You should have a working knowledge of the following terms:
Introduction and Goals
It has long been noted that offspring resemble their parents in many ways, and that many traits are 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 also is 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
Types of Reproduction
Organisms reproduce in one of two general ways: asexually or sexually. In asexual reproduction individuals (e.g., bacteria, Hydra) produce genetically identical offspring termed clones. For example, many plants can be cloned by vegetative propagation of root and stem cuttings.
Asexual reproduction - binary fission in bacteria (Click image to enlarge).
Asexual reproduction is sometimes advantageous to organisms because it is generally quick and requires less energy. 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. 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 sometimes manifest themselves as differences in physical appearance, physiological functioning, etc.
Figure. 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 eukaryotic organisms have their genetic material subdivided 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 normal humans, any cell that is not a sperm or ovum 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). 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.
This figure illustrates the human life cycle. Adults produce gametes (reproductive cells termed sperm and eggs) by meiosis (a type of nuclear division), 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. 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 (another type of nuclear division; discussed in more detail here) and grows to form a new individual.
In a second type of life cycle, as seen in most fungi and some protists (unicellular eukaryotes and their somewhat simple multicellular relatives), the diploid zygote produces haploid cells by meiosis; these develop into free-living, haploid multicellular organisms. These haploid organisms then produce modified gametes by mitosis (they are already haploid, therefore, they cannot undergo meiosis), and these gametes unite to form diploid zygotes. Some plants and algae exhibit yet another type of life cycle, which will be described when we discuss plants and algae later in this course. Although these three types of life cycles differ in the timing of fertilization and meiosis, they all produce genetic variation in their offspring. Next we will examine the process of meiosis and see how genetic variation arises.
Organisms such as humans have two haploid sets of chromosomes, one set inherited from a male in the sperm and the second set from a female in the egg. Thus, we have a duplicate set of each of our 23 chromosomes. A pair of duplicate chromosomes is known as a homologous pair. Each generation, human males and females reduce their chromosome numbers in half, 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. Meiosis has three essential phases, (1) pairing of homologous chromosomes, (2) locking of the chromosomes together via crossing over, and disjunction or segregation of the chromosomes into two daughter cells. As shown in this figure, cells go from the diploid to haploid state through an initial replication of chromosomes (resulting in sister chromatids and nonsister chromatids), 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. Overview of meiosis: How meiosis reduces chromosome number. (Click image to enlarge)
The first source of genetic variation, independent assortment, 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. This figure 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 pairs, not the total number of chromosomes; remember that sex chromosomes are not homologous.
Figure. 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. 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. 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 between pairs of homologous chromosomes. If we also consider random fertilization, the amount of genetic variation increases more. 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 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 enable some individuals 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.
This tutorial demonstrated how traits are passed from one generation to the next. 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.
The process of random segregation of chromosomes during meiosis provides for an incredible amount of potential variation. Crossing over in prophase I adds another level of variation to the genetic character of gametes, and this will be explored in more detail later.