Sexual Reproduction
Chapter
9
Outline
	9.1	The Basics of Meiosis
	•	In males and females, the body cells are diploid and have 23 pairs of chromosomes; one pair is the sex chromosomes.129
	•	The human life cycle includes mitosis (for growth and repair) and meiosis (for the production of gametes).129
	•	The gametes are haploid and have only one chromosome from each pair of chromosomes. Fertilization restores the number of chromosomes.129
	•	Meiosis produces genetically different gametes. After fertilization, a zygote has a different combination of chromosomes than either parent.130–31 
	9.2	The Phases of Meiosis
	•	Meiosis I and meiosis II have four phases each.132
	•	Following meiosis I, the daughter cells are haploid, and following meiosis II, the chromosomes are no longer duplicated.
132–33
	9.3	Meiosis Compared to Mitosis
	•	Meiosis reduces the chromosome number during the production of gametes. Mitosis keeps the chromosome number constant during growth and repair of tissues.134
	•	In humans and many other animals, meiosis is part of the production of sperm in males and eggs in females.134
	9.4	Abnormal Chromosome Inheritance
	•	An error during meiosis can produce gametes that have too many or too few chromosomes.136
	•	A person with Down syndrome has three number 21 chromosomes, usually because the egg had two copies instead of one.136
	•	Turner females are XO, and Klinefelter males are XXY.137
Because of a process called meiosis, two individuals can create offspring that are genetically different from themselves and from each other. In humans, more than 70 trillion different genetic combinations are possible from the mating of two individuals.
Meiosis is the type of cell division that occurs during sexual reproduction. The process plus maturation result in four cells, called gametes. In humans, the male gametes are sperm, and the female gametes are eggs. Meiosis that helps produce sperm is called spermatogenesis, and meiosis that helps produce eggs is called oogenesis.
Although spermatogenesis and oogenesis follow the same steps overall, they differ in some ways. One major difference pertains to the age at which the process begins and ends. In males, sperm production does not begin until puberty, but then continues throughout a male’s lifetime. In females, the process of producing eggs has started before the female is even born and ends around the age of 50, a time called menopause. 
Another difference concerns the number of gametes that can be produced. In males, sperm production is unlimited, whereas in females, the number of potential egg cells is usually set at birth. Furthermore, meiosis in women need not go to completion, and very few cells ever finish the entire process. Many of the birth control methods available to females, such as the pill or “the patch,” work by manipulating hormones so that cells called oocytes do not complete meiosis and an egg is never released for fertilization. 
In this chapter, you will see how meiosis is involved in the production of gametes. You will also learn how meiosis compares with mitosis, the cell division process discussed in Chapter 8. 
9.1		The Basics of Meiosis
Human beings, like most other animals, practice sexual reproduction in which two parents pass chromosomes to their offspring. Because each child receives a unique combination of the parents’ chromosomes, children are not exactly like either parent. How does sexual reproduction bring about the distribution of chromosomes to offspring in a way that ensures not only the correct number of chromosomes but also a unique combination?
Let’s begin by examining the chromosomes of one of the parents—for instance, the father. To view the chromosomes, a cell can be photographed just prior to division so that a picture of the chromosomes is obtained. The picture can be entered into a computer and the chromosomes electronically arranged by numbered pairs. The members of a pair are called homologous chromosomes, or homologues, because they have the same size, shape, and constriction (location of the centromere) (Fig. 9.1). Homologous chromosomes also have the same characteristic banding pattern upon staining because they contain genes for the same traits. Alternate forms of a gene are called alleles. For example, if a gene codes for the trait of finger length, the allele on one homologue may be for short fingers, and the allele at the same location on the other homologue may be for long fingers. Both sister chromatids of the same homologue carry the same alleles because sister chromatids are identical. One member of a homologous pair was derived from a chromosome inherited from the male parent, and the other member was derived from a chromosome inherited from the female parent.
Both males and females normally have 23 pairs of chromosomes, but one of these pairs is of unequal length in males. The larger chromosome of this pair is the X chromosome, and the smaller is the Y chromosome. In contrast, females have two X chromosomes. The X and Y chromosomes are called the sex chromosomes because they contain the genes that determine gender. The other chromosomes, known as autosomes, include all the pairs of chromosomes except the X and Y chromosomes. Twenty-three pairs of chromosomes, or 46 altogether, is called the diploid (2n) number of chromosomes in humans. Half this number is the haploid (n) number of chromosomes. 
