CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

9. Cell Division—Proliferation and Reproduction

9.9. Genetic Diversity—The Biological Advantage of Sexual Reproduction

Cell division allows organisms to reproduce either asexually or sexually. There are advantages and disadvantages to both. Asexual reproduction always produces organisms that are genetically identical to the parent. A single organism, separated from others of its kind, can still reproduce if it can reproduce asexually. Organisms that can reproduce only sexually are at a disadvantage, because they require two different organisms to reproduce. Also, sexually reproducing populations tend to grow at a much slower rate than do asexually reproducing populations. However, asexually reproducing populations could be wiped out by a single disease or a change in living conditions, because the members of the population are genetically similar.

Sexual reproduction offers an advantage over asexual reproduction. Populations that have a large genetic diversity are more likely to survive. When living conditions change or a disease occurs, some members of the population are more likely to survive if the population consists of many, genetically different individuals.

One reason for learning meiosis is to see how the events of meiosis and fertilization create genetic variation within a population. Haploid cells from two different individuals combine to form new, unique combinations of genetic information. Each new organism, with its unique combination of genetic information, may be important to the survival of the species.

Genetic diversity in a population is due to differences in the types of genes present in individual organisms. Although all the members of the population have the genes for the same basic traits, the exact information coded in the genes may vary from individual to individual. An allele is a specific version of a gene. Examples of alleles are: blood type A versus blood type O, dark versus light skin, normal versus sickle-cell hemoglobin, and attached versus free earlobes.

Five factors create genetic diversity in offspring by creating either new alleles or new combinations of alleles: mutation, crossing-over, segregation, independent assortment, and fertilization.

Mutation

Several types of mutations were discussed in chapter 8: point mutations and chromosomal aberrations. In point mutations, a change in a DNA nucleotide results in the production of a different protein. In chromosomal aberrations, genes are rearranged. Both types of mutations can create new proteins. Both types of mutations increase genetic diversity by creating new alleles.

Recall that epigenetic modifications to both DNA and histones are also able to be passed on through mitosis and, in some cases, meiosis. These result in different forms of gene expression displayed through determination.

Crossing-Over

The second source of variation is crossing-over. Crossing-over is the exchange of equivalent portions of DNA between homologous chromosomes, which occurs during prophase I while homologous chromosomes are synapsed. Remember that a chromosome is a double strand of DNA. To break chromosomes and exchange pieces of them, bonds between sugars and phosphates are broken. This is done at comparable locations on both chromatids, and the two pieces switch places. After switching places, the two pieces of DNA are bonded together by re-forming the bonds between the sugar and the phosphate molecules.

Crossing-over allows new combinations of genetic information to occur. While mutations introduce new genetic information to the population, crossing-over introduces new combinations of previously existing information. An organism receives one set of genetic information from each of its parents. Each gamete contains chromosomes that have crossed-over and therefore contains some of the father’s and some of the mother’s genes. As a result, traits from the mother and from the father can be inherited on a single piece of DNA.

Examine figure 9.33 carefully to note precisely what occurs during crossing-over. This figure shows a pair of homologous chromosomes close to each other. Each gene occupies a specific place on the chromosome, its locus. Homologous chromosomes contain an identical order of genes, and chromosomes may contain thousands of genes.

FIGURE 9.33. Synapsis and Crossing-Over

(a) While pairs of homologous chromosomes are in synapsis, (b) one part of 1 chromatid can break off and be exchanged for an equivalent part of its homologous chromatid. (c) As a result, new combinations of genetic information are created.

Notice in figure 9.34 that, without crossing-over, only two kinds of genetically different gametes result. Two of the four gametes have one type of chromosome, whereas the other two have the other type of chromosome. With crossing-over, four genetically different gametes are formed. With just one cross-over, the number of genetically different gametes is doubled.

FIGURE 9.34. Variations Resulting from Crossing-Over

Cells with identical genetic information are boxed together. (a) These cells resulted from meiosis without crossing-over. Only two unique cell types of cells were produced. Cell type 1—Diabetes, dark skin color. Cell type 2—Normal insulin, light skin color. (b) These cells had one cross-over. From one cross-over, the number of genetically unique gametes doubled from two to four. Type 1—Diabetes, dark skin color. Type 2—Normal insulin, dark skin color. Type 3—Diabetes, light skin color. Type 4—Normal insulin, light skin.

