5.01 MEIOSIS
The steps of the human life cycle are typical of many sexually reproducing animals. Indeed, fertilization and meiosis are also the hallmarks of sexual reproduction in plants, fungi, and protists.
Fertilization and meiosis alternate in sexual life cycles, maintaining a constant number of chromosomes in a species from one generation to the next.
As you have learned so far in this chapter, the phenotype of an organism can be affected by small-scale changes involving individual genes.
Random mutations are the source of all new alleles, which can lead to new phenotypic traits.
Introduction
Sometimes similarities between parents and children, and between siblings, make it obvious that they are related. Other times, you may not see any obvious similarities in their physical traits. Each of these couples has two children represented among these four randomly arranged photos.
In each generation, the number of chromosome sets is halved during meiosis but doubles at fertilization. For humans, the number of chromosomes in a haploid cell is 23, consisting of one set ;
; the number of chromosomes in the diploid zygote and all somatic cells arising from it is 46, consisting of two sets .
Crossing Over and Synapsis During Prophase I
Prophase I of meiosis is a very busy time. The prophase I cell shown in Figure 10.8 is at a point fairly late in prophase I, when pairing of homologous chromosomes, crossing over, and chromosome condensation have already taken place.
Cells of the gametophyte give rise to gametes by mitosis. Fusion of two haploid gametes at fertilization results in a diploid zygote, which develops into the next sporophyte generation.
Therefore, in this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces a sporophyte. The name alternation of generations fits well for this type of life cycle.
Parents endow their offspring with coded information in the form of hereditary units called
genes
Genetics
is the scientific study of heredity and inherited variation. In this unit, you'll learn about genetics at multiple levels, from organisms to cells to molecules.
A life cycle
the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.
In humans, each somatic cell has 46 chromosomes. Before mitosis begins, the chromosomes duplicate. During mitosis, the chromosomes become condensed enough to be visible under a light microscope.
. At this point, they can be distinguished from one another by their size, the position of their centromeres, and the pattern of colored bands produced by certain chromatin-binding stains.
However, children are not identical copies of either parent or of their siblings. Along with inherited similarity, there is also
variation.
The two chromosomes referred to as X and Y are an important exception to the general pattern of homologous chromosomes in human somatic cells. Typically, human females have a homologous pair of X chromosomes (XX), while males have one X and one Y chromosome (XY; see Figure 10.3).
. Only small parts of the X and Y are homologous. Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X. Due to their role in sex determination (see Concept 12.2), the X and Y chromosomes are called sex chromosomes. The other chromosomes are called autosomes.
Errors in meiosis or damaging agents such as radiation can cause breakage of a chromosome, which can lead to four types of changes in chromosome structure (Figure 12.14). A deletion occurs when a chromosomal fragment is lost. The affected chromosome is then missing certain genes.
A broken fragment may become reattached as an extra segment to a sister or nonsister chromatid, producing a duplication of a portion of that chromosome. A chromosomal fragment may also reattach to the original chromosome but in the reverse orientation, producing an inversion. A fourth possible result of chromosomal breakage is for the fragment to join a nonhomologous chromosome, a rearrangement called a translocation.
The frequency of Down syndrome increases with the age of the mother. While the disorder occurs in just 0.04% (4 out of 10,000 births) of children born to women under age 30, the risk climbs to 0.92% (92 out of 10,000) for mothers at age 40 and is even higher for older mothers. The correlation of Down syndrome with maternal age has not yet been explained. Most cases result from nondisjunction during meiosis I, and some research points to an age-dependent abnormality in meiosis. Medical experts recommend that prenatal screening for trisomies in the embryo be offered to all pregnant women due to its low risk and useful results.
A law passed in 2008 stipulates that medical practitioners give accurate, up-to-date information about any prenatal or postnatal diagnosis received by parents and that they connect parents with appropriate support services.
Females with trisomy X (XXX), which occurs once in approximately 1,000 live female births, are healthy and have no unusual physical features other than being slightly taller than average. Females with trisomy X are at risk for learning disabilities but are fertile. Monosomy X, called Turner syndrome, occurs about once in every 2,500 female births and is the only known viable monosomy in humans.
Although these X0 individuals are phenotypically female, they are sterile because their sex organs do not mature. When provided with estrogen replacement therapy, girls with Turner syndrome do develop secondary sex characteristics. Most have typical intelligence.
