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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Mendelian genetics in eukaryotic life cycles

So far, we have mainly been considering diploid organisms—organisms with two homologous chromosome sets in each cell. As we have seen, the diploid condition is designated 2n, where n stands for the number of chromosomes in one chromosome set. For example, the pea cell contains two sets of seven chromosomes, so 2n = 14. The organisms that we encounter most often in our daily lives (animals and flowering plants) are diploid in most of their tissues.

However, a large part of the biomass on the earth comprises organisms that spend most of their life cycles in a haploid condition, in which each cell has only one set of chromosomes. Important examples are most fungi and algae. Bacteria could be considered haploid, but they form a special case because they do not have chromosomes of the type that we have been considering. (Bacterial cycles are discussed in Chapter 10.) Also important are organisms that are haploid for part of their life cycles and diploid for another part. Such organisms are said to show alternation of generations, referring to the alternation of the 2n and n stages. All types of plants show alternation of generations; however, the haploid stage of flowering plants and conifers is an inconspicuous specialized structure dependent on the diploid part of the plant. Other types of plants, such as mosses and ferns, have independent haploid stages.

Do all these various life cycles show Mendelian genetics? The answer is that Mendelian inheritance patterns characterize any species that has meiosis as part of its life cycle, because Mendelian laws are based on the process of meiosis. All the groups of organisms mentioned, except bacteria, undergo meiosis as part of their cycles.

Diploids

Figure 3-14 summarizes the diploid cycle, the cycle of most animals (including humans). The adult body is composed of diploid cells, and meiosis takes place in specialized diploid cells, the meiocytes, that are found in the gonads (testes and ovaries). The products of meiosis are the gametes (eggs or sperm). Fusion of haploid gametes forms a diploid zygote, which, through mitosis, produces a multicellular organism.

Figure 3-14. The diploid life cycle.

Figure 3-14

The diploid life cycle.

Meiosis in a dihybrid is shown in Figure 3-15. The genotype of the cell is A/a ; B/b, and the two allelic pairs, A/a and B/b, are shown on two different chromosome pairs. The hypothetical cell has four chromosomes: a pair of homologous long chromosomes and a pair of homologous short ones. Such size differences between pairs are common.

Figure 3-15. Meiosis in a diploid cell of genotype A/a ; B/b, showing how the segregation and assortment of different chromosome pairs give rise to the 1:1:1:1 Mendelian gametic ratio.

Figure 3-15

Meiosis in a diploid cell of genotype A/a ; B/b, showing how the segregation and assortment of different chromosome pairs give rise to the 1:1:1:1 Mendelian gametic ratio.

Parts 4 and 4′ of Figure 3-15 show that two equally frequent spindle attachments to the centromeres in the first anaphase result in two different allelic segregation patterns. Meiosis then produces four cells of the genotypes shown from each of these segregation patterns. Because segregation patterns 4 and 4′ are equally common, the meiotic product cells of genotypes A ; B, a ; b, A ; b, and a ; B are produced in equal frequencies. In other words, the frequency of each of the four genotypes is 1/4. This gametic distribution is that postulated by Mendel for a dihybrid, and it is the one that we insert along one edge of the Punnett square. The random fusion of these gametes results in the 9:3:3:1 F2 phenotypic ratio.

The diagram in Figure 3.15 shows exactly why chromosomal behaviors produce the Mendelian ratios. Notice that Mendel’s first law (equal segregation) describes what happens to a pair of alleles when the pair of homologs that carry them separate into opposite cells at the first meiotic division. Notice also that Mendel’s second law (independent assortment) results from the independent segregation of different pairs of homologous chromosomes. Mitosis in the same dihybrid is shown in Figure 3-16.

Figure 3-16. Mitosis in a diploid cell of genotype A/a ; B/b.

Figure 3-16

Mitosis in a diploid cell of genotype A/a ; B/b. The heterozygous genes are on separate chromosome pairs.

