Human Physiology, 14th edition (2016)

702

20.1 SEXUAL REPRODUCTION

A particular gene on the Y chromosome induces the

embryonic gonads to become testes. The embryonic tes-

tes secrete testosterone, which induces the development

of male accessory sex organs and external genitalia. The

absence of testes in a female embryo causes the develop-

ment of the female accessory sex organs.

by chance; mechanisms must exist to transmit the blueprint

(genetic code) from one generation to the next. Sexual repro-

duction, in which genes from two individuals are combined in

random and novel ways with each new generation, offers the

further advantage of introducing great variability into a pop-

ulation. This diversity of genetic constitution helps to ensure

that some members of a population will survive changes in the

environment over evolutionary time.

In sexual reproduction, germ cells, or gametes (sperm

and ova), are formed within the gonads (testes and ovaries)

by a process of reduction division, or meiosis (chapter 3; see

fig. 3.30). During this type of cell division, the normal number

of chromosomes in human cells—46—is halved, so that each

gamete receives 23 chromosomes. Fusion of a sperm cell and

ovum (egg cell) in the act of fertilization results in restoration

of the original chromosome number of 46 in the zygote, or fer-

tilized egg. Growth of the zygote into an adult member of the

next generation occurs by means of mitotic cell divisions, as

described in chapter 3. When this individual reaches puberty,

mature sperm or ova will be formed by meiosis within the

gonads so that the life cycle can be continued ( fig. 20.1 ).

Sex Determination

Autosomal and Sex Chromosomes

Each zygote inherits 23 chromosomes from its mother and 23

chromosomes from its father. This does not produce 46 different

chromosomes, but rather 23 pairs of homologous chromosomes.

The members of a homologous pair, with the important excep-

tion of the sex chromosomes, look like each other and contain

similar genes (such as those coding for eye color, height, and

Linda was diagnosed with endometriosis, for which she

received a series of shots and birth control pills. Her

symptoms subsided over some months, and she eventu-

ally stopped that treatment and decided to get pregnant.

However, she had difficulty getting pregnant and experi-

enced painful menstruations over the following year. She

noticed that the hair on her head was thinning, though

her body hair was growing. Suspecting that she had uter-

ine fibroids, she went to her gynecologist, who performed

an ultrasound. He told her that she might have fibroids,

but her symptoms were most likely caused by polycys-

tic ovarian syndrome. He advised her to lose weight and

exercise. A year later, Linda was overjoyed when her

pregnancy test result was positive.

The new terms and concepts you will encounter

include:

• Pulsatile secretion of GnRH, and hormonal changes
  during the menstrual cycle


• Endometrial and ovarian changes during the
  menstrual cycle


• hCG and pregnancy tests

Clinical Investigation

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Explain how the chromosomal sex of the zygote
   affects the formation of the gonads.


2. Describe the development of male and female
   accessory sex organs.

“A chicken is an egg’s way of making another egg.” Phrased
in more modern terms, genes are “selfish.” Genes, according
to this view, do not exist in order to make a well-functioning
chicken (or other organism). The organism, rather, exists and
functions so that the genes can survive beyond the mortal life
of individual members of a species. Whether or not one accepts
this rather cynical view, it is clear that reproduction is one of
life’s essential functions.
The incredible complexity of structure and function in liv-
ing organisms could not be produced in successive generations

Figure 20.1 The human life cycle. Numbers in

parentheses indicate the haploid state (23 chromosomes) or

diploid state (46 chromosomes).

Adult male Adult female

Testes Ovaries

Meiosis

Ovum

(23)

Sperm

(23)

Fertilization

Zygote

(46)

Growth by

mitosis