; its price is shared weakness. Shuffling (pips) explores new combinations — right when improvement or insurance is wanted; its price is that most deals are unremarkable.

**Exercise 54.8 ★★.**

Compile [Method 54.8](#met-g8-sexual-reproduction-dossier) for the frog (all pieces are in [Example 23.2](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#ex-g4-animal-reproduction-song) and [Example 8.7](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-frog)).

**Solution of Exercise 54.8.**

[Gametes](#def-g8-sexual-reproduction-gametes): [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) from the [female](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), sperm from the [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), shed in the pond. Meeting: external [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) in water, the [males](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)’ song convening the pond. Lodging: [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in jelly, provisioned small. Numbers and care: thousands, uncared for. Season: spring — May’s chorus.

**Exercise 54.9 ★★.**

Compile the dossier for the dandelion — with one warning: many dandelions actually set [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) without [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), cloning by [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed). Which entries go strange, and what does the oddity teach about dossiers?

**Solution of Exercise 54.9.**

The meeting entry collapses — no [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) at all — yet the lodging, numbers and season entries read normally: plumed [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) by the thousand, in summer. The oddity teaches that the dossier is a checklist, not an assumption: fill it from evidence, and a “[seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed)” need not certify a shuffle. (The clone-by-seed dandelion gets a copying bet with a [dispersal](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#def-g6-how-plants-spread-dispersal) service — both worlds.)

**Exercise 54.10 ★★.**

A vineyard grows one famous grape variety by [cuttings](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#met-g4-plants-without-seeds-cutting) for two centuries; a pest arrives that relishes exactly that variety. Retell [Example 54.6](#ex-g8-sexual-reproduction-bets) as this vineyard’s story.

**Solution of Exercise 54.10.**

Two centuries of [cuttings](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#met-g4-plants-without-seeds-cutting) made the vineyard one shared constitution — excellence banked, weakness shared. The pest that relishes that constitution finds every vine equally to its taste: no varied individuals, no reserve, the copying bet called in at the worst table. The rescue, historically as in the exercise, is the other bet: crossing in resistant stock — the shuffle.

**Exercise 54.11 ★★.**

“A [gamete](#def-g8-sexual-reproduction-gametes) is half a beginning.” Justify the phrase twice: from the core’s step 2, and from the shuffle.

**Solution of Exercise 54.11.**

From the core: a [gamete](#def-g8-sexual-reproduction-gametes) alone dies — only [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps), joining two, makes the [zygote](#prop-g8-sexual-reproduction-core) that lives and divides. From the shuffle: each [gamete](#def-g8-sexual-reproduction-gametes) carries only one parent’s half of a combination that does not exist until the joining deals it.

**Exercise 54.12 ★★★.**

Why might a [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) that can do both — the strawberry, with [runners](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-runner) *and* [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) — keep both? Assign each mode its service, using [Remark 39.8](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#rem-g6-how-plants-spread-twospeeds) and [Remark 54.7](#rem-g8-sexual-reproduction-reserve).

**Solution of Exercise 54.12.**

The [runners](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-runner) are the short-range, sure service: exact copies densely occupying the good ground next door — this year’s proven [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), banked. The [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) are the long-range, exploring service: shuffled combinations scattered far, stocking the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’ reserve for changed [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) elsewhere. Keeping both buys the present and the future at once.

## 54.5 Problem: The Breeding Program

**Problem 54.1.**

Weekend problem — a plant breeder’s year, run on the theory

A breeder wants a new apple: crisp like variety A, blight-resistant like variety B. Her tools are exactly this chapter’s.

**Part I — The cross.**

1. She takes [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) from A’s [stamens](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) with a brush and touches it to B’s [pistils](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) , then bags the [flowers](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) against bees. Name each step’s role in the core — and why the bags matter ( [Method 31.8](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#met-g5-flower-to-fruit-diagnose) used the same brush).
2. What grows in B’s [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) after her visit, and what two inheritances does each pip combine?
3. She sows three hundred pips. Why does she expect three hundred *different* trees, by [Proposition 54.5](#prop-g8-sexual-reproduction-shuffle) ?
4. Most [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) will be unremarkable, a few worse than either parent, perhaps one just right. Which example of the chapter predicted exactly this spread?

