Introduction and Learning Objectives
It has long been noted that offspring resemble their parents in many ways, and that traits are often passed from one generation to the next. This is evident in the physical similarities of siblings, and in offspring and their parents. Although there are definite similarities between parents and offspring (and between offspring), there is also variation. After all, children never look entirely like either of their parents because they have inherited traits from both parents. Variation between generations is an important feature of natural selection because it provides the genetic basis of differential reproductive success. This tutorial will examine how chromosomes are passed from parents to offspring, and how this process gives rise to genetic variation in offspring. By the end of this tutorial you should be familiar with:
- Asexual and sexual reproduction
- Sexual life cycles
- The two sources of genetic variation during meiosis
- Discuss sexual versus asexual reproduction
- Identify the major features of an animal life cycle
- Describe the role of meiosis in eukaryotes, and its effect on chromosome number
- Draw the major features of meiosis
- Diagram the process of independent assortment and calculate the number of different chromosome combinations independent assortment can produce
- Diagram crossing-over in a homologous pair of chromosomes
- Demonstrate how events in meiosis affect heredity
Types of Reproduction
Organisms reproduce in one of two ways: asexually or sexually. In asexual reproduction, individuals (e.g., bacteria (Fig. 1), Hydra) produce genetically identical offspring or clones. The bacterium in Figure 1 is dividing via binary fission - a cloning process. The two bacteria that result from this process will be genetically identical to each other (unless a mutation occurs).
Figure 1. Asexual reproduction - binary fission in bacteria (Click image to enlarge).
Asexual reproduction is sometimes advantageous to organisms because it is relatively fast, requires less energy, and avoids the potential for creating a bad combination of genes. The major disadvantage of asexual reproduction is that offspring are genetically identical to the parent, and this lack of genetic variation may be detrimental if conditions change and genetic diversity becomes valuable. This situation could occur with the introduction of a new disease or predator, changing environmental conditions, or other factors.
In sexual reproduction the genetic material from two parents combines to form offspring who are genetically distinct from their parents (and who are also distinct from other siblings produced by the same parents). Even though sexual reproduction requires more energy and time than asexual reproduction, it provides offspring with a unique genetic makeup that may provide them with a selective advantage in an unpredictable environment. The key to understanding how this genetic variation is maintained lies in examining the behavior of chromosomes during the sexual life cycle.
In many of the organisms we will discuss, the exchange (or recombination) of genetic information is obligatorily coupled to reproduction, which is why such an organism is characterized as undergoing sexual reproduction. However, the exchange (or recombination) of genetic information is not always coupled to reproduction. For example, conjugation in bacteria can be accompanied by a brief exchange of genetic material without division; so, bacteria can engage in a form of sex (i.e., exchange of genetic material) known as conjugation and also undergo asexual reproduction (binary fission). Sex and reproduction are not always linked.
The inheritance of traits is due to the transmission of genes from parents to offspring, and these genes code for the physical similarities that we observe. A gene is a heritable unit that codes for the production of a protein. In cells, nuclear DNA is organized into chromosomes. Genes occupy specific positions on chromosomes. More than one form of a gene may exist, and these alternative forms of the gene (alleles) result in the formation of slightly different proteins and enzymes that result in differences between individuals (for example, in physical appearance or physiological functioning).
Figure 2. Organization of chromosomes (Click image to enlarge). Public domain image
By passing chromosomes from parents to offspring, genes are transmitted from one generation to the next resulting in similarities between parents and offspring. All organisms have their genetic material organized into chromosomes, although the number of chromosomes differs by species (e.g., humans have 46 chromosomes, dogs have 78, cats have 34, and mosquitoes have only 6). Regardless of the number of chromosomes, the underlying principles of inheritance are the same for all species. To better understand the scale and relationship of nucleotide sequences, genes and chromosomes, examine the figure illustrating the Comparative Scale of Mapping.
In humans, any cell that is not a sperm or egg contains 46 chromosomes. These 46 chromosomes actually occur as 23 pairs of chromosomes, and the two chromosomes in a pair are identical in length and shape (except in males, who have one pair of nonidentical chromosomes: an X and a Y chromosome). Figure 3 is a human karyotype - an image of the 46 human chromosomes arranged into pairs. Is this karyotype from a male or a female? The two chromosomes in the pair also have the same genes; if a gene for hair color is located at one location on one chromosome (commonly referred to as a locus; plural, loci), its homologous chromosome will have the hair color gene at the same location. Importantly, the homologues may or may not have the same allele for the gene. This is because one homologue comes from each parent.