Aneuploid conditions involving sex chromosomes appear to upset the genetic balance less than those involving autosomes. This may be because the Y chromosome carries relatively few genes. Also, extra copies of the X chromosome simply become inactivated as Barr bodies.
An extra X chromosome in a male, producing an XXY genotype, occurs approximately once in every 500 to 1,000 live male births. People with this disorder, called Klinefelter syndrome, have male sex organs, but the testes are small and the man is sterile. Even though the extra X is inactivated, some breast enlargement and other female body characteristics are common. Affected individuals may have learning disabilities. About one of every 1,000 males is born with an extra Y chromosome (XYY). These males undergo typical sexual development and do not exhibit any well-defined syndrome.
Some multicellular organisms are also capable of reproducing asexually (Figure 10.2). Because the cells of the offspring arise via mitosis in the parent, the offspring is usually genetically identical to its parent.
An individual that reproduces asexually gives rise to a clone, an individual or group of individuals that are genetically identical to the parent. Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations, which we will discuss in Concept 14.5.
As shown in this figure, the two members of a homologous pair associate along their length, aligned allele by allele. The DNA molecules of a maternal and a paternal chromatid are broken at precisely matching points. A zipper-like structure called the synaptonemal complex forms, and during this attachment (synapsis), the DNA breaks are closed up so that a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa.
At least one crossover per chromosome must occur, in conjunction with sister chromatid cohesion, in order for the homologous pair to stay together as it moves to the metaphase I plate, for reasons that will be explained shortly.
Haploid vs. Diploid
Every species has a specific number of chromosomes in its cells. Human cells contain 46 chromosomes, paired up as two sets of 23. The number 46 is a human's diploidnumber. Their cells contain two homologous sets of chromosomes, 23 chromosomes from the male parent that correspond to 23 chromosomes from the female parent. The cells involved in sexual reproduction, called gametes, contain half the number of chromosomes as diploid cells. These haploid cells contain a single set of chromosomes to pass on to their offspring. The process for producing gametes, meiosis, reduces the diploid number of chromosomes by half to produce haploid sperm and egg cells. The haploid number (abbreviated n) for a human is 23.This means 23 chromosomes are in the male sperm and 23 chromosomes are in the female ova. Diploid and haploid numbers are represented by the algebraic symbols 2N and N respectively. The diploid cell of a given organism always contains twice the number of chromosomes as the haploid cell, so if we know one number, we can determine the other. For example, if an organism's haploid number (N) is 4, the diploid number (2N) is 2 × 4, or 8. If the diploid number is 20, the haploid number is 20/2, or 10. Through sexual reproduction, haploid cells from each parent fuse together in a process called fertilization. The union of these gametes forms a diploid zygote. It is important that the resulting gametes only have one set of chromosomes to prevent the doubling of chromosomes when two gametes come together during fertilization. If the chromosomes were doubled, the type of species created would be different from the parent species.
Learning Objectives
Explain how meiosis results in the transmission of chromosomes from one generation to the next. Describe similarities and/or differences between the phases and outcomes of mitosis and meiosis.
Except for small amounts of DNA in mitochondria and chloroplasts, the DNA of a eukaryotic cell is packaged into chromosomes within the nucleus. Every species has a characteristic number of chromosomes.
For example, humans have 46 chromosomes in their somatic cells—all the cells of the body except the gametes and their precursors. Each chromosome consists of a single long DNA molecule elaborately coiled in association with various proteins. One chromosome includes several hundred to a few thousand genes, each of which is a precise sequence of nucleotides along the DNA molecule. A gene's specific location along the length of a chromosome is called the gene's locus (plural, loci; from the Latin, meaning "place"). Our genetic endowment (our genome) consists of the genes and other DNA that make up the chromosomes we inherited from our parents
Only organisms that reproduce asexually have offspring that are exact genetic copies of themselves. In asexual reproduction, a single individual (like a yeast cell or an amoeba; see Figure 9.2a) is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes.
For example, single-celled eukaryotic organisms can reproduce asexually by mitotic cell division, in which DNA is copied and allocated equally to two daughter cells. The genomes of the offspring are virtually exact copies of the parent's genome.
