---
title: "Diversification of Living Things"
book: "High School Biology"
subject: biology
language: en
chapter: 24
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/24-diversification-of-living-things
---

# Chapter 24 — Diversification of Living Things

Bread wheat has 42 [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), in six sets of seven: it is the sum of three wild grasses that crossed, twice, in the fields of the first farmers. A python has three hundred vertebrae and a mouse thirty, yet they build their backbones with the same [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene). A lichen on a rock is two organisms, a fungus and an alga, that have lived as one for so long that neither can be found alone. And a young chimpanzee learns to crack nuts with a stone by watching its mother, in a forest where the chimpanzees a hundred kilometres away have never learnt. [Mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) and the shuffle of the last chapter are not the only ways living things become different; this chapter surveys the others.

## 24.1 More than mutation

**Proposition 24.1 (Sources of diversification).**

Point [mutations](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) and the sexual shuffle create new [alleles](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) and new combinations of them. Beyond these, the diversity of living things has been produced by processes that change the genome on a larger scale — duplicating [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), importing [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) from other [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species), adding whole sets of [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) — by changes in the way existing [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are used during development, by associations between [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species), and, in animals, by behaviour transmitted without any [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) at all.

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

## 24.2 New genes from old

**Proposition 24.2 (Gene duplication and divergence).**

An unequal [crossing-over](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#prop-g12-meiosis-genetic-shuffling-crossingover) ([Chapter 23](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#ch-g12-meiosis-genetic-shuffling)) or a copying error can leave a [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) with two copies of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene). One copy continues its original job; the other, freed of that constraint, accumulates [mutations](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) and may acquire a new function — or be lost. Repeated over evolutionary time, this produces *gene families*: sets of related [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) descended from one ancestor, whose degrees of similarity record the order of the duplications.

**Evidence.** The three [opsins](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors) and rhodopsin of [Chapter 21](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#ch-g11-the-eye) are one such family; the globins are another: the several [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) for the chains of haemoglobin — one used in the embryo, one in the foetus, two in the adult — and the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of myoglobin, the oxygen store of muscle, lie in clusters on two [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), are 40% to 80% identical in sequence, and their tree matches the order in which the [vertebrate](https://one-course.com/books/biology/2/en/chapter/6-body-plans-and-common-ancestry#def-g10-common-ancestry-bodyplan) groups appeared. Genomes carry hundreds of such families, and a sizeable fraction of every genome consists of duplicated segments. ∎

![How a gene family grows. One gene is duplicated; the copies diverge by mutation until each does a different job; further duplications repeat the process. The globin family supplies different haemoglobin chains at different stages of life.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/fig-a80e39d3abf6.svg)

*How a [gene family](#prop-g12-diversification-of-life-duplication) grows. One [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is duplicated; the copies diverge by [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) until each does a different job; further duplications repeat the process. The globin family supplies different haemoglobin chains at different stages of life.*

**Proposition 24.3 (Horizontal gene transfer).**

[Genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) can also enter a genome from another [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species). Bacteria exchange plasmids freely ([Chapter 18](https://one-course.com/books/biology/2/en/chapter/18-bacteria-and-antibiotic-resistance#ch-g11-antibiotic-resistance)); viruses carry fragments of their hosts’ [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) from one [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to another and leave copies of their own [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) behind; and comparison of genomes shows that even animals and plants have acquired [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) this way. About 8% of the human genome is of viral origin, and at least one of those [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), once a virus’s tool for fusing [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), now builds the placenta.

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

**Proposition 24.4 (Hybridisation and polyploidy).**

Two related [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) can sometimes cross; the hybrid carries one [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) set from each and is usually sterile, since its [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) have no partners to pair with at [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis). If, by an error of division, the hybrid doubles its [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), every [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) gains a partner, [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis) works, and a new fertile [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) — a *polyploid* — exists at once, isolated from both parents. Half of all flowering plant [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) have polyploidy in their history; bread wheat, cotton, tobacco and the potato are among them.