Figure 3. Human karyotype (Click to enlarge) Public Domain Image.
Figure 4 illustrates the human life cycle. Adults produce gametes (reproductive cells termed sperm and eggs) by meiosis, and each gamete contains only one-half the number of chromosomes found in the nonreproductive cells of the parent. The nonreproductive cells (containing 46 chromosomes) are called diploid cells, and are abbreviated as "2n." The gametes (containing 23 chromosomes) are called haploid cells, and are abbreviated as "n." When a haploid ovum and a haploid sperm unite through fertilization, a diploid zygote is produced. This life cycle describes how diploid (2n) adults produce haploid (n) gametes, which then unite to form a new diploid (2n) zygote.
Figure 4. The Human Life Cycle (Click image to enlarge)
This alternation of meiosis and fertilization is found in the sexual life cycles of all sexually reproducing organisms, but its timing can differ considerably depending on the organism. The human life cycle typifies the most common life cycle found in animals, where the gametes are the only haploid cells. They combine to form a zygote, which divides by mitosis and grows to form a new individual.
Diploid organisms have two sets of chromosomes, one set inherited from the father and the second set from the mother. Thus, each diploid individual has a duplicate set of each chromosome. A pair of duplicate chromosomes is known as a homologous pair. Each generation, in the production of sperm and eggs, males and females reduce their chromosome numbers in half (to the haploid number), but must insure that each gamete has a complete set of half of the chromosomes.
The primary function of meiosis in diploid organisms is the proper distribution of homologous chromosomes into gametes. Meiosis has three essential phases, (1) pairing of homologous chromosomes, (2) synapsis of the chromosomes together (resulting in crossing over), and (3) segregation of the chromosomes into daughter cells. As shown in Figure 5, a diploid cell that goes through meiosis produces 4 haploid daughter cells. Just as in mitosis, before the cell goes through meiosis, the chromosomes must replicate themselves (resulting in sister chromatids and nonsister chromatids). Replication is followed by two divisions called meiosis I and meiosis II. At the conclusion of meiosis, four daughter cells are formed, each with one-half the normal number of chromosomes. One consequence of meiosis is that new genetic diversity arises due to two factors: the independent assortment of homologous chromosomes to gametes, and crossing over (recombination) in prophase I of meiosis. Before we examine these two factors, you must first familiarize yourself with the process of meiosis. To do so, view the illustrations of meiosis, and then return to this page.
Now that you are familiar with meiosis, let's examine the factors that lead to genetic diversity in this process.
Figure 5. Overview of meiosis: How meiosis reduces chromosome number. (Click image to enlarge)
The first source of genetic variation, independent assortment (Fig. 6), occurs in metaphase I, when the homologous chromosomes line up on the metaphase plate. Orientation of the pairs is random; therefore, which chromosome (maternal or paternal) goes to each daughter cell is random. A particular daughter cell has a 50% chance of getting either the maternal or paternal chromosome for each homologous pair. Figure 6 illustrates two pairs of homologous chromosomes and how their random alignment at the metaphase plate can lead to different combinations of chromosomes in the daughter cells that are produced. The chromosome pairs align independently of one another. Therefore, the number of different combinations of chromosomes in the daughter cells is equal to 2n, where n = the haploid number of chromosomes (or the number of chromosome pairs). In this example there are two chromosomes, so there are 22 = 4 different chromosome combinations in the daughter cells. If we look at the whole human genetic complement (n = 23), the number of different combinations is 223 = 8,388,608. Due only to the independent assortment of chromosomes in meiosis I, over 8 million genetically distinct gametes can be produced.
When making these computations, calculate the number of possibilities using the number of chromosome pairs, not the total number of chromosomes.
Figure 6. The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I.(Click image to enlarge)
A second source of genetic variation during meiosis is the exchange of genetic material between the maternal and paternal chromosomes, a process called crossing over or recombination (Fig. 7). While the homologous chromosomes are paired together in prophase I, pieces of one chromosome may be exchanged with the identical portion of the other chromosome. This means that the resulting chromosomes are not entirely maternal or paternal, but rather a mixture of both. In humans, crossing over occurs about 2-3 times per chromosome pair, between nonsister chromatids only (not between sister chromatids).
Figure 7. The results of crossing over during meiosis. (Click image to enlarge)
View this animation of crossing over to reinforce this concept.
