---
title: "Becoming Male or Female"
book: "High School Biology"
subject: biology
language: en
chapter: 19
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/19-becoming-male-or-female
---

# Chapter 19 — Becoming Male or Female

At seven weeks after fertilisation, a human embryo has a pair of gonads that are neither testes nor ovaries, and two complete sets of ducts side by side — one that could become the male tract, one that could become the female one. Nothing visible distinguishes an embryo that will be a boy from one that will be a girl. Over the following weeks one set of ducts grows and the other withers, the gonads take one form or the other, and by the fourth month the anatomy is decided. The decision is taken by a single [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), and carried out by two hormones. This chapter follows that sequence, from the [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) received at fertilisation to the changes of [puberty](#prop-g11-becoming-male-female-puberty).

## 19.1 Sex chromosomes

**Definition 19.1 (Sex chromosomes).**

Of the 23 pairs of human [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), 22 are alike in the two sexes; the 23rd pair is made of the *sex chromosomes*: two X [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) in a female (XX), one X and one Y in a male (XY). The Y is small, carries few [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), and is transmitted only from father to son; every egg carries an X, and a sperm carries either an X or a Y, so the father’s sperm decides the chromosomal sex of the child, with equal chances.

**Proposition 19.2 (The undifferentiated stage).**

Until the seventh week, embryos of both chromosomal sexes are identical in their reproductive anatomy: a pair of undifferentiated gonads, each beside two ducts — the *Wolffian duct*, the precursor of the male tract (epididymis, vas deferens), and the *Müllerian duct*, the precursor of the female tract (oviduct, uterus). Sexual development is the transformation of this common plan into one of two forms; it is the differentiation of the gonad that starts it.

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

![From the common plan to two tracts. The testis, by its two hormones, keeps the Wolffian duct and removes the Müllerian one; in the absence of those hormones the Müllerian duct develops and the Wolffian one regresses.](https://one-course.com/images/onecourse/chapters/biology-2/g11-becoming-male-female/fig-a4e9428625c9.svg)

*From the common plan to two tracts. The testis, by its two hormones, keeps the Wolffian duct and removes the Müllerian one; in the absence of those hormones the Müllerian duct develops and the Wolffian one regresses.*

## 19.2 The gene that decides

**Proposition 19.3 (SRY and the gonad).**

The Y [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) carries a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), *SRY*, expressed in the undifferentiated gonad in the seventh week. Its [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) switches on the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that turn the gonad into a *testis*. In its absence — in an XX embryo — the gonad becomes an *ovary* some weeks later. The presence of [SRY](#prop-g11-becoming-male-female-sry), not the number of X [chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), decides the gonad.

**Evidence.** Rare XY individuals whose [SRY gene](#prop-g11-becoming-male-female-sry) carries a [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) develop ovaries and a female body; rare XX individuals who carry a fragment of the Y bearing [SRY](#prop-g11-becoming-male-female-sry), transferred onto an X, develop testes and a male body. Mice: an XX embryo into which the mouse *Sry* [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) has been inserted develops as a male. One [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of the Y is sufficient, and necessary, to make a testis. ∎

**Proposition 19.4 (Hormones of the foetal testis).**

Once formed, the testis secretes two hormones. *Testosterone* maintains the Wolffian ducts and makes them develop into the male internal tract, and shapes the external genitals in the male form. *Anti-Müllerian hormone* (AMH) makes the Müllerian ducts regress. An ovary secretes neither at this stage: without testosterone the Wolffian ducts regress, and without AMH the Müllerian ducts become the oviducts, the uterus and the upper vagina. The female tract is thus the form the ducts take in the absence of testicular hormones.

**Evidence.** Jost (1947) removed the gonads of rabbit foetuses in the womb before differentiation: every foetus, XX or XY, developed a female tract. Grafting a testis beside the gonads of a castrated foetus produced a male tract; implanting a crystal of testosterone alone kept the Wolffian ducts but did not remove the Müllerian ones, which persisted beside them. So the testis makes two products — one that builds the male ducts (testosterone) and one, later identified as AMH, that removes the female ones — and the ovary is not needed for the female tract to form. ∎

![Jost’s three experiments on rabbit foetuses, and their reading. The female tract needs no hormone; the male tract needs testosterone to build it and a second testicular hormone, AMH, to remove the female ducts.](https://one-course.com/images/onecourse/chapters/biology-2/g11-becoming-male-female/fig-3063199a2c0e.svg)