**Evidence.** Wheat: the wild einkorn has $2n = 14$; emmer, $2n = 28$, contains the einkorn set and the set of another wild grass, [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) by [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome); bread wheat, $2n = 42$, adds the set of a third. The three sets can be identified in the bread wheat [karyotype](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) and matched to the living wild [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species), and the crosses can be repeated in the laboratory. A cordgrass that appeared on the coast of Europe in the nineteenth century, fertile and vigorous, has the doubled [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) set of a sterile hybrid of two [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) that had never met before ships brought one across the Atlantic. ∎

![The origin of bread wheat: two hybridisations, each followed by a doubling of the chromosomes. Each letter is a set of seven chromosomes from one wild species; bread wheat carries six sets.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/fig-b3f71b456c58.svg)

*The origin of bread wheat: two hybridisations, each followed by a doubling of the [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information). Each letter is a set of seven [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) from one wild [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species); bread wheat carries six sets.*

## 24.3 Same genes, different uses

**Proposition 24.5 (Developmental genes and the timing of their expression).**

The [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that lay out an animal’s [body plan](https://one-course.com/books/biology/2/en/chapter/6-body-plans-and-common-ancestry#def-g10-common-ancestry-bodyplan) during development — which end is the head, where the limbs go, how many segments the trunk has — are shared, with little change in sequence, across the animals: the same family of *developmental genes* organises a fly, a mouse and a human. What differs between [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) is not so much these [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) as *where, when and how strongly* they are switched on. A change in the regulation of a [developmental gene](#prop-g12-diversification-of-life-development) can alter the shape of a body part or the number of repeated parts without changing any [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein).

**Evidence.** The [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that define the regions of the trunk are expressed in the same order from head to tail in a mouse and a python; in the python the region that makes ribbed vertebrae extends along almost the whole body, and the region that would make limbs never receives the signal to do so: three hundred vertebrae and no legs, from the same [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) expressed over different lengths. Among Darwin’s finches, the depth of the beak follows the level at which one growth signal is expressed in the embryonic beak: more signal, deeper beak. Raising that signal artificially in a chicken embryo produces a deep, finch-like beak. ∎

![Same genes, different extents. The regional genes that pattern the trunk are the same in a mouse and a python; in the python the "ribbed trunk" state is expressed along almost the whole axis and the limb-forming signal is never given.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/fig-dcbebf48015d.svg)

*Same [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), different extents. The regional [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that pattern the trunk are the same in a mouse and a python; in the python the "ribbed trunk" state is expressed along almost the whole axis and the limb-forming signal is never given.*

**Example 24.6 (Beaks by the dose).**

Thirteen [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) of finch on one group of islands have beaks from needle-thin to nutcracker-deep, and eat accordingly. Their beak [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are the same; the embryos of the deep-beaked [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) express one growth signal earlier and more strongly in the tissue that becomes the beak. A single change in the timing or amount of one signal, not a new [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), separates an insect-eater from a seed-crusher — which is why such differences can arise in a few thousand generations.

![Four of Darwin’s finches, drawn by John Gould in 1845 from the specimens the Beagle brought back: the same bird in every other respect, with beaks from a nutcracker to a needle. One growth signal, dosed differently in the embryo, separates them. Engraving, public domain.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/img-bd51f01356a3.jpg)

*Four of Darwin’s finches, drawn by John Gould in 1845 from the specimens the *Beagle* brought back: the same bird in every other respect, with beaks from a nutcracker to a needle. One growth signal, dosed differently in the embryo, separates them. Engraving, public domain.*

## 24.4 Living together: symbiosis

**Definition 24.7 (Symbiosis).**

A *symbiosis* is a lasting, close association of two [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) from which both benefit. A lichen is a fungus housing algal [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that feed it by [photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) while it shelters and waters them; a coral is an animal housing algae in its tissues; the roots of most plants are wrapped in fungi that supply minerals in exchange for sugar; the guts of animals hold bacteria that digest what the animal cannot. The association produces organisms, structures and ways of life that neither partner could achieve alone.

![Lichens on a granite boulder. Each patch is a fungus and an alga grown into one body: a form of life, able to colonise bare rock, that neither partner can take alone.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/img-2c8841044032.jpg)

*Lichens on a granite boulder. Each patch is a fungus and an alga grown into one body: a form of life, able to colonise bare rock, that neither partner can take alone.*

**Proposition 24.8 (The symbiosis inside every cell).**

The [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of every [eukaryotic cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-prokaryote), and the [chloroplasts](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of plant [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), descend from free-living bacteria that were engulfed by an ancestral [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) some two billion years ago and stayed. The [eukaryotic cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-prokaryote) itself is the outcome of a [symbiosis](#def-g12-diversification-of-life-symbiosis).