Haploids

Figure 3-17 shows the basic haploid life cycle, found in many fungi and algae. Here, the organism is haploid. How can meiosis possibly take place in a haploid organism? After all, meiosis requires the pairing of two homologous chromosome sets. The answer is that all haploid organisms that undergo meiosis create a temporary diploid stage that provides the meiocytes. In some cases, such as in yeast, unicellular, haploid, mature individuals fuse to form a diploid meiocyte, which then undergoes meiosis. In other cases, specialized cells from different parents fuse to give rise to the meiocytes. Note that these fusing cells are properly called gametes, so we see that, in these cases, gametes arise from mitosis. Meiosis, as usual, produces haploid products, which are called sexual spores. The sexual spores in some species become new unicellular adults; in other species, they each develop through mitosis into a multicellular haploid individual. A cross between two haploid organisms includes only one meiosis, whereas a cross between two diploid organisms includes a meiosis in each diploid organism. As we shall see, this simplicity makes haploids very attractive for genetic analysis. In haploids, mitosis proceeds as shown in Figure 3-18.

Figure 3-17. The haploid life cycle.

Figure 3-17

The haploid life cycle.

Figure 3-18. Mitosis in a haploid cell of genotype A ; b.

Figure 3-18

Mitosis in a haploid cell of genotype A ; b. The genes are on separate chromosomes.

Let’s consider a cross in a specific haploid. A convenient organism for demonstration is the orange-colored bread mold Neurospora crassa. This fungus is a multicellular haploid in which the cells are joined end to end to form hyphae, or threads of cells. The hyphae grow through the substrate and send up aerial branches that bud off haploid cells known as conidia (asexual spores). Conidia can detach and disperse to form new colonies or, alternatively, they can act as paternal gametes and fuse with a maternal structure of a different individual (Figure 3-19). However, that different individual must be of the opposite mating type. In fungi there are no true sexes, and all haploid cultures develop similarly. However, populations contain distinct genetically determined mating types. In Neurospora, there are two mating types, called A and a, and the meiotic (sexual) part of the life cycle can take place only if two haploids of different mating type unite. Mating types can be thought of as “physiological sexes,” and, although this definition is inadequate, it is a useful phrase that stresses the unseen difference between mating types.

Figure 3-19. The life cycle of Neurospora crassa, the orange bread mold.

Figure 3-19

The life cycle of Neurospora crassa, the orange bread mold. Self-fertilization is not possible in this species: there are two mating types, determined by the alleles A and a of one gene. A cross will succeed only if it is A ×  (more...)

A maternal gamete waits inside a specialized knot of hyphae, and eventually a haploid maternal and a haploid paternal nucleus pair up and divide mitotically to produce numerous pairs. The pairs eventually fuse to form diploid meiocytes. Meiosis takes place and, in each meiocyte, four haploid nuclei, which represent the four products of meiosis, are produced. For an unknown reason, these four nuclei divide mitotically, resulting in eight nuclei, which develop into eight football-shaped sexual spores called ascospores. The ascospores are shot out of a flask-shaped fruiting body that has developed from the knot of hyphae that originally contained the maternal gametic cell. The ascospores can be isolated, each into a culture tube, where each ascospore will grow into a new culture by mitosis (Figure 3-20).

Figure 3-20. (a) A Neurospora cross made in a petri plate (at the left).

Figure 3-20

(a) A Neurospora cross made in a petri plate (at the left). The many small black spheres are fruiting bodies in which meiosis has taken place; the ascospores (sexual spores) were shot as a fine dust into the condensed moisture on the lid (which has been removed (more...)

What characters can be studied in such an organism? One character is the color of the conidia. Variants of the normal orange color can be found. Figure 3-21 shows a normal culture and some color variants, including an albino. Another possible character to study is the compactness of the culture, and one pair of contrasting phenotypes are normal spreading growth and a densely branching variant called colonial.

Figure 3-21. Genetically determined color variants of the fungus Neurospora.