*Jost’s three experiments on rabbit foetuses, and their reading. The female tract needs no hormone; the male tract needs testosterone to build it and a second testicular hormone, AMH, to remove the female ducts.*

**Example 19.5 (The cascade in order).**

Fertilisation by a Y-bearing sperm; week 7, [SRY](#prop-g11-becoming-male-female-sry) expressed, the gonad becomes a testis; weeks 8 to 12, testosterone and AMH: Wolffian ducts kept, Müllerian ducts gone, external genitals shaped male; then quiescence until [puberty](#prop-g11-becoming-male-female-puberty). Fertilisation by an X-bearing sperm: no [SRY](#prop-g11-becoming-male-female-sry); weeks 10 to 12, the gonad becomes an ovary; no testicular hormones: Müllerian ducts develop, Wolffian ducts regress, external genitals take the female form. Each step depends on the one before, and a failure at any step changes everything downstream.

## 19.3 Puberty

**Proposition 19.6 (Puberty).**

The reproductive organs, formed before birth, remain inactive through childhood. At *puberty* — around eleven to thirteen years, earlier in girls — a centre in the brain begins to release, in pulses, a hormone that makes the pituitary gland secrete two hormones, LH and FSH, into the blood. These act on the gonads: the testis begins to make sperm and to secrete testosterone in adult amounts; the ovary begins its cycles and secretes oestrogens. The sex hormones then produce the *secondary sexual characteristics*: growth of the reproductive organs, body hair, voice, breasts, the distribution of muscle and fat, the growth spurt and then the closure of the growth plates of the bones. [Puberty](#prop-g11-becoming-male-female-puberty) is complete when the individual can reproduce.

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

![Sex hormones in the blood through childhood and adolescence, as a percentage of the adult level (rounded averages; individual timing varies by two or three years). Both are near zero until the brain gives the signal.](https://one-course.com/images/onecourse/chapters/biology-2/g11-becoming-male-female/fig-fb7e1d8d7f73.svg)

*Sex hormones in the blood through childhood and adolescence, as a percentage of the adult level (rounded averages; individual timing varies by two or three years). Both are near zero until the brain gives the signal.*

**Example 19.7 (Reading the curves).**

Oestrogens rise from about 9 in girls, testosterone from about 11 in boys; each reaches half its adult level within two years of starting and its plateau two or three years later. The growth spurt follows the rise by about a year, and the bones stop growing when the hormone reaches adult level — which is why girls, starting earlier, also stop growing earlier and end shorter on average.

## 19.4 Sex, in its several senses

**Proposition 19.8 (Distinguishing the levels).**

"Male" and "female" apply at several levels that usually agree but are distinct:

- *chromosomal* sex: XX or XY, fixed at fertilisation;
- *gonadal* sex: ovary or testis, decided by [SRY](#prop-g11-becoming-male-female-sry) ;
- *anatomical* sex: the tracts and external genitals, shaped by the hormones of the foetal gonad, and the secondary characteristics, shaped at [puberty](#prop-g11-becoming-male-female-puberty) ;
- *gender identity* , the sense a person has of being a man, a woman or neither, and *sexual orientation* , whom a person is attracted to — both facts about the person that are not deduced from the previous levels and are not chosen.

The biology of this chapter describes the first three; it neither determines nor judges the last.

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

**Example 19.9 (When the levels disagree).**

About one person in a few thousand is born with a combination the cascade does not usually produce: an XY person whose [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) lack the receptor for testosterone develops a female body, with testes that never descended and no uterus; an XX person with a testis-forming translocation of [SRY](#prop-g11-becoming-male-female-sry) develops a male body; an XY person with a non-working [SRY](#prop-g11-becoming-male-female-sry) develops as female. These variations show, better than the ordinary case, that each level is the outcome of a specific step — a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), a hormone, a receptor — and that the levels are separate facts.

**Method 19.10 (Reasoning about a case).**

1. [Chromosomes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) : is [SRY](#prop-g11-becoming-male-female-sry) present and working?
2. Gonad: testis if [SRY](#prop-g11-becoming-male-female-sry) acts, ovary otherwise.
3. Hormones of the foetal gonad: testosterone and AMH, or neither; and can the tissues respond (receptors)?
4. Ducts: Wolffian kept and Müllerian removed if both hormones act; Müllerian developed if neither acts; both if testosterone acts without AMH.
5. [Puberty](#prop-g11-becoming-male-female-puberty) : which sex hormone the gonad secretes decides the secondary characteristics — again through receptors.