**Evidence.** [Mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and [chloroplasts](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) have their own [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), a small circle like a bacterium’s, and their own [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle), of the bacterial kind, sensitive to the [antibiotics](https://one-course.com/books/biology/2/en/chapter/18-bacteria-and-antibiotic-resistance#def-g11-antibiotic-resistance-antibiotic) that block bacterial but not eukaryotic [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle); they multiply by dividing in two, and cannot be made anew by the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); they are wrapped in two membranes, the inner one resembling a bacterial membrane; and their [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are closest, in sequence, to those of specific groups of living bacteria — purple bacteria for [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle), cyanobacteria for [chloroplasts](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle). ∎

![The origin of mitochondria. A bacterium engulfed by an ancestral cell was kept and multiplied inside it; its descendants are the mitochondria of every eukaryote, still carrying their own bacterial DNA and ribosomes. Chloroplasts arose the same way from a photosynthetic bacterium.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/fig-d2fe78ad8415.svg)

*The origin of [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle). A bacterium engulfed by an ancestral [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) was kept and multiplied inside it; its descendants are the [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of every eukaryote, still carrying their own bacterial [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) and [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle). [Chloroplasts](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) arose the same way from a photosynthetic bacterium.*

## 24.5 Diversity without genes: behaviour

**Proposition 24.9 (Transmitted behaviour).**

In many animals, part of what an individual does is learnt from others — by imitation, teaching or copying — and passed on in turn: a *culture*, transmitted without [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene). Populations of the same [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) can therefore differ in behaviour as they differ in [alleles](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), and a behaviour can spread through a population in a generation rather than over the many that a [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) would need.

**Evidence.** Chimpanzee communities in different forests use different tools — stones to crack nuts here, sticks to fish for termites there, leaves as sponges elsewhere — and the differences do not follow genetic relatedness or the availability of materials; a young chimpanzee acquires its community’s set by watching its elders. Songbirds raised in isolation sing a crude song; raised hearing adults, they sing the local dialect, which changes across a range like a human accent. A troop of macaques on an island learnt to wash sweet potatoes in the sea from one young female within a decade, and its descendants still do. ∎

![A chimpanzee fishing for termites with a prepared stick. The technique is learnt by watching, and differs from one community to another: a tradition, transmitted without genes.](https://one-course.com/images/onecourse/chapters/biology-2/g12-diversification-of-life/img-340269f0ce1d.jpg)

*A chimpanzee fishing for termites with a prepared stick. The technique is learnt by watching, and differs from one community to another: a tradition, transmitted without [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene).*

**Method 24.10 (Naming the source of a difference).**

Faced with a difference between two organisms or populations:

1. Is it in the sequence of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) ( [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) , [allele](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) )? In the number of copies of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) (duplication)? In the presence of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) from elsewhere (horizontal transfer)? In the number of [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) sets (polyploidy)?
2. Is it in when, where or how much a shared [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is expressed during development (regulation)?
3. Is it the result of an association with another [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) ( [symbiosis](#def-g12-diversification-of-life-symbiosis) )?
4. Is it learnt (behaviour transmitted by culture) — inherited, but not genetically?

More than one answer is often right; the questions are the inventory of this chapter.

**Remark 24.11 (Diversification and selection).**

Every process here produces variation; none of them decides what survives. A [polyploid](#prop-g12-diversification-of-life-polyploidy) wheat, a python’s expression pattern, a lichen, a nut-cracking tradition each persisted because it worked where it arose. The sorting of variation — selection, and chance — is the subject of the next chapter; this one has laid out how much there is to sort, and how many ways there are of making it.

## 24.6 Exercises

**Exercise 24.1 ★.**

List five sources of diversification besides point [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation).

**Solution of Exercise 24.1.**

[Gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) duplication and divergence; [horizontal gene transfer](https://one-course.com/books/biology/2/en/chapter/18-bacteria-and-antibiotic-resistance#prop-g11-antibiotic-resistance-variation); hybridisation with polyploidy; changes in the regulation of [developmental genes](#prop-g12-diversification-of-life-development); [symbiosis](#def-g12-diversification-of-life-symbiosis); transmitted behaviour.