The Human Life Cycle
The term life cycle in sexually reproducing organisms refers to all the reproductive events that occur from one generation to the next. The human life cycle involves two types of nuclear division: mitosis and meiosis (Fig. 9.2).
During development and after birth, mitosis is involved in the continued growth of the child and the repair of tissues at any time. As a result of mitosis, each somatic (body) cell has the diploid number of chromosomes. 
During sexual reproduction, a special type of nuclear division called meiosis reduces the chromosome number from the diploid to the haploid number in such a way that the gametes (sperm and egg) have one chromosome derived from each homologous pair of chromosomes. In males, meiosis is a part of spermatogenesis, which occurs in the testes and produces sperm. In -females, meiosis is a part of oogenesis, which occurs in the ovaries and produces eggs. After the sperm and egg join during fertilization, the resulting cell, called the zygote, again has homologous pairs of chromosomes. The zygote then undergoes mitosis and differentiation of cells to become a fetus, and eventually a new human being. 
Meiosis is important because, if it did not halve the chromosome number, the gametes would contain the same number of chromosomes as the body cells, and the number of chromosomes would double with each new generation. Within a few generations, the cells of sexually reproducing organisms would be nothing but chromosomes! Because of meiosis, however, the chromosome number stays constant in each generation.
Overview of Meiosis
Meiosis results in four daughter cells because it consists of two divisions, called meiosis I and meiosis II (Fig. 9.3). Before meiosis I begins, each chromosome has duplicated and is composed of two sister chromatids. During meiosis I, the homologous chromosomes of each pair come together and line up side-by-side. What draws them together is still unknown. This so-called synapsis results in a tetrad, an association of four chromatids that stay in close proximity until they separate. Following synapsis and during meiosis I, the homologous chromosomes of each pair separate. This separation means that one chromosome from each homologous pair goes to each daughter nucleus. No rules restrict which chromosome goes to which daughter nucleus, and therefore, all possible combinations of chromosomes may occur within the gametes.
Following meiosis I, the daughter nuclei have half the number of chromosomes, and the chromosomes are still -duplicated. Therefore, no duplication of chromosomes is needed between meiosis I and meiosis II. The chromosomes are dyads because each one is composed of two sister chromatids. During meiosis II, the sister chromatids of each dyad in the two daughter nuclei separate, and the -resulting four new daughter cells have the haploid number of chromosomes. If the parent cell has 4 chromosomes, then following meiosis, each daughter cell has 2 chromosomes. (Remember that counting the centromeres tells you the number of chromosomes in a nucleus.)
Because of meiosis, the gametes have all possible combinations of chromosomes. Notice in Figure 9.3 that the final pair of daughter cells above does not have the same chromosomes as the pair below. Why not? Because the homologous chromosomes of each pair separated during meiosis I. Other chromosome combinations are possible in addition to those depicted. It’s possible that the daughter cells could have only red or only blue chromosomes in Figure 9.3. (The red and blue signify that the chromosomes have come from different parents.) 
Crossing-Over
Aside from allowing gametes to have different chromosomes, sexual reproduction can also lead to a difference in alleles on the chromosomes. This change is not due to mutations, but results from “shuffling” of the genetic material. A tetrad, as you just learned, is made up of the sister chromatids of two homologous chromosomes. When a tetrad forms during synapsis, the nonsister chromatids may exchange genetic material, an event called crossing-over (Fig. 9.4). 
Recall that the homologous chromosomes carry genes for traits, such as finger length, but that one allele might call for short fingers, for example, while the other allele calls for long fingers. Therefore, when the nonsister chromatids exchange genetic material, the sister chromatids then have a different combination of alleles, and the resulting gametes will be genetically different. So in Figure 9.4, even though two of the gametes have the same chromosomes, these chromosomes may not have the same combination of alleles as before because of crossing-over. Crossing-over increases the variability of the gametes and, therefore, of the offspring.