In fact, crossing-over can occur at a number of points on a chromosome; that is, there can be more than one cross-over per chromosome pair. Because crossing-over can occur at almost any point along the length of the chromosome, great variation is possible (figure 9.35).

FIGURE 9.35. Multiple Cross-Overs

Crossing-over can occur several times between the chromatids of one pair of homologous chromosomes.

The closer two genes are to each other on a chromosome (i.e., the more closely they are linked), the more likely they will stay together, because the chance of crossing- over occurring between them is lower than if they were far apart. Thus, there is a high probability that they will be inherited together. The farther apart two genes are, the more likely it is that they will be separated during crossing- over. This fact enables biologists to map the order of gene loci on chromosomes.

Segregation

Recall that segregation is the process during which the alleles on homologous chromosomes separate during meiosis I. Review figure 9.34; the normal insulin allele and the diabetes allele are both present in the diploid cell. However, following meiosis the normal insulin allele and the diabetes allele are segregated into separate haploid cells away from the other allele. Half of this individual’s gametes would carry genetic information for normal functional insulin. The other half of the individual’s gametes would carry genetic information for nonfunctional insulin (diabetes). Consider if this individual’s mate had the same genetic makeup. If the mate also had one normal gene for insulin production and one abnormal gene for diabetes, that person also would produce two kinds of gametes. Because of segregation, this couple could produce children that were genetically different from themselves. If both parents contributed a gamete that carried diabetes, their child would be diabetic. Other combinations of gametes would result in children without diabetes. Segregation increases genetic diversity by allowing parents to produce children that are genetically different from their parents and from their siblings.

Independent Assortment

So far in discussing genetic diversity, we have dealt with only one pair of chromosomes. Now let’s consider how genetic variation increases when we add a second pair of chromosomes. Independent assortment is the segregation of homologous chromosomes independent of how other homologous pairs segregate.

Figure 9.36 shows chromosomes with traits for the garden pea plant. The chromosomes carrying alleles for flower color (P = purple; p = white) always separate from each other. The second pair of chromosomes with the information for seed texture also separates. Because the pole to which an individual chromosome moves is determined randomly, half the time the chromosomes divide so that the trait for purple flowers and the trait for round-smooth seeds move in one direction, whereas the trait for white flowers and the trait for wrinkled seeds move in the opposite direction. An equally likely alternative is that, the trait for purple flowers and the trait for wrinkled seeds go together toward one pole of the cell, whereas the trait for white flowers and the trait for round-smooth seeds go to the other pole. With two pairs of homologous chromosomes there are four possible kinds of cells produced by independent assortment during meiosis.

FIGURE 9.36. The Independent Assortment of Homologous Chromosome Pairs

The orientation of one pair of chromosomes on the equatorial plane does not affect the orientation of another pair of chromosomes. Note that different possible arrangements of chromosomes can be compared on the left and right side of this figure. Comparing the sets of cells that result from each initial arrangement will show the new genetic combinations that result from independent assortment.

With three pairs of homologous chromosomes, there are eight possible kinds of cells produced as a result of independent assortment. The number of possible chromosomal combinations of gametes is calculated by using the expression 2ⁿ, where n equals the number of pairs of chromosomes. With three pairs of chromosomes, n equals 3, so 2ⁿ = 2³ = 2 x 2 x 2 = 8. With 23 pairs of chromosomes, as in human cells, 2ⁿ = 2²³ = 8,388,608. More than 8 million genetically different kinds of sperm cells or egg cells are possible from a single human parent. This number doesn’t consider the additional possible sources of variation, such as mutation and crossing-over. Thus, when genetic variation due to mutation and crossing-over is added, the number of different gametes become incredibly large.

Fertilization

Because of the large number of genetically different gametes resulting from independent assortment, segregation, mutation, and crossing-over, an incredibly large number of types of offspring can result. Because humans can produce millions of genetically different gametes, the number of kinds of offspring possible is infinite for all practical purposes, and each offspring is unique, with the exception of identical twins.

9.9. CONCEPT REVIEW

28. How much variation as a result of independent assortment can occur in cells with the following diploid numbers: 2, 4, 6, 8, and 22?

29. What are the major sources of variation in the process of meiosis?