**Exercise 24.2 ★.**

What is a [gene family](#prop-g12-diversification-of-life-duplication), and how does one arise?

**Solution of Exercise 24.2.**

A set of related [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) descended from one ancestral [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) by successive duplications, each copy then diverging by [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) to a distinct function ([opsins](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors), globins).

**Exercise 24.3 ★.**

Why is a hybrid between two [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) usually sterile, and how can [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) doubling make it fertile?

**Solution of Exercise 24.3.**

Its [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), one set from each parent [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species), have no homologues to pair with at [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis), so gametes are unbalanced. Doubling gives every [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) an identical partner: pairing and [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis) become regular and the plant is fertile.

**Exercise 24.4 ★.**

Give three lines of evidence that [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) descend from bacteria.

**Solution of Exercise 24.4.**

Their own circular [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information); bacterial-type [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) sensitive to antibacterial [antibiotics](https://one-course.com/books/biology/2/en/chapter/18-bacteria-and-antibiotic-resistance#def-g11-antibiotic-resistance-antibiotic); multiplication by division; a double membrane; sequences closest to a group of living bacteria.

**Exercise 24.5 ★.**

What is meant by a culture in animals? Give an example.

**Solution of Exercise 24.5.**

A behaviour learnt from other members of the group and passed on without [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene): nut-cracking with stones in some chimpanzee communities and not others.

**Exercise 24.6 ★★.**

Emmer wheat has $2n = 28$ and einkorn $2n = 14$. How many [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) has their hybrid, and how many pairs can form at its [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis)? Explain its sterility.

**Solution of Exercise 24.6.**

$14 + 7 = 21$ [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information): 7 A from einkorn, 7 A and 7 B from emmer. Seven pairs (A with A) can form; the 7 B [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) have no partner and are distributed at random, so the gametes are unbalanced: sterile.

**Exercise 24.7 ★★.**

The $\alpha$ and $\beta$ globin [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are 50% identical; the $\beta$ and $\gamma$ [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) 80%. Which duplication is older? Draw the tree.

**Solution of Exercise 24.7.**

The $\alpha$–$\beta$ split (50% identical) is older than the $\beta$–$\gamma$ split (80%): $\alpha$ branches first, then $\beta$ and $\gamma$ separate from each other.

**Exercise 24.8 ★★.**

Explain how a python can have no legs although it carries the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that build legs in a lizard.

**Solution of Exercise 24.8.**

The limb [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are present but the signal that switches them on in the embryo’s flank is not given, or is given and then stopped; the difference is in the regulation of expression, not in the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene).

**Exercise 24.9 ★★.**

Two finch [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) differ in beak depth by a factor of two but have identical beak [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene). Where does the difference lie, and why can it arise quickly?

**Solution of Exercise 24.9.**

In the amount and timing of a growth signal in the embryonic beak: a regulatory difference. A change in regulation needs only a small [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) in a control sequence, whereas a new [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) would need many; it can arise and be selected within thousands of generations.

**Exercise 24.10 ★★.**

A lichen grown from its fungus alone is a shapeless mould; from its alga alone, a green film. Explain in what sense the lichen is a new kind of organism.

**Solution of Exercise 24.10.**

Its form, its capacity to colonise bare rock, its resistance to drought and its slow growth belong to neither partner alone; they emerge from the association. The lichen is a compound organism with properties of its own.

**Exercise 24.11 ★★.**

A population of chimpanzees cracks nuts; a neighbouring one, across a river, does not, although nuts and stones abound on both sides. Using [Method 24.10](#met-g12-diversification-of-life-name), decide the source of the difference and justify.

**Solution of Exercise 24.11.**

Transmitted behaviour: materials are available on both sides, the populations are closely related, and the difference follows the river — a barrier to contact and hence to learning, not to [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) or resources.

**Exercise 24.12 ★★★.**

A [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of the human genome is nearly identical to a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of a virus and to nothing in any other mammal except the primates. Propose its history, and say how you would test it.