Enduring Understanding
Heritable information provides for continuity of life.
Alterations of chromosome number and structure are associated with a number of serious human disorders. As described earlier, nondisjunction in meiosis results in aneuploidy in gametes and any resulting zygotes. Although the frequency of aneuploid zygotes may be quite high in humans, most of these chromosomal alterations are so disastrous to development that the affected embryos are spontaneously aborted long before birth.
However, some types of aneuploidy appear to upset the genetic balance less than others, so individuals with certain aneuploid conditions can survive to birth and beyond. These individuals have a set of traits—a syndrome—characteristic of the type of aneuploidy. Genetic disorders caused by aneuploidy can be diagnosed before birth by genetic testing of the fetus.
Several steps of meiosis closely resemble corresponding steps in mitosis. Meiosis, like mitosis, is preceded by the duplication of chromosomes.
However, this single duplication is followed by not one but two consecutive cell divisions, called meiosis I and meiosis II. These two divisions result in four daughter cells (rather than the two daughter cells of mitosis), each with only half as many chromosomes as the parent cell—one set, rather than two.
The only cells of the human body not produced by mitosis are the gametes, which develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males (see Figure 10.5).
Imagine what would happen if human gametes were made by mitosis: They would be diploid like the somatic cells.
The transmission of hereditary traits has its molecular basis in the replication of DNA, which produces copies of genes that can be passed from parents to offspring.
In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. During fertilization, male and female gametes (sperm and eggs) unite, passing on genes of both parents to their offspring.
The overview of meiosis in Figure 10.7 shows, for a single pair of homologous chromosomes in a diploid cell, that both members of the pair are duplicated and the copies sorted into four haploid daughter cells. Recall that sister chromatids are two copies of one chromosome, closely associated all along their lengths; this association is called sister chromatid cohesion.Together, the sister chromatids make up one duplicated chromosome
In contrast, the two chromosomes of a homologous pair are individual chromosomes that were inherited from each parent. Homologs appear alike in the microscope, but they may have different versions of genes at corresponding loci; each version is called an allele of that gene (see Figure 11.4). For example, one chromosome may have an allele for freckles, but the homologous chromosome may have an allele for the absence of freckles at the same locus. Homologs are not associated with each other in any obvious way except during meiosis
Sister chromatids stay together due to sister chromatid cohesion, mediated by cohesin proteins. In mitosis, this attachment lasts until the end of metaphase, when enzymes cleave the cohesins, freeing the sister chromatids to move to opposite poles of the cell. In meiosis, sister chromatid cohesion is released in two steps, one at the start of anaphase I and one at anaphase II.
In metaphase I, the two homologs of each pair are held together by cohesion between sister chromatid arms in regions beyond points of crossing over, where stretches of sister chromatids now belong to different chromosomes. The combination of crossing over and sister chromatid cohesion along the arms results in the formation of a chiasma. Chiasmata hold homologs together as the spindle forms for the first meiotic division. At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologs to separate. At anaphase II, the release of sister chromatid cohesion at the centromeres allows the sister chromatids to separate. Thus, sister chromatid cohesion and crossing over, acting together, play an essential role in the lining up of chromosomes by homologous pairs at metaphase I.
Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a nondisjunction, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II (Figure 12.13).
In nondisjunction, one gamete receives two of the same type of chromosome and another gamete receives no copy. The other chromosomes are usually distributed normally.
A diploid embryo that is homozygous for a large deletion (or has a single X chromosome with a large deletion, in a male) is usually missing a number of essential genes, a condition that is typically lethal. Duplications and translocations also tend to be harmful.
In reciprocal translocations, in which segments are exchanged between nonhomologous chromosomes, and in inversions, the balance of genes is not abnormal—all genes are present in their normal doses. Nevertheless, translocations and inversions can alter phenotype because a gene's expression can be influenced by its location among neighboring genes, which can have devastating effects.
One aneuploid condition, Down syndrome, affects approximately one out of every 830 children born in the United States (Figure 12.15). Down syndrome is usually the result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21. Down syndrome includes characteristic facial features, short stature, correctable heart defects, and developmental delays.