**Part II — Fixing the winner.**

5. Year eight: one tree bears crisp, blight-resistant apples. If she sows its pips, what happens to the prized combination — and why?
6. Instead she multiplies the winner by grafting — which reproduction is that, and what does it guarantee?
7. Her whole future orchard will be that one tree’s copies. Name the risk she has accepted, and the chapter’s phrase for it.
8. Propose the standard insurance: what should her orchard also keep, and why does the strategy echo [Remark 54.7](#rem-g8-sexual-reproduction-reserve) ?

**Part III — The theory, audited.**

9. Her program used both of the orchard’s bets in sequence. Say which phase used which, and what each bought her.
10. A colleague breeds sticklebacks the same way — choosing parents, raising the shuffled young, keeping the best. List the dossier entries ( [Method 54.8](#met-g8-sexual-reproduction-dossier) ) where [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) practice differs from apple practice, and the core steps where it cannot differ.
11. The breeder says: “sex proposes, selection disposes — I only choose among what the shuffle deals.” Connect her slogan to the variety-reserve of [Remark 54.7](#rem-g8-sexual-reproduction-reserve) .
12. One sentence to close: what did the program need [sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) *for* , and what did it need [vegetative reproduction](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-vegetative) for?

**Solution of Problem 54.1.**

**1.** The brush plays pollinator: carrying [male](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) [gametes](#def-g8-sexual-reproduction-gametes) from A to B’s [pistils](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) — the meeting step. The bags exclude bees so no stray [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) adds a third parent: control of the cross — the fair test’s spirit at the [flower](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts).

**2.** [Seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) — each pip an [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) from one of B’s [egg cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) fertilized by one of A’s [pollen](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): A’s and B’s inheritances combined.

**3.** Because every pip is a different deal of the shuffle: different [gametes](#def-g8-sexual-reproduction-gametes) on both sides, so three hundred different combinations of crispness, resistance and all else.

**4.** The orchard’s two bets ([Example 54.6](#ex-g8-sexual-reproduction-bets)): most [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) unremarkable, some worse, a rare one better — the shuffle’s known spread.

**5.** The combination is shuffled away again: the winner’s pips deal *new* mixtures of its inheritance with its pollen-parent’s, and crisp-and-resistant scatters into fragments. Sowing the winner un-wins it.

**6.** Grafting — [vegetative reproduction](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-vegetative): one parent, exact copies. It guarantees the prized combination, tree after tree.

**7.** The shared-weakness risk of the copying bet: one constitution orchard-wide — the vineyard’s story waiting to happen.

**8.** A reserve of variety: other varieties interplanted, and breeding lines maintained from [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) — so that when a new blight or climate calls, the shuffle has stock to deal from. The species-level reserve, kept deliberately at orchard scale.

**9.** Phase one (the cross and the three hundred [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle)): the shuffle — it *found* the combination. Phase two (grafting the winner): the copy — it *kept* the combination. Exploration, then banking.

**10.** Differing entries: meeting (external, in water, no brush needed — the breeder just pairs the parents), lodging ([eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in a nest, not [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform)), care (the father fans the clutch), season. Invariant: [gametes](#def-g8-sexual-reproduction-gametes) made, [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) joining two into a [zygote](#prop-g8-sexual-reproduction-core), the [zygote](#prop-g8-sexual-reproduction-core) dividing into a shuffled individual — the core cannot differ, or it would not be [sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual).

**11.** The shuffle stocks the table — every generation deals combinations no one designed; the breeder’s choosing is just the reserve principle run on purpose: where nature’s changed [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) pick the suited, she picks the crisp and resistant. No variety dealt, nothing to choose.

**12.** [Sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) was needed to *create* the new combination; [vegetative reproduction](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-vegetative) to *multiply it unchanged*.