**Solution of Exercise 24.12.**

A virus inserted its [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) into the genome of a primate ancestor’s germ [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) some tens of millions of years ago; the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) was inherited and kept. Test: the same [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) should sit at the same chromosomal position in all primates and be absent from that position in other mammals; its sequence should be closest to the virus’s.

**Exercise 24.13 ★★★.**

[Antibiotics](https://one-course.com/books/biology/2/en/chapter/18-bacteria-and-antibiotic-resistance#def-g11-antibiotic-resistance-antibiotic) that block bacterial [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) have side effects on the patient’s [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) at high doses. Explain why, and what this says about the ancestry of [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle).

**Solution of Exercise 24.13.**

Mitochondrial [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) are of the bacterial type and bind the same [antibiotics](https://one-course.com/books/biology/2/en/chapter/18-bacteria-and-antibiotic-resistance#def-g11-antibiotic-resistance-antibiotic): at high doses the drug slows the [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)’s [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) synthesis. The sensitivity is inherited from the [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)’s bacterial ancestor.

**Exercise 24.14 ★★★.**

Explain why a new [polyploid](#prop-g12-diversification-of-life-polyploidy) [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) is isolated from its parents from the first generation, while a new [allele](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) takes many generations to spread. What does this imply about the speed at which plant [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) can form?

**Solution of Exercise 24.14.**

The [polyploid](#prop-g12-diversification-of-life-polyploidy)’s [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) number no longer matches either parent’s, so crosses with them give sterile offspring: isolation is immediate. A new [allele](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) must spread through a population over many generations before it distinguishes a group. A plant [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) can therefore arise in a single generation.

**Exercise 24.15 ★★★.**

"All diversity comes from [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation)." Discuss in a paragraph: in what sense the statement is true at bottom, and in what senses the processes of this chapter go beyond it.

**Solution of Exercise 24.15.**

At bottom, every new sequence — a duplicated copy that diverges, a regulatory change, the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of a transferred plasmid — began as a [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) somewhere. But duplication, transfer and polyploidy move and multiply whole [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) and genomes at once; regulatory change produces new forms without new [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein); [symbiosis](#def-g12-diversification-of-life-symbiosis) combines genomes of different [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species); and culture transmits variation with no [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) at all. [Mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) supplies the letters; these processes rearrange whole pages and books.

## 24.7 Problem: Three Grasses in a Loaf

**Problem 24.1.**

Weekend problem — the history of bread wheat reconstructed from chromosomes, a python’s genes read beside a mouse’s, and the bacterium inside every cell put on trial

Einkorn (AA), goat grass 1 (BB) and goat grass 2 (DD) each have $2n = 14$. Emmer is AABB and bread wheat AABBDD.

**Part I — The [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) of wheat.**

1. Give the number of [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) in emmer and in bread wheat.
2. The hybrid of einkorn and goat grass 1 has 14 [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) , 7 of each kind. How many pairs of homologues form at its [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis) ? What are its gametes like?
3. Explain why a doubling of its [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) restores fertility: count the pairs.
4. The hybrid of emmer and goat grass 2 has 21 [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) . Which of them can pair, and why is it sterile?
5. After doubling, bread wheat has 42. How many pairs form at its [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis) , and how many [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) does a pollen grain carry?

**Part II — Reading the history.**

6. Bread wheat carries three copies of most [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) , one on each set. Explain why a [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) destroying one copy usually has no visible effect, and what this allows over time.
7. Emmer was cultivated 10 000 years ago and bread wheat appears in archaeological sites 8 000 years ago. Where and when did the second hybridisation probably occur? What had to be growing side by side?
8. Laboratory crosses of emmer with goat grass 2, followed by chemical doubling, give fertile plants resembling bread wheat. Which conclusion of this history does that confirm?
9. Bread wheat cannot cross with einkorn to give fertile offspring. Is it a separate [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) ? Justify with the definition of [Chapter 5](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#ch-g10-biodiversity-scales) .
10. A crop breeder wants to introduce a disease-resistance [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) from goat grass 2 into bread wheat. Explain why this is easier than from an unrelated plant.