Figure 3-21

Genetically determined color variants of the fungus Neurospora. The orange wild-type color is shown at right; the variants are, starting at left, albino, yellow, and brown. Their genotypes are wild type (al+ · ylo+ ·  (more...)

Let’s cross a spreading, orange culture with a colonial, albino culture of opposite mating type. We isolate asco-spores (progeny) and grow each one into a culture. We would find the following progeny phenotypes and proportions:

Image ch3e1.jpg

In total, half the progeny are spreading and half are colonial. Thus, this phenotypic difference must be determined by the alleles of one gene that have segregated equally at meiosis. We can call these alleles col+ (spreading) and col (colonial). The same logic can be applied to the other character: half the progeny are orange and half are albino, so the phenotypic difference in color also is determined by a pair of alleles, which we can name al+ (orange) and al (albino). We can represent the parents and the four progeny types as follows:

Image ch3e2.jpg

The 1:1:1:1 ratio is a result of equal segregation and independent assortment, as illustrated in the following branch diagram:

Image ch3e3.jpg

So we see that, even in such a lowly organism, Mendel’s laws are still in operation.

Alternating haploid and diploid

In an organism with alternation of generations, the life cycle comprises two stages: one diploid and one haploid. One stage is usually more prominent than the other. For example, what we recognize as a fern plant is the diploid sporophyte stage. This stage is the one that undergoes meiosis, producing sexual spores. However, the organism also has a small, independent, photosynthetic haploid stage that is usually much more difficult to spot on the forest floor. This haploid stage is the gametophyte stage, which produces gametes by mitosis. In contrast, the green moss plant is the haploid gametophyte stage, and the brownish stalk that grows up out of this plant is a dependent, diploid sporophyte that is effectively parasitic on the gametophyte.

In flowering plants, the main green stage is the diploid sporophyte. The haploid gametophytes of flowering plants are extremely reduced and dependent on the diploid. These gametophytes are found in the flower. In the anther and the ovary, meiocytes undergo meiosis, and the resulting haploid products of meiosis are called spores. A spore undergoes a few mitotic divisions to produce a small, multicellular gametophyte. The male gametophyte of seed plants is known as a pollen grain. Figure 3-22 shows that, in flowering plants, cells of the gametophytes act as eggs or sperm in fertilization. The generalized cycle of alternation of generations is shown in Figure 3-23.

Figure 3-22. Alternation of generations of corn.

Figure 3-22

Alternation of generations of corn. The male gametophyte arises from a meiocyte in the tassel. The female gametophyte arises from a meiocyte in the ear shoot. One sperm cell from the male gametophyte fuses with an egg nucleus of the female gametophyte, (more...)

Figure 3-23. The alternation of diploid and haploid stages in the life cycle of plants.

Figure 3-23

The alternation of diploid and haploid stages in the life cycle of plants.

In mosses and ferns, the sperm cells are motile and travel from one gametophyte to another in a film of water to effect fertilization. Let us consider a cross that we might make in a moss. The character to be studied can pertain to the gametophyte or the sporophyte. Assume that we have a gene whose alleles affect the “leaves” of the gametophyte, with w causing wavy edges and w+ causing smooth edges. Also assume that a separate gene affects the color of the sporophyte, with r causing reddish coloration and r+ causing the normal brown coloration. We fertilize a smooth-leaved w+ gametophyte that also bears the unexpressed allele r by transferring onto it male gametes from a wrinkly- leaved w gametophyte, also carrying r+ (Figure 3-24). Hence, the cross is w+ ; r × w ; r+.

Figure 3-24. Mendelian genetics in a hypothetical cross in a moss.

Figure 3-24

Mendelian genetics in a hypothetical cross in a moss. Only the haploid gametophyte expresses the w+ or w, and only the diploid sporophyte expresses the r+ or r alternatives.