**Remark 19.11 (One gene, and everything after it).**

The whole difference between the two anatomies traces back to whether one [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) on one [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) is present in the seventh week. What follows is a cascade of ordinary [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) biology — [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) expression, hormones, receptors — of the kind every chapter of this year has described. The plan the cascade starts from is common to both sexes, and the female form is the one it builds when the testis does not intervene.

## 19.5 Exercises

**Exercise 19.1 ★.**

Which parent’s gamete decides the chromosomal sex of a child, and why are the chances equal?

**Solution of Exercise 19.1.**

The father’s: every egg carries an X, while half the sperm carry an X and half a Y, so XX and XY are equally likely.

**Exercise 19.2 ★.**

Describe the reproductive anatomy of a seven-week embryo.

**Solution of Exercise 19.2.**

Two undifferentiated gonads, each with two ducts beside it: a Wolffian duct (future male tract) and a Müllerian duct (future female tract). Identical in XX and XY embryos.

**Exercise 19.3 ★.**

What does the [SRY gene](#prop-g11-becoming-male-female-sry) do, and where is it?

**Solution of Exercise 19.3.**

On the Y [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome); expressed in the undifferentiated gonad around week 7, its [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) switches on the [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that make the gonad a testis.

**Exercise 19.4 ★.**

Name the two hormones of the foetal testis and give the effect of each on the ducts.

**Solution of Exercise 19.4.**

[Testosterone](#prop-g11-becoming-male-female-hormones) maintains and develops the Wolffian ducts into the male tract; anti-Müllerian hormone makes the Müllerian ducts regress.

**Exercise 19.5 ★.**

What triggers [puberty](#prop-g11-becoming-male-female-puberty), and what are the secondary sexual characteristics?

**Solution of Exercise 19.5.**

A brain centre releases pulses of a hormone that makes the pituitary secrete LH and FSH, which activate the gonads. Secondary sexual characteristics: the changes produced by the sex hormones — body hair, voice, breasts, muscle and fat distribution, the growth spurt, growth of the reproductive organs.

**Exercise 19.6 ★★.**

In Jost’s experiments, what did the castrated XY foetus show, and what did it prove?

**Solution of Exercise 19.6.**

It developed a female tract, despite its Y [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome): the male tract requires the testis’s hormones, and the female tract forms without any gonad.

**Exercise 19.7 ★★.**

Why did the testosterone implant alone leave both sets of ducts in place? What did that reveal?

**Solution of Exercise 19.7.**

[Testosterone](#prop-g11-becoming-male-female-hormones) builds the male ducts but does not remove the female ones; the testis must secrete a second substance for that — AMH.

**Exercise 19.8 ★★.**

An XX mouse embryo receives the *Sry* [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) by [transgenesis](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#prop-g10-universal-dna-universal). Predict its gonads, its ducts and its external anatomy, and say what it cannot do as an adult.

**Solution of Exercise 19.8.**

Testes (Sry acts); Wolffian ducts kept, Müllerian regressed; male external anatomy. Sterile: sperm production requires [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of the Y that the transgene does not supply.

**Exercise 19.9 ★★.**

From the hormone figure, at what age does each hormone reach half its adult value? When does each plateau?

**Solution of Exercise 19.9.**

Oestrogens: 50% at about 11, plateau at about 14–15. [Testosterone](#prop-g11-becoming-male-female-hormones): 50% at about 13, plateau at about 16–17.

**Exercise 19.10 ★★.**

Explain why girls stop growing earlier than boys, using the curves.

**Solution of Exercise 19.10.**

The bones stop growing when the sex hormone reaches its adult level; oestrogens reach it about two years before testosterone does, so girls’ growth plates close earlier.

**Exercise 19.11 ★★.**

An XY embryo carries a [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) that destroys the [SRY](#prop-g11-becoming-male-female-sry) [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein). Follow the cascade and describe the person at birth and at [puberty](#prop-g11-becoming-male-female-puberty).