The Importance of Meiosis
Meiosis is important first of all because the chromosome number stays constant in each new generation of individuals. When a haploid sperm fertilizes a haploid egg, the new individual has the diploid number of chromosomes. 
Second, the new individual is ensured a different combination of alleles than either parent because:
	1.	Crossing-over can result in different alleles on the sister chromatids of a homologous pair of chromosomes.
	2.	Meiosis produces gametes that have all possible combinations of the haploid number of chromosomes. 
	3.	At fertilization, a new combination of chromosomes can occur, and a zygote can have any one of a vast number of combinations of chromosomes. In humans, (223)2 or 70,368,744,000,000 chromosomally different zygotes are possible, even assuming no crossing-over. 
9.2		The Phases of Meiosis
The same four stages of mitosis-—prophase, metaphase, anaphase, and telophase—occur during both meiosis I (Fig. 9.5) and meiosis II (Fig. 9.6).
The First Division—Meiosis I
To help you recall the events of meiosis, keep in mind what meiosis accomplishes—namely, the production of gametes that have a reduced chromosome number and are genetically different from each other and from the parent cell. During Prophase I, the nuclear envelope fragments and the nucleolus disappears as the spindle appears but more important the condensing homologous chromosomes undergo synapsis to produce tetrads. The formation of tetrads helps prepare the homologous chromosomes for separation; it also allows crossing-over to occur between nonsister chromatids. Crossing-over “shuffles” the alleles on chromosomes. 
During metaphase I, the tetrads are attached to the spindle and aligned at the spindle equator. It does not matter which homologous chromosome faces which pole; therefore, all possible combinations of chromosomes can occur in the gametes. Following separation of the homologous chromosomes during anaphase I and re-formation of the nuclear envelopes during telophase, the daughter nuclei are haploid: Each daughter cell contains only one chromosome from each homologous pair. The chromosomes are dyads, and each still has two sister chromatids. No replication of DNA occurs during a period of time called -interkinesis.
The Second Division—Meiosis II
When you think about it, the events of meiosis II are the same as those for mitosis, except the cells are haploid. At the beginning of prophase II, a spindle appears while the nuclear envelope fragments and the nucleolus disappears. Dyads (one dyad from each pair of homologous chromosomes) are present, and each attaches to the spindle. During metaphase II, the dyads are lined up at the spindle equator. During anaphase II, the sister chromatids of each dyad separate and move toward the poles. Each pole receives the same number and kinds of chromosomes. In telophase II, the spindle disappears as nuclear envelopes form. 
During cytokinesis, the plasma membrane pinches off to form two complete cells, each of which has the haploid, or n, number of chromosomes. The gametes are genetically dissimilar because they can contain different combinations of chromosomes and because crossing-over changed which alleles are together on a chromosome. Because each cell from meiosis I undergoes meiosis II, four daughter cells -altogether are produced from the original diploid parent cell.
9.3		Meiosis Compared to Mitosis
Figure 9.7 compares meiosis to mitosis. Notice that:
	•	Meiosis requires two nuclear divisions, but mitosis requires only one nuclear division.
	•	Meiosis produces four daughter nuclei, and there are four daughter cells following cytokinesis. Mitosis followed by cytokinesis results in two daughter cells.
	•	Following meiosis, the four daughter cells are haploid and have half the chromosome number as the parent cell. Following mitosis, the daughter cells have the same chromosome number as the parent cell.
	•	Following meiosis, the daughter cells are genetically dissimilar to each other and to the parent cell. Following mitosis, the daughter cells are genetically identical to each other and to the parent cell.
The specific differences between these nuclear divisions can be categorized according to process and occurrence.
Process
To summarize the processes, Table 9.1 compares meiosis I to mitosis and Table 9.2 compares meiosis II to mitosis.
Meiosis I Compared to Mitosis
The following events distinguish meiosis I from mitosis:
	•	During meiosis I, tetrads form, and crossing-over occurs during prophase I. These events do not occur during mitosis.
	•	During metaphase I of meiosis, tetrads align at the spindle equator. The paired chromosomes have a total of four chromatids each. During metaphase in mitosis, dyads align at the spindle equator. 