Individuals with Down syndrome have an increased chance of developing leukemia and Alzheimer's disease but have a lower rate of high blood pressure, atherosclerosis (hardening of the arteries), stroke, and many types of solid tumors. Although people with Down syndrome have an average life span that is shorter than typical, most, with proper medical treatment, live to middle age and beyond. Many live independently or at home with their families, are employed, and are valuable contributors to their communities. Almost all males and about half of females with Down syndrome are sexually underdeveloped and sterile.
Meiosis I:
Interphase I: At the end of interphase, as the cell prepares for meiosis, the chromosomes replicate. This process is similar to what occurs before mitosis. For each original chromosome, a set of genetically identical sister chromatids is formed. Prophase I: Chromosomes condense and become more visible. Each set of sister chromatids pair up with its homologous pair of chromosomes, forming a set of four chromatids. At several places along their length, non-sister chromatids (chromatids that belong to homologous chromosomes but are not identical sister chromatids) undergo a process called crossing over. The chromatids of homologous chromosomes crossover one another to hold the set of four chromatids together. Then the crossed sections of the chromatids are exchanged, forming new combinations of alleles in the cell. As prophase I continues, the cell prepares for a division of the nucleus similar to what happens in mitosis. Spindle fibers form between the centrioles, which are at opposite ends of the cell. The nuclear membrane and the nucleolus disappear. Prophase I is the longest stage of meiosis, often taking up to 90% of the time required for the entire process to occur. Metaphase I: The homologous sets of four chromatids line up along the center of the cell. Spindle fibers from one pole attach to each set of sister chromatids, while spindle fibers from the other pole attach to the homologous sets. Anaphase I: The spindle fibers pull the chromosomes apart, as in mitosis. However, the sister chromatid pairs remain attached together at their centromeres and move together as single set to the same end of the cell. It is the homologous sets of chromosomes that are separated in this stage of meiosis. Notice that this is different than mitosis. Telophase I: Each pole now has a haploid set of chromosomes, but each chromosome still has two chromatids. Cytokinesis I: The cell splits into two daughter cells. Due to the crossover between haploid chromosomes in prophase I, the two daughter cells produced by meiosis I have sets of chromosomes that are different from each other and from the diploid cell that entered meiosis. Interphase II: In some species, the daughter cells go through a period of time, called interphase II, before the second stage of meiosis begins. In other species, the daughter cells immediately begin meiosis II after cytokinesis. Unlike the first division, the daughter cells do not undergo chromosome replication before entering meiosis II.
Large-scale chromosomal changes can also affect an organism's phenotype. Physical and chemical disturbances, as well as errors during meiosis, can damage chromosomes in major ways or alter their number in a cell.
Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortion (miscarriage) of a fetus, and individuals born with these types of genetic defects commonly exhibit various developmental disorders. Plants appear to tolerate such genetic defects better than animals do.
Disorders Caused by Structurally Altered Chromosomes
Many deletions in human chromosomes, even in a heterozygous state, cause severe problems. One such syndrome, known as cri du chat ("cry of the cat"), results from a specific deletion in chromosome 5. A child born with this deletion has a small head with unusual facial features and has a cry that sounds like the mewing of a distressed cat. Such individuals usually die in infancy or early childhood. Chromosomal translocations can also occur during mitosis; some have been implicated in certain cancers, including chronic myelogenous leukemia (CML). This disease occurs when a reciprocal translocation happens during mitosis of pre-white blood cells. The exchange of a large portion of chromosome 22 with a small fragment from a tip of chromosome 9 produces a much shortened, easily recognized chromosome 22, called the Philadelphia chromosome (Figure 12.16). Such an exchange causes cancer by creating a new "fused" gene that leads to uncontrolled cell cycle progression
Polyploidy is fairly common in plants; the spontaneous origin of polyploid individuals plays an important role in the evolution of plants (see Figure 22.9).
Many of the plant species we eat are polyploid; for example, bananas are triploid, wheat is hexaploid (6n), and strawberries are octoploid (8n).
Essential Knowledge
Meiosis is a process that ensures the formation of haploid gamete cells in sexually-reproducing diploid organisms. Mitosis and meiosis are similar in the way chromosomes segregate but differ in the number of cells produced and the genetic content of the daughter cells.