**Solution of Exercise 19.11.**

No testis: the gonad becomes an ovary-like gonad; no testosterone or AMH: Müllerian ducts develop, Wolffian regress, female external anatomy at birth. At [puberty](#prop-g11-becoming-male-female-puberty) the gonads, lacking normal eggs, secrete little; without hormone treatment secondary characteristics develop poorly.

**Exercise 19.12 ★★★.**

An XY foetus has working testes but [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that lack the receptor for testosterone; AMH acts normally. Predict the gonads, each set of ducts, the external anatomy and the secondary characteristics at [puberty](#prop-g11-becoming-male-female-puberty) (oestrogens are partly made from testosterone).

**Solution of Exercise 19.12.**

Testes ([SRY](#prop-g11-becoming-male-female-sry) works). Wolffian ducts regress (no response to testosterone); Müllerian ducts regress (AMH acts): no internal tract of either kind. External anatomy female (its male form needs testosterone action). At [puberty](#prop-g11-becoming-male-female-puberty), testosterone converted into oestrogens produces breasts and a female build, without body hair.

**Exercise 19.13 ★★★.**

In some reptiles the temperature of the egg, not a [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome), decides the sex. Explain in what sense the cascade after the gonad could be the same as in mammals, and which step differs.

**Solution of Exercise 19.13.**

Once a gonad has become a testis or an ovary, its hormones could shape the ducts and the body exactly as in mammals; what differs is the first step — a temperature-sensitive process replaces [SRY](#prop-g11-becoming-male-female-sry) as the switch that decides the gonad.

**Exercise 19.14 ★★★.**

The Y [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) carries fewer than a hundred [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) and the X about a thousand. Explain why a person needs at least one X but can lack a Y, and why Y-linked traits pass only from father to son.

**Solution of Exercise 19.14.**

The X carries hundreds of [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) needed by every [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), so no embryo survives without one; the Y carries almost nothing except the testis-determining and sperm-related [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), so XX individuals lack nothing they need. A Y passes only to sons because daughters receive the father’s X.

**Exercise 19.15 ★★★.**

Explain why the statement "the female form is the default of development" is a statement about hormones, not about importance, and what it says about the evolution of the two sexes from a common plan.

**Solution of Exercise 19.15.**

It means that the female tract develops when no testicular hormone intervenes — a fact about which signals are needed, not a ranking. It reflects a common embryonic plan on which one lineage of [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), the testis, imposes an alternative by two hormones: the two sexes are two outcomes of one programme, differing by a switch.

## 19.6 Problem: Jost’s Rabbits

**Problem 19.1.**

Weekend problem — the experiments that found the two hormones of the testis, reconstructed step by step, then applied to three human cases and to the timing of puberty

In the rabbit, the gonads differentiate around day 19 of gestation and the ducts around day 22. Jost operated on foetuses in the womb on day 19 and examined them at birth (day 30).

**Part I — The experiments.**

1. Foetuses of both chromosomal sexes, gonads removed on day 19: all are born with oviducts and a uterus and without male ducts. What does this show about the ovary’s role, and about the testis’s?
2. Why was it essential to operate before day 22?
3. Foetuses castrated on day 19, with a testis grafted into the abdomen: male ducts, no female ducts. What does the graft supply, and how does it reach the ducts?
4. Foetuses castrated on day 19, with a crystal of testosterone implanted: male ducts present, female ducts also present. What does testosterone do, and what does it not do?
5. Deduce that the testis must make a second substance, and state its effect. How could its existence later be confirmed?

**Part II — One gonad removed.**

6. A single testis is removed from an XY foetus on day 19. Predict the ducts on each side of the body, knowing that the hormones act mostly on the nearer side.
7. A testis is grafted beside one ovary of an XX foetus. Predict each side.
8. A crystal of testosterone is implanted beside one ovary of an XX foetus. Predict each side.
9. Why do these one-sided experiments strengthen the conclusion of Part I?
10. Explain why an XX foetus with a testis graft would nevertheless be sterile as an adult.

**Part III — Three human cases.** Apply [Method 19.10](#met-g11-becoming-male-female-case) to each.

11. An XX person carries, on one X [chromosome](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-chromosome) , a fragment of Y containing [SRY](#prop-g11-becoming-male-female-sry) .
12. An XY person carries a non-working [SRY](#prop-g11-becoming-male-female-sry) .
13. An XY person with normal testes has [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that cannot respond to AMH.
14. In which of the three cases is the person likely to be fertile? Justify.
15. What do the three cases show about the relation between chromosomal sex and anatomical sex?