	•	During anaphase I of meiosis, the homologous chromosomes of each tetrad separate, and dyads (with centromeres intact) move to opposite poles. During anaphase of mitosis, sister chromatids separate, becoming daughter chromosomes that move to opposite poles.
Meiosis II Compared to Mitosis
The events of meiosis II are just like those of mitosis except that in meiosis II, the cells have the haploid number of chromosomes.
Occurrence
Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms. In humans, meiosis occurs only in the testes and ovaries, where it is involved in the production of gametes. The function of meiosis is to provide gamete variation and to keep the chromosome number constant generation after generation. With fertilization, the full chromosome number is restored. Because unlike gametes fuse, fertilization also leads to variation among the offspring.
Mitosis is more common because it occurs in all tissues during embryonic development and also during growth and repair. The function of mitosis is to keep the chromosome number constant in all the cells of the body so that every cell has the same genetic material. Consider that reproductive cloning results in an individual with the same genes as the parent because a single diploid nucleus gives genes to the new individual (see Fig. 12.2).
9.4		Abnormal Chromosome Inheritance
The normal number of chromosomes in human cells is 46, but occasionally humans are born with an abnormal number of chromosomes because improper meiosis occurred. Nondisjunction occurs during meiosis I when both members of a homologous pair go into the same daughter cell, or during meiosis II when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete (Fig. 9.8). If an egg that ends up with 24 chromosomes instead of 23 is fertilized with a normal sperm, the result is a trisomy, so called because one type of chromosome is present in three copies. If an egg that has 22 chromosomes instead of 23 is fertilized by a normal sperm, the result is a monosomy, so called because one type of chromosome is present in a single copy.
Down Syndrome
Down syndrome is trisomy 21, in which an individual has three copies of chromosome 21. In most instances, the egg contained two copies of this chromosome instead of one. (In 23% of the cases studied, however, the sperm had the extra chromosome 21.)
Down syndrome is easily recognized by the following characteristics: short stature; an eyelid fold; stubby fingers; a wide gap between the first and second toes; large, fissured tongue; round head; palm crease; and unfortunately, mental retardation, which can sometimes be severe (Fig. 9.9).
The chance of a woman having a Down syndrome child increases rapidly with age, starting at about age 40. The frequency of Down syndrome is 1 in 800 births for mothers under 40 years of age and 1 in 80 for mothers over 40 years of age. However, most Down syndrome babies are born to women younger than age 40, because this is the age group having the most babies. 
Abnormal Sex Chromosome Number
Nondisjunction during oogenesis or spermatogenesis can result in gametes that have too few or too many X or Y chromosomes. Figure 9.8 can be used to illustrate nondisjunction of the sex chromosomes during oogenesis if we assume that the chromosomes shown represent X chromosomes. 
Just as having an abnormal number of autosomes can result in a condition such as Down syndrome, having additional X or Y chromosomes, or lacking them, also causes certain syndromes. Newborns with an abnormal sex chromosome number are more likely to survive than those with an abnormal autosome number, and the explanation is quite surprising. Normal females, like normal males, have only one functioning X chromosome. The other X chromosome (or any extra X chromosomes) become an inactive mass called a Barr body (after the person who discovered it).
In humans, the presence of a Y chromosome, not the number of X chromosomes, almost always determines maleness. The SRY (a gene on the Y chromosome) on the short arm of the Y chromosome produces a hormone called testis-determining factor, which plays a critical role in the development of male genitals. No matter how many X chromosomes are involved, an individual with a Y chromosome is a male, assuming a functional SRY is on the Y chromosome.
A person with Turner syndrome (XO) is a female. The O signifies the absence of a second sex chromosome. Turner syndrome females are short, with a broad chest and webbed neck. The ovaries, oviducts, and uterus are very small and underdeveloped. Turner females do not undergo puberty or menstruate, and their breasts do not develop (Fig. 9.10a). However, some have given birth following in vitro fertilization using donor eggs. These women usually have normal intelligence and can lead fairly normal lives if they receive hormone supplements.
A person with Klinefelter syndrome (XXY) is a male. A Klinefelter male has two or more X chromosomes in addition to a Y chromosome. The extra X chromosomes become Barr bodies. 