A Comparison of Mitosis and Meiosis
Meiosis produces four cells and reduces the number of chromosome sets from two to one, whereas mitosis produces two cells and conserves the number of sets. Meiosis produces cells that differ genetically from the parent cell and from each other, whereas mitosis produces daughter cells that are genetically identical to the parent cell.
If either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a particular chromosome, a condition known as aneuploidy. Fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the zygote (so that the cell has chromosomes); the aneuploid zygote is said to be monosomic for that chromosome. If a chromosome is present in triplicate in the zygote (so that the cell has chromosomes), the aneuploid cell is trisomic for that chromosome.
Mitosis will subsequently transmit the anomaly to all embryonic cells. Monosomy and trisomy are estimated to occur in 10-25% of human conceptions and are the main reason for pregnancy loss. If the organism survives, it usually has a set of traits caused by the abnormal dose of the genes associated with the extra or missing chromosome. Down syndrome is an example of trisomy in humans that will be discussed later. Nondisjunction can also occur during mitosis. If such an error takes place early in embryonic development, then the aneuploid condition is passed along by mitosis to a large number of cells and is likely to have a substantial effect on the organism.
Some organisms have more than two complete chromosome sets in all somatic cells. The general term for this chromosomal alteration is polyploidy; the specific terms triploidy (3n) and tetraploidy (4n) indicate three and four chromosomal sets, respectively.
One way a triploid cell may arise is by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes. Tetraploidy could result from the failure of a 2n zygote to divide after replicating its chromosomes. Subsequent normal mitotic divisions would then produce a 4n embryo.
Meiosis II:
Prophase II: The chromosomes are still paired together as sister chromatids in each of the daughter cells formed during meiosis I. Spindle fibers begin to form in the cells. Metaphase II: The sister chromatid pairs line up across the middle of the cell, and spindle fibers attach to the centromeres at the center of each chromatid pair. Anaphase II: The centromeres that are holding each sister chromatid pair together separate. The individual chromosomes move along the spindle fibers toward opposite ends of the cell. Telophase II: Nuclear membranes form around each haploid (N) set of chromosomes. Cytokinesis: The cells divide, forming a total of four haploid (N) daughter cells from the original diploid (2N) cell that entered meiosis I.
A third type of life cycle occurs in most fungi and some protists, including some algae (Figure 10.6c). After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing. Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism.
Subsequently, the haploid organism carries out further mitoses, producing the cells that develop into gametes. The only diploid stage found in these species is the single-celled zygote.
Three events unique to meiosis occur during meiosis I:
Synapsis and crossing over. During prophase I, duplicated homologs pair up, and crossing over occurs as described previously and in Figure 10.9. Neither pairing nor crossing over occurs in mitosis. Alignment of homologous pairs at the metaphase plate. At metaphase I of meiosis, pairs of homologs are positioned at the metaphase plate, rather than individual chromosomes as in metaphase of mitosis. Separation of homologs. At anaphase I of meiosis, the duplicated chromosomes of each homologous pair move toward opposite poles, but the sister chromatids of each duplicated chromosome remain attached. In anaphase of mitosis, by contrast, sister chromatids separate.
Each sexually reproducing species has a characteristic diploid and haploid number. For example, the fruit fly Drosophila melanogaster has a diploid number (2n) of 8 and a haploid number (n) of 4, while for dogs, 2n is 78 and n is 39.
The chromosome number generally does not correlate with the size or complexity of a species' genome; it simply reflects how many linear pieces of DNA make up the genome, which is a function of the evolutionary history of that species (see Concept 18.5)
The genes we inherit from our mothers and fathers are our genetic link to our parents, and they account for family resemblances such as shared eye color or freckles. Our genes program specific traits that emerge as we develop from fertilized eggs into adults.
The genetic program is written in the language of DNA, the polymer of four different nucleotides (see Concepts 1.1 and 3.6). Inherited information is passed on in the form of each gene's specific sequence of DNA nucleotides, much as printed information is communicated in the form of meaningful sequences of letters
Meiosis I reduces the number of chromosome sets per cell from two (diploid) to one set (haploid). During the second meiotic division (meiosis II), the sister chromatids separate, producing haploid daughter cells. The mechanisms for separating sister chromatids in meiosis II and mitosis are virtually identical.
The molecular basis of chromosome behavior during meiosis continues to be a focus of intense research. In the Scientific Skills Exercise, you can work with data that track the amount of DNA in cells during meiosis.