As previously described, meiosis gives rise to genetic variation through the independent assortment of chromosomes and recombination (or crossing-over) between pairs of homologous chromosomes. If we also consider random fertilization, the amount of genetic variation increases greatly. Let's consider the example of a human couple having a child. Over 8 million different genetic combinations are possible in the father's sperm, and an equal number in the mother's ovum. The number of unique genetic combinations would be (8 million)(8 million) = 64 trillion when a sperm and ovum randomly unite to form a zygote. Therefore, sexual reproduction is an effective way to create offspring with genetic variation.
Why is this important? Populations evolve and adapt through natural selection. In order for natural selection to operate, genetic variation must be present. Genetic variation leads to heterogeneity between generations; if the environment favors one variant, then this advantageous variant will increase in frequency over generational time. If environmental conditions change, individuals migrate, or new predators or pathogens are introduced, then genetic variation may allow a new variant to do better in these new conditions. Therefore, genetic variation is "insurance" for organisms against changing conditions because it helps to insure that some of their offspring will survive if conditions change.
A Real Life Example: How a lack of genetic variation can cause problems
The bananas you buy in the grocery store are seedless and sterile – they do not reproduce sexually (plant breeders produce them through asexual propagation methods). Some scientists are concerned that a common disease of bananas, Black Sigatoka fungus, may wipe out most bananas in the world and they would no longer be a readily available food (in many parts of the world, bananas are a staple food crop).
- Why are bananas more susceptible to being wiped out than a food crop that reproduces sexually?
- Specifically, what aspects of sexual reproduction produce genetic variation?
In each generation, sexually reproducing eukaryotes undergo a series of reproductive events that can be summarized by a life cycle. There are some differences in the details of various life cycles, but in essence the life cycle describes how the genetic composition of the cells involved in reproduction alternates between a 1n (haploid) and 2n (diploid) state. Superficially, this seems simple and straightforward; that is, two haploid gametes fuse (at some point in the life cycle), resulting in a diploid (2n) organism; one or more cells in this 2n organism then (at some point) undergo meiosis, giving rise to haploid gametes that potentially go on to form the next generation, etc. However, this is only part of the importance of the life cycle.
The mechanisms involved in the haploid/diploid/haploid transitions lead to genetic variation. All eukaryotic organisms have the majority of their DNA arranged in a set of chromosomes (found within the nucleus). Humans have 46 chromosomes (2n). Our gametes have 23 chromosomes (1n). Each of us received half of our chromosomes from each parent. However, the combination of 23 chromosomes received from each of our parents was not the same complement that our siblings received (which explains why most siblings look similar but nonidentical). This scenario provides a major source of variation between generations (as does crossing over between paternal and maternal chromosomes).
Each of our 23 haploid chromosomes has a similar but nonidentical partner termed a homologue. It is essential to remember that homologues are not identical. Therefore, the diploid composition can be described as 23 pairs of homologous chromosomes. During meiosis, these homologues will be segregated into four progeny cells, each of which will have a haploid chromosome complement. The cellular process by which these similar, yet nonidentical, chromosomes are physically moved during meiosis results in a random distribution of the 23 chromosomes into the haploid cells. Because this process occurs randomly, we can use statistics to predict the number of possible outcomes.
In metaphase I of meiosis, the replicated chromosomes appear situated in a row with the homologues lined up opposite one another. Importantly, how they pair is random. For example, the paternal homologue of chromosome 1 may line up on one side, whereas the paternal homologue of chromosome 2 may line up on the other side (remember, then the maternal pair would be on opposite sides as well). The odds that the paternal member of a given homologous pair lines up on one side is equal to 50% (or 0.5 or 1:1); i.e., it goes either on one side or the other. The odds that the paternal members of two chromosomes line up on the same side is then a product of their individual probabilities; i.e., 0.5 x 0.5 = 0.25 (or 1:4). What would be the odds of all 23 paternal chromosomes lining up on one side? The solution is 223 or 1 in 8 million. This is also the odds of any other specific combination occurring during metaphase I. In metaphase II, the segregated homologues will separate. There is only one way this will occur, so the probabilities outlined for the events at metaphase I can be extended to the entire process of meiosis.
Let's put together the events occurring during meiosis for both parents. If the combination for a specific arrangement of chromosomes in each parent is 8 million, what are the odds of a specific diploid arrangement occurring? (Fertilization is somewhat random and therefore can be described statistically.) Again, the probability is a product of their independent probabilities, which is 8 million x 8 million = 64 trillion.
After reading this tutorial, you should have a working knowledge of the following terms:
Questions? Send your instructor a message through ANGEL!