In Klinefelter males, the testes and prostate gland are underdeveloped. There is no facial hair, but some breast development may occur (Fig. 9.10b). Affected individuals have large hands and feet and very long arms and legs. They are usually slow to learn but not mentally retarded unless they inherit more than two X chromosomes. 
As with Turner syndrome, it is best for parents to know as soon as possible that their child has Klinefelter syndrome because much can be done to help the child lead a normal life.
The Chapter in Review
Summary
9.1		The Basics of Meiosis
Parents pass chromosomes to their children, who resemble them but not exactly because they receive a unique combination of their parents’ chromosomes and alleles. 
Diploid Versus HaploidDiploid cells in humans have 22 homologous pairs of autosomes and 1 pair of sex chromosomes for a total of 46 chromosomes. Males are XY and females are XX. Haploid cells have 22 autosomes and one sex chromosome, either an X or a Y for a total of 23 chromosomes.
The Human Life Cycle
The human life cycle has two types of nuclear division:
	•	Mitosis ensures that every body cell has 23 pairs of chromosomes. It occurs during growth and repair. 
	•	Meiosis occurs during formation of gametes. It ensures that the gametes are haploid and have 23 chromosomes, one from each of the pairs of chromosomes. 
Overview of Meiosis
Meiosis has two nuclear divisions called meiosis I and meiosis II. Therefore, meiosis results in four daughter cells. 
	•	Meiosis ISynapsis and formation of tetrads leads to separation of homologous chromosomes. Because no restrictions govern which member goes to which pole, the gametes will contain all possible combinations of chromosomes. 
	•	Meiosis IIThe chromosomes are still duplicated. The chromatids separate, forming daughter chromosomes. The daughter cells are haploid. If the parent cell has 4 chromosomes, each of the daughter cells has 2 chromosomes. All possible combinations of chromosomes occur among the daughter cells.
Crossing-OverCrossing-over between nonsister chromatids during meiosis I “shuffles” the alleles on the chromosomes so that each sister chromatid of a homologue has a different mix of alleles. 
Importance of MeiosisMeiosis is important because the chromosome number stays constant between the generations of individuals and because the daughter cells, and therefore the gametes, are genetically different. 
9.2		The Phases of Meiosis
Meiosis I 
	•	Prophase ITetrads form, and crossing-over occurs as chromosomes condense; the nuclear envelope fragments. 
	•	Metaphase ITetrads align at the spindle equator. Either homologue can face either pole.
	•	Anaphase IHomologues of each tetrad separate, and dyads move to the poles. 
	•	Telophase IDaughter nuclei are haploid, having received one duplicated chromosome from each homologous pair. 
Meiosis II 
	•	Prophase IIChromosomes condense, and the nuclear envelope fragments.
	•	Metaphase IIThe haploid number of dyads align at the spindle equator.
	•	Anaphase IISister chromatids separate, becoming daughter chromosomes that move to the poles.
	•	Telophase IIFour haploid daughter cells are genetically different from each other and from the parent cell. 
9.3		Meiosis Compared to Mitosis
The following diagrams show the differences between meiosis and mitosis. Duplication occurs before each begins.
Meiosis 
	•	In humans, meiosis occurs in the testes and ovaries where it produces the gametes.
	•	Because meiosis has two nuclear divisions, it produces four daughter cells. The daughter cells are haploid and genetically different from each other and from the parent cell. 
Mitosis 
	•	Mitosis occurs in the body cells and accounts for growth and repair. Because mitosis has one nuclear division, it produces two daughter cells. The daughter cells are diploid and are genetically identical to each other and to the parent cell.
Meiosis I Compared with Mitosis 
Meiosis I differs from mitosis in the following ways:
	•	During prophase I, tetrads form and crossing-over occurs. No such events occur in mitosis.
	•	During metaphase I, tetrads are at the spindle equator. During mitosis, dyads are at the spindle equator.
	•	During anaphase I, homologous chromosomes separate. During mitosis, sister chromatids separate.
Meiosis II Compared with Mitosis 
	•	The events of meiosis II are the same as those of mitosis except that the cells undergoing meiosis II are haploid, whereas those undergoing mitosis are diploid.