**Part IV — [Puberty](#prop-g11-becoming-male-female-puberty) in numbers.** From the hormone figure, take the growth spurt to begin one year after the hormone reaches 20% of its adult level and the bones to stop growing when it reaches 100%.

16. Estimate the age at which the growth spurt begins in girls and in boys, and the age at which growth stops.
17. How many years of growth does each sex have between the start of the spurt and its end?
18. A boy grows $8\,\mathrm{cm}$ a year during the spurt and a girl $7\,\mathrm{cm}$ . Estimate the height gained during the spurt by each, and comment on the average difference in adult height (about $13\,\mathrm{cm}$ ).
19. A girl whose oestrogen rise begins at 7 will grow fast early and stop early. Explain why doctors sometimes delay a very early [puberty](#prop-g11-becoming-male-female-puberty) with drugs.
20. State the result: the two hormones Jost’s rabbits revealed, the effect of each, and the one [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) that switches on the organ that makes them.

**Solution of Problem 19.1.**

**1.** The ovary is not needed for the female tract, which forms without any gonad; the testis is needed for the male tract and for the removal of the female ducts.

**2.** After day 22 the ducts have already started to differentiate under the gonad’s influence; the experiment must act before the decision.

**3.** The testis’s hormones, carried by the blood to the ducts: the graft, far from the ducts, still masculinises them.

**4.** [Testosterone](#prop-g11-becoming-male-female-hormones) builds the male ducts; it does not remove the female ones.

**5.** A second testicular substance must make the Müllerian ducts regress; it could be confirmed by extracting it from testes and implanting it alone, expecting female ducts to disappear and male ones not to form — which is what AMH does.

**6.** On the operated side, no testosterone or AMH: female duct, no male duct. On the intact side: male duct, female duct regressed.

**7.** On the graft’s side, male duct and no female duct; on the other side, female duct only.

**8.** On the crystal’s side, both ducts present; on the other side, the female duct only.

**9.** Each foetus serves as its own control: the difference between the two sides can only be due to the local presence of the testis, the testosterone or their absence.

**10.** Its ovaries are removed or overridden and the grafted testis, from another animal, makes no sperm in a foreign host; the tract is male but no gametes are produced.

**11.** [SRY](#prop-g11-becoming-male-female-sry) present: testis; testosterone and AMH: male tract, male anatomy; at [puberty](#prop-g11-becoming-male-female-puberty), testosterone: male characteristics. Sterile (no Y [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) for sperm).

**12.** No working [SRY](#prop-g11-becoming-male-female-sry): no testis; no hormones: female tract and anatomy; poorly developed gonads.

**13.** Testes; Wolffian ducts developed (testosterone acts); Müllerian ducts persist too (no response to AMH): a male with, in addition, a uterus and oviducts; male anatomy and [puberty](#prop-g11-becoming-male-female-puberty).

**14.** Only the third: functional testes and male ducts, although the persisting uterus may hinder the descent of the testes. The first lacks sperm [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene); the second has no functional gonads.

**15.** Chromosomal sex predicts anatomical sex only through the steps between: a working [SRY](#prop-g11-becoming-male-female-sry), working hormones, working receptors. Change any step and the two levels separate.

**16.** Girls: 20% at 10, spurt from about 11, stop at about 15. Boys: 20% at about 12.5, spurt from about 13.5, stop at about 17.

**17.** Girls about 4 years, boys about 3.5 — but boys start from a taller base, having grown at the childhood rate two years longer.

**18.** Boys about $3.5 \times 8 = 28\,\mathrm{cm}$, girls $4 \times
7 = 28\,\mathrm{cm}$: the spurts are similar; the adult difference comes mostly from the two extra years of childhood growth before the boys’ spurt.

**19.** An early rise brings the growth plates to closure years early: the child grows fast now but stops at a short adult height. Delaying the hormones prolongs childhood growth.

**20.** [Testosterone](#prop-g11-becoming-male-female-hormones), which builds the male ducts, and anti-Müllerian hormone, which removes the female ones; both made by the testis, an organ switched on by the single [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) [SRY](#prop-g11-becoming-male-female-sry).