Plants and some species of algae exhibit a second type of life cycle called alternation of generations (Figure 10.6b). This type includes both diploid and haploid stages that are multicellular.
The multicellular diploid stage is called the sporophyte. Meiosis in the sporophyte produces haploid cells called spores. Unlike a gamete, a haploid spore doesn't fuse with another cell but divides mitotically, generating a multicellular haploid stage called the gametophyte.
The occurrence of pairs of homologous chromosomes in each human somatic cell is a consequence of our sexual origins. We inherit one chromosome of a pair from each parent. Thus, the 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes—a maternal set (from our mother) and a paternal set (from our father).
The number of chromosomes in a single set is represented by n. Any cell with two chromosome sets is called a diploid cell and has a diploid number of chromosomes, abbreviated 2n. For humans, the diploid number is , the number of chromosomes in our somatic cells. In a cell in which DNA synthesis has occurred, all the chromosomes are duplicated, and therefore each consists of two identical sister chromatids, associated closely at the centromere and along the arms. (Even though the chromosomes are duplicated, we still say the cell is diploid, or 2n. This is because it has only two sets of information regardless of the number of chromatids, which are merely copies of the information in one set.) Figure 10.4helps clarify the various terms that we use to describe duplicated chromosomes in a diploid cell.
Deletions and duplications are especially likely to occur during meiosis. In crossing over, nonsister chromatids sometimes exchange unequal-sized segments of DNA, so that one partner gives up more genes than it receives (see Figure 18.12).
The products of such an unequal crossover are one chromosome with a deletion and one chromosome with a duplication.
Most genes program cells to synthesize specific enzymes and other proteins, whose cumulative action produces an organism's inherited traits.
The programming of these traits in the form of DNA is one of the unifying themes of biology.
The human life cycle begins when a haploid sperm from the father fuses with a haploid egg from the mother (Figure 10.5). This union of gametes, culminating in fusion of their nuclei, is called fertilization.
The resulting fertilized egg, or zygote, is diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines. As a human develops into a sexually mature adult, mitosis of the zygote and its descendant cells generates all the somatic cells of the body. Both chromosome sets in the zygote and all the genes they carry are passed with precision to the somatic cells.
Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types. This becomes clear when images of the chromosomes are arranged in pairs, starting with the longest chromosomes.
The resulting ordered display is called a karyotype (Figure 10.3). The two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called homologous chromosomes (or homologs) or a homologous pair. Both chromosomes of each pair carry genes controlling the same inherited characters. For example, if a gene for eye color is situated at a particular locus on a certain chromosome, its homologous chromosome will also have a version of the eye-color gene at the equivalent locus.
Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). For humans, the haploid number is .
The set of 23 consists of the 22 autosomes plus a single sex chromosome. An unfertilized egg contains an X chromosome; a sperm contains either an X or a Y chromosome.
Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies, depending on the species.
These variations can be grouped into three main types of life cycles. In the type that occurs in humans and most other animals, gametes are the only haploid cells (Figure 10.6a). Meiosis occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization. After fertilization, the diploid zygote divides by mitosis, producing a multicellular organism that is diploid.
In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents. In contrast to a clone, offspring of sexual reproduction vary genetically from their siblings and both parents:
They are variations on a common theme of family resemblance, not exact replicas. Genetic variation like that shown in Figure 10.1 is an important consequence of sexual reproduction. What mechanisms generate this genetic variation? The key is the behavior of chromosomes during the sexual life cycle.
At the next round of fertilization, when two gametes fused, the normal chromosome number of 46 would double to 92, and each subsequent generation would double the number of chromosomes yet again. This does not happen, however, because in sexually reproducing organisms, gamete formation involves a type of cell division called meiosis.
This type of cell division reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at fertilization. As a result of meiosis, each human sperm and egg is haploid . Fertilization restores the diploid condition by combining two sets of chromosomes, and the human life cycle is repeated, generation after generation
Note that either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. Only diploid cells, however, can undergo meiosis: Haploid cells can't because they already have a single set of chromosomes that cannot be further reduced.
Though the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental result: genetic variation among offspring. A closer look at meiosis will reveal the sources of this variation.
The transmission of traits from one generation to the next is called inheritance
heredity