9.4		Abnormal Chromosome Inheritance
Nondisjunction accounts for the inheritance of an abnormal chromosome number. Nondisjunction can occur during meiosis I if homologous chromosomes fail to separate, and both go into one daughter cell and the other daughter cell receives neither. It can also occur during meiosis II if chromatids fail to separate, and both go into one daughter cell and the other daughter cell receives neither. 
Down SyndromeIn Down syndrome, an example of an autosomal syndrome, the individual inherits three copies of chromosome 21. 
Abnormal Sex Chromosome Number
Examples of syndromes caused by inheritance of abnormal sex chromosomes are Turner syndrome (XO) and Klinefelter syndrome (XXY).
Turner SyndromeAn XO individual inherits only one X chromosome. This individual survives because even in an XX individual, one X becomes a Barr body.
Klinefelter SyndromeAn XXY individual inherits two X (or more) chromosomes and also a Y chromosome. This individual survives because one (or more) X chromosome becomes a Barr body.
Thinking Scientifically
	1.	Some plants contain mutations in the genes that control the movement of chromosomes at meiosis. These mutant plants produce gametes that are not reduced in chromosome number. So, egg and sperm cells have the same number of chromosomes as the plants that produced them. Suppose a mutant plant containing 10 pairs of chromosomes is crossed with a normal plant (also containing 10 pairs of chromosomes). How many chromosomes would their offspring have? If two mutant plants were crossed, how many chromosomes would their offspring have?
	2.	In the nineteenth century, physicians noticed that people with Down syndrome were often the youngest children in large families. Some physicians suggested that the disorder is due to “maternal reproductive exhaustion.” How would you disprove this hypothesis? What might be a reasonable explanation for the relationship between maternal age and incidence of Down syndrome?
Testing Yourself
Choose the best answer for each question.
	1.	A human cell contains _________ pair(s) of sex chromosomes.
	a. 1	c. 22
	b. 2	d. 23
	2.	Mitosis _________ chromosome number, while meiosis 
_________ the chromosome number of the daughter cells.
	a. maintains, increases	c. increases, decreases
	b. increases, maintains	d. maintains, decreases
For questions 3–6, match the items to those in the key. Answers can be used more than once.
Key:
	a. haploid number
	b. 2 times the haploid number
	c. diploid number
	d. 2 times the diploid number
	3.	How many chromatids are in a cell as it enters meiosis? 
	4.	How many chromosomes are in each daughter cell following meiosis I? 
	5.	How many chromatids are in each daughter cell following meiosis I? 
	6.	How many chromosomes are in each daughter cell following meiosis II? 
	7.	The two major functions of meiosis are to
	a.	
maintain chromosome number and create genetically uniform products.
	b.	
reduce chromosome number and create genetically uniform products.
	c.	
reduce chromosome number and create genetically variable products.
	d.	
maintain chromosome number and create genetically variable products.
For questions 8–15, match the items to those in the key. 
Key:
	a. prophase I
	b. metaphase I
	c. anaphase I
	d. telophase I
	e. prophase II
	f. metaphase II
	g. anaphase II
	h. telophase II
	8.	A cleavage furrow forms, resulting in haploid nuclei. Each chromosome contains two chromatids. 
	9.	Tetrads form, and crossing-over occurs. 
	10.	Dyads align at the spindle equator. 
	11.	Four haploid daughter cells are created. 
	12.	Homologous chromosomes move to opposite poles. 
	13.	Sister chromatids separate. 
	14.	Tetrads align on the spindle equator. 
	15.	Chromosomes in haploid nuclei condense. 
	16.	At each letter in the following diagram, indicate how mitosis differs from meiosis.
	17.	The condition that results when an egg carrying 23 chromosomes unites with a sperm carrying 22 chromosomes is called
	a. monosomy.	c. trisomy.
	b. isomy.	d. tetrasomy.
	18.	An individual with Turner syndrome is conceived when a normal gamete unites with
	a.	
an egg that was produced by nondisjunction during oogenesis.
	b.	
a sperm that was produced by nondisjunction during spermatogenesis.
	c. Either a or b is correct.
	d. Both a and b are correct.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
The dizzying array of assisted reproductive technologies has progressed from simple in vitro fertilization to the ability to freeze eggs, sperm, or even embryos for future use. Older women who never had the opportunity to freeze their eggs can still have children if they use donated eggs—perhaps today harvested from a fetus.