A diploid sporophyte of genotype w+/w ; r+/r develops on the gametophyte, and it is brown because reddish is recessive. Cells of this sporophyte act as meiocytes, and sexual spores (products of meiosis) are produced in the following proportions:

Image ch3e4.jpg

We can directly classify only the leaf character in these gametophytes, and we would have to make the appropriate intercrosses to determine whether each individual is r+ or r.

Once again, Mendel’s laws dictate the inheritance patterns. These patterns may be deduced simply by keeping track of the ploidy in each part of the cycle and applying the simple Mendelian ratios.

MESSAGE

Mendelian laws apply to meiosis in any organism and may be generally stated as follows:

1.

At meiosis, the alleles of a gene segregate equally into the haploid products of meiosis.

2.

At meiosis, the alleles of one gene segregate independently of the alleles of genes on other chromosome pairs.

The molecular basis of mitosis and meiosis

We know that, at the genetic level, a chromosome is a single DNA molecule. Cell and nuclear division are made possible by the DNA replication that takes place during S phase before division. This replication creates two sister chromatids. The formation of chromatids at the DNA level is shown in Figure 3-25. The events of mitosis and meiosis can be represented at the DNA level, as diagrammed for a diploid heterozygote in Figure 3-26.

Figure 3-25. Chromatid formation and its underlying DNA replication.

Figure 3-25

Chromatid formation and its underlying DNA replication. Three different diploid and two different haploid genotypes are represented. The wild-type allele is called b+ and a mutant allele is b. At the mutant site, a single base pair changes from GC in (more...)

Figure 3-26. Simplified representation of mitosis and meiosis at the DNA level.

Figure 3-26

Simplified representation of mitosis and meiosis at the DNA level. Both are shown starting with a diploid cell of genotype a+/a.

Adhesion and pairing also are key molecular features. At both mitosis and meiosis, sister chromatids remain attached along their length until pulled apart at anaphase of mitosis or anaphase II of meiosis. This sister chromatid adhesion is due to special adhesive proteins. Pairing of homologous chromosomes at meiosis is accomplished by molecular assemblages called synaptonemal complexes along the middle of paired sister chromatids (Figure 3-27). Although the existence of synaptonemal complexes has been known for some time, the precise working of these structures is still a topic of research.

Figure 3-27. Synaptonemal complexes.

Figure 3-27

Synaptonemal complexes. (a) In Hyalophora cecropia, a silk moth, the normal male chromosome number is 62, giving 31 synaptonemal complexes. In the individual shown here, one chromosome (center) is represented three times; such a chromosome is termed (more...)

The motive force of mitosis and meiosis is produced by the nuclear spindle fibers (Figure 3-28). In nuclear division, spindle fibers form that are parallel to the cell axis and that connect the poles of the cells. Spindle fibers are polymers of a protein called tubulin. The chromosomal centromere is a specific DNA sequence that is essential for chromatid movement during division. Each centromere acts as a site at which a multiprotein complex called the kinetochore binds. The kinetochores in turn act as the sites for attachment to microtubules. From one to many microtubules from one pole attach to one kinetochore, and a similar number from the opposite pole attach to the other kinetochore. The spindle fibers then pull the chromosomes to opposite poles. The spindle apparatus and the complex of kinetochores and centromeres determine the fidelity of nuclear division.

Figure 3-28. Fluorescent label of the nuclear spindle (green) and chromosomes (blue) in mitosis: (a) before the chromatids are pulled apart; (b) during the pulling apart.

Figure 3-28

Fluorescent label of the nuclear spindle (green) and chromosomes (blue) in mitosis: (a) before the chromatids are pulled apart; (b) during the pulling apart. (From J. C. Waters, R. W. Cole, and C. L. Rieder, J. Cell Biol. 122, 1993, 361; courtesy of C. (more...)

The key features of mitosis and meiosis are compared in Figure 3-29. In the next sections, we probe deeper into the way that a chromosome is structured.

Figure 3-29. Comparison of the main features of mitosis and meiosis.

Figure 3-29

Comparison of the main features of mitosis and meiosis.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21836

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