Which of the following best explains how meiosis followed by fertilization ensures genetic variation

Learning Outcomes

Understand how meiosis contributes to genetic diversity

The gametes produced in meiosis aren’t genetically identical to the starting cell, and they also aren’t identical to one another. As an example, consider the meiosis II diagram above, which shows the end products of meiosis for a simple cell with a diploid number of 2n = 4 chromosomes. The four gametes produced at the end of meiosis II are all slightly different, each with a unique combination of the genetic material present in the starting cell.

As it turns out, there are many more potential gamete types than just the four shown in the diagram, even for a simple cell with with only four chromosomes. This diversity of possible gametes reflects two factors: crossing over and the random orientation of homologue pairs during metaphase of meiosis I.

Crossing over. The points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. If meiosis happens many times, as it does in human ovaries and testes, crossovers will happen at many different points. This repetition produces a wide variety of recombinant chromosomes, chromosomes where fragments of DNA have been exchanged between homologues.
Random orientation of homologue pairs. The random orientation of homologue pairs during metaphase of meiosis I is another important source of gamete diversity.

What exactly does random orientation mean here? Well, a homologous pair consists of one homologue from your paternal parent and one from your maternal parent, and you have 23 pairs of homologous chromosomes all together, counting the X and Y as homologous for this purpose. During meiosis I, the homologous pairs will separate to form two equal groups, but it’s not usually the case that all the paternally inherited chromosomes will go into one group and all the maternally inherited chromosomes into the other.

Instead, each pair of homologues will effectively flip a coin to decide which chromosome goes into which group. In a cell with just two pairs of homologous chromosomes, like the one at right, random metaphase orientation allows for 22 = 4 different types of possible gametes. In a human cell, the same mechanism allows for 223 = 8,388,608 different types of possible gametes. And that’s not even considering crossovers!

Given those kinds of numbers, it’s very unlikely that any two sperm or egg cells made by a person will be the same. It’s even more unlikely that you and your sibling(s) will be genetically identical, unless you happen to be identical twins, thanks to the process of fertilization (in which a unique egg from the maternal parent combines with a unique sperm from the paternal parent, making a zygote whose genotype is well beyond one-in-a-trillion!).

Meiosis and fertilization create genetic variation by making new combinations of gene variants (alleles). In some cases, these new combinations may make an organism more or less fit (able to survive and reproduce), thus providing the raw material for natural selection. Genetic variation is important in allowing a population to adapt via natural selection and thus survive in the long term.

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What helps ensure the survival of a species?

Genetic variation. It is this variation that is the essence of evolution. Without genetic differences among individuals, "survival of the fittest" would not be likely. Either all survive, or all perish.

Sexual reproduction results in infinite possibilities of genetic variation. In other words, sexual reproduction results in offspring that are genetically unique. They differ from both parents and also from each other. This occurs for a number of reasons.

When homologous chromosomes form pairs during prophase I of meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome.
When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This called is called independent assortment. It results in gametes that have unique combinations of chromosomes.
In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring. This is known as random fertilization.

All of these mechanisms working together result in an amazing amount of potential variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!

See Sources of Variation at http://learn.genetics.utah.edu/content/variation/sources/ for additional information.

Crossing-over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recall during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad. At this point, the chromatids are very close to each other and some material from two chromatids switch chromosomes, that is, the material breaks off and reattaches at the same position on the homologous chromosome (Figure below). This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes. This process is also known as recombination.

Crossing-over. A maternal strand of DNA is shown in red. A paternal strand of DNA is shown in blue. Crossing over produces two chromosomes that have not previously existed. The process of recombination involves the breakage and rejoining of parental chromosomes (M, F). This results in the generation of novel chromosomes (C1, C2) that share DNA from both parents.

In humans, there are over 8 million configurations in which the chromosomes can line up during metaphase I of meiosis. It is the specific processes of meiosis, resulting in four unique haploid cells, that result in these many combinations. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Together with random fertilization, more possibilities for genetic variation exist between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. In humans, over 8 million (223) chromosome combinations exist in the production of gametes in both the male and female. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing-over. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations!

See How Cells Divide: Mitosis vs. Meiosis at http://www.pbs.org/wgbh/nova/miracle/divide.html for an animation comparing the two processes.

Sexual reproduction has the potential to produce tremendous genetic variation in offspring.
This variation is due to independent assortment and crossing-over during meiosis, and random union of gametes during fertilization.

Use this resource to answer the questions that follow.

Genetic Variation at http://www.eoearth.org/view/article/152942/.

What is meant by genetic variation?
Would natural selection occur without genetic variation? Explain your response.
What causes genetic variation?
How would genetic variation result in a change in phenotype?
What are the sources of genetic variation? Explain your response.

What is crossing-over and when does it occur?
Describe how crossing-over, independent assortment, and random fertilization lead to genetic variation.
How many combinations of chromosomes are possible from sexual reproduction in humans?
Create a diagram to show how crossing-over occurs and how it creates new gene combinations on each chromosome.

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