Legal complications abound, ranging from which mother has first claim to the child-—the surrogate mother, the woman who donated the egg, or the primary caregiver—to which partner has first claim to frozen embryos following a divorce. Legal issues involving who has the right to use what techniques have rarely been discussed, much less decided upon. Some clinics will help anyone, male or female, no questions asked, as long as they have the ability to pay. Most clinics are heading toward being able to do any type of procedure, including guaranteeing the sex of the child and making sure the child will be free from a particular genetic disorder. It would not be surprising if, in the future, zygotes could be engineered to have any particular trait desired by the parents.
Even today, eugenic (good gene) goals are evidenced by the fact that reproductive clinics advertise for egg and sperm donors, primarily in elite college newspapers. The questions become, is it too late for us as a society to make ethical decisions about reproductive issues? Should we come to a consensus about what techniques should be allowed and who should be able to use them? Should a woman investigate a couple before she has a child for them?
Understanding the Terms
allele128
autosome129
Barr body137
crossing-over131
diploid (2n) number129
Down syndrome136
dyad130
haploid (n) number129
homologous 
chromosome128
homologue128
interkinesis133
Klinefelter syndrome137
life cycle129
meiosis129
meiosis I130
meiosis II130
nondisjunction136
oogenesis129
sex chromosome129
spermatogenesis129
synapsis130
tetrad130
Turner syndrome137
zygote129
Match the terms to these definitions:
a. _______________ The set of events that produces egg cells.
b. _______________ A chromosome consisting of two sister chromatids.
c. _______________ The side-by-side pairing of chromosomes in early meiosis.
d. _______________ The exchange of genetic material between nonsister chromatids.
e. _______________ Improper separation of chromosomes at meiosis.
f. _______________ The syndrome expressed by an XO individual.
g. _______________ An inactive X chromosome.
A couple can potentially create 70,368,744,000,000 genetically different zygotes.
In females, egg production begins before birth; in males, sperm production begins at puberty.
Division mistakes during meiosis can cause conditions such as Down syndrome.
Figure 9.8Nondisjunction during meiosis.
Because of nondisjunction, gametes either lack a chromosome or have an extra chromosome. Nondisjunction can occur (a) during meiosis I if homologous chromosomes fail to separate, and (b) during meiosis II if the sister chromatids fail to separate completely. 
Figure 9.9Down syndrome.
Down syndrome occurs when the egg or the sperm has an extra chromosome 21 due to nondisjunction in either meiosis I or meiosis II. Characteristics include a wide, rounded face and narrow, slanting eyelids. Mental retardation to varying degrees is usually present.
Figure 9.5Meiosis I.
As a result of meiosis I, the daughter nuclei are haploid and are genetically dissimilar. They may contain a different combination of chromosomes and a different combination of alleles on the sister chromatids.
Figure 9.3Overview of meiosis.
Following duplication of chromosomes, the parent cell undergoes two divisions, meiosis I and meiosis II. During meiosis I, homologous chromosomes separate, and during meiosis II, chromatids separate. The final daughter cells are haploid. (The blue chromosomes were originally inherited from one parent, and the red chromosomes were originally inherited from the other parent.) 
Figure 9.1Homologous chromosomes.
In body cells, the chromosomes occur in pairs called homologous chromosomes. In this micrograph of stained chromosomes from a human cell, the pairs have been numbered. These chromosomes are duplicated, and each one is composed of two  sister chromatids. 
Figure 9.10Abnormal sex chromosome number.
a. A female with Turner syndrome (XO) has a short thick neck, short stature, and lack of breast development. b. A male with Klinefelter syndrome (XXY) has immature sex organs and some development of the breasts.
Figure 9.7Meiosis compared to mitosis.
Figure 9.6Meiosis II.
During meiosis II, daughter chromosomes consisting of one chromatid each move to the poles. Following meiosis II, there are four haploid daughter cells. Comparing the number of centromeres in the daughter cells with the number in the parent cell at the start of meiosis I verifies that the daughter cells are haploid. Notice that the daughter cells are genetically dissimilar from each other and from the parent cell. 
Figure 9.4Crossing-over.
When homologous chromosomes are in synapsis, the nonsister chromatids exchange genetic material. This illustration shows only one crossover per chromosome pair, but the average is slightly more than two per homologous pair in humans. Following crossing-over, the sister chromatids of a dyad may no longer be identical and instead may have different combinations of alleles.
Figure 9.2Life cycle of humans.
Meiosis in males is a part of sperm production, and meiosis in females is a part of egg production. When a haploid sperm fertilizes a haploid egg, the zygote is diploid. The zygote undergoes mitosis as it eventually develops into a newborn child. Mitosis continues throughout life during growth and repair.
Check Your Progress
	1.	
Describe how a zygote could receive an abnormal chromosome number.
	2.	
Contrast monosomy with trisomy.
Answers:1. If nondisjunction occurs and improper separation has occurred in meiosis I or II, the gametes and then the zygote have an abnormal chromosome number.2. Monosomy results when an individual is missing one chromosome. Trisomy results when an individual has one extra chromosome.
Check Your Progress
Why does meiosis, but not mitosis, produce daughter cells with half the number of chromosomes? 
Answer:The homologous chromosomes pair and separate only during metaphase I of meiosis and not during metaphase of mitosis.
Check Your Progress
	1.	Describe the significance of tetrad formation during meiosis I.
	2.	How does the chromosome number differ between a cell that is entering meiosis and a cell that has completed meiosis I?
Answers:1. Tetrad formation helps prepare homologous chromosomes for separation and allows crossing-over to occur.2. A cell that has completed meiosis I has half as many chromosomes as it did at the beginning of meiosis.
Check Your Progress
	1.	Describe the chromosome number of the next generation if meiosis did not occur.
	2.	Describe the major differences between meiosis I and meiosis II. 
	3.	What are two ways in which meiosis results in gametes that are genetically different? 
Answers:1. The chromo-some number would double in the next generation.2. During meiosis I, the parent cell is diploid, the homologous chromosomes separate, and the daughter cells are haploid. During meiosis II, the parent cell is haploid, the chromatids separate, and the daughter cells have the same number of chromosomes as the parent cell.3. The daughter cells have different combinations of the haploid number of chromosomes, and crossing-over alters the types of alleles on the daughter chromosomes. 
Check Your Progress
	1.	Describe what is meant by homologous chromosomes.
	2.	
Explain why homologous chromosomes occur in pairs.
Answers:1. Homologous chromosomes have the same size, shape, centromere position, and genes.2. One member of the pair came from the maternal parent and the other from the paternal parent.
Check Your Progress
Explain how mitosis and meiosis contribute to the human life cycle.
Answer:Mitosis is responsible for growth before and after birth and for repair of tissues. Meiosis results in gametes that unite at fertilization, producing the first cell of a new human being.

Table 9.1	Meiosis I Compared 
to Mitosis	

Meiosis I	Mitosis 	

Prophase I	Prophase	
Pairing of homologous chromosomes; 
crossing-over	No pairing of chromosomes; 
no crossing-over	
Metaphase I	Metaphase	
Tetrads at spindle equator	Dyads at spindle equator	
Anaphase I	Anaphase	
Homologues of each tetrad separate, and dyads move to poles	Sister chromatids separate, becoming daughter chromosomes that move to the poles	
Telophase I	Telophase	
Two haploid daughter cells not identical to parent cell	Two diploid daughter cells, identical to the parent cell	

Table 9.2	Meiosis II Compared 
to Mitosis	

Meiosis II	Mitosis 	

Prophase II	Prophase	
No pairing of chromosomes	No pairing of chromosomes	
Metaphase II	Metaphase	
Haploid number of dyads at spindle equator	Diploid number of dyads at spindle equator	
Anaphase II	Anaphase	
Sister chromatids separate, becoming daughter chromosomes that move to the poles	Sister chromatids separate, becoming daughter chromosomes that move to the poles	
Telophase II	Telophase	
Four haploid daughter cells, not genetically identical to each other or the parent cell	Two daughter cells, genetically identical to the parent cell