**Part III — The python’s [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene).**

11. The regional [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of the trunk are 98% identical between mouse and python. What does that say about the source of the difference in their bodies?
12. In the mouse embryo the "ribbed trunk" [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is active over 13 segments; in the python over about 300. Name the kind of change involved.
13. The python embryo forms tiny buds where hindlimbs would be, which then stop growing. What does this suggest about the limb [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) and the signal that would activate them?
14. Mice engineered to express the "ribbed trunk" [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) over the lumbar region grow extra ribs there. What does this experiment add to the comparison?
15. Compare the two kinds of change: bread wheat’s and the python’s. Which changed the genome’s size, which its use?

**Part IV — The bacterium inside.**

16. List four features of [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) that fit a bacterial origin, and for each say what you would expect if [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) had instead been built by the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) from scratch.
17. Mitochondrial [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) carries only 37 [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) , while the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) needs about 1500 [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) . Where are the other [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) , and what happened to them over two billion years?
18. Every [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of a person comes from the egg, none from the sperm. What follows for the inheritance of mitochondrial [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) , and for tracing maternal ancestry?
19. A lichen and a [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) are both symbioses. In what way has the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) ’s gone further?
20. State the result: the three [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) in a loaf and the number of [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) each contributed; the one word that names what differs between the python’s and the mouse’s use of their shared [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) ; and the [organelle](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) that is a former bacterium.

**Solution of Problem 24.1.**

**1.** Emmer 28, bread wheat 42.

**2.** None: the A and B [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) are not [homologous](https://one-course.com/books/biology/2/en/chapter/6-body-plans-and-common-ancestry#def-g10-common-ancestry-homology). Gametes receive random assortments of the 14, almost never a complete set.

**3.** With 28 [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), each A has an identical A partner and each B a B: 14 pairs, regular [meiosis](https://one-course.com/books/biology/2/en/chapter/23-meiosis-and-genetic-shuffling#def-g12-meiosis-genetic-shuffling-meiosis), balanced gametes of 14.

**4.** None can pair: 7 A, 7 B and 7 D [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), each without a homologue. Sterile.

**5.** 21 pairs; a pollen grain carries 21.

**6.** The other two copies still supply the [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein). Redundant copies are free to accumulate [mutations](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) and diverge — duplication on the scale of a whole genome.

**7.** In fields of the region where emmer was grown, between 10 000 and 8 000 years ago, with wild goat grass 2 growing as a weed among the crop.

**8.** That the two steps — hybridisation and doubling — are sufficient to produce bread wheat from its parents: the history can be re-run.

**9.** Yes: it cannot produce fertile offspring with einkorn, so by the definition it is a distinct [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species), although it descends from it.

**10.** Goat grass 2’s D [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) are [homologous](https://one-course.com/books/biology/2/en/chapter/6-body-plans-and-common-ancestry#def-g10-common-ancestry-homology) to the wheat’s D set and pair with them, so a cross followed by back-crosses can move the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) in; an unrelated plant’s [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) would not pair at all.

**11.** The [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) themselves hardly differ; the difference in the bodies must lie in how the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are used.

**12.** A change in the regulation of expression — the extent of the region along the axis where the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is active.

**13.** The limb [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are present and begin to act; the signal that sustains limb growth is missing, so the buds stop. Again regulation, not the loss of the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene).

**14.** It shows that changing where the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is expressed is sufficient to change the body region: the correlation between expression and form is causal.

**15.** Bread wheat’s change added whole sets to the genome (its size); the python’s changed where shared [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) are used (their regulation).

**16.** Own circular [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) (expected: none, all [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) nuclear); bacterial [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) (expected: eukaryotic [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)); multiplication by division (expected: assembly from parts); double membrane (expected: a single membrane like other [organelles](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)). And their [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) resemble bacterial [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) (expected: nuclear-type [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene)).

**17.** In the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle): over time most of the bacterium’s [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) were transferred to the nuclear genome, and the [organelle](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)’s own genome shrank to a remnant.

**18.** Mitochondrial [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) pass only from mother to child; a person’s mitochondrial [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) traces the unbroken maternal line.

**19.** The [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) has given up most of its [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) to its host and cannot exist outside the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); the lichen’s partners keep their genomes and can, with difficulty, be separated.

**20.** Einkorn (14, A), goat grass 1 (14, B), goat grass 2 (14, D): 42 [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information); "regulation"; the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle).