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2.39: Genetic Variation
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.
Independent Assortment and Random Fertilization
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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Frequently Asked Questions About how does sexual reproduction lead to genetic variation
If you have questions that need to be answered about the topic how does sexual reproduction lead to genetic variation, then this section may help you solve it.
Test your knowledge of how sexual reproduction increases genetic diversity.
Asexual reproduction does not have variation from sperm and egg because the offspring is an exact replica of a single parent, whereas sexual reproduction does because the sperm and egg have different combinations of genes than their parent organisms.
Does genetic variation always rise as a result of sexual reproduction?
Contrary to popular belief, sexual reproduction does not always produce more genetically diverse offspring, which is a common misconception among those who believe that sexual reproduction is essential to evolution.
How does the sexual life cycle affect the amount of genetic diversity?
Meiosis is a process that increases genetic variation. During fertilization, one gamete from each parent is combined to form a zygote, and because meiosis involves independent assortment and recombination, each gamete has a different set of DNA, resulting in a zygote with a distinct set of genes.
Why is there more variation in sexual reproduction?
Because genetic material from parents of two different species is involved, offspring produced through sexual reproduction are more variable than those produced through asexual reproduction.
Do sexual relations lessen genetic diversity?
These results suggest that reducing the rate of appearance of genetic variation and the speed at which new species emerge may increase biodiversity in the long-term. This leads to the paradoxical result that sexual reproduction can increase genetic variation but reduce species diversity.
What leads to genetic diversity?
Genetic variations that alter gene activity or protein function can introduce different traits in an organism. Genetic variations can arise from gene variants (also called mutations) or from a normal process in which genetic material is rearranged as a cell prepares to divide (known as genetic recombination).
Which three factors lead to genetic variation?
Mutation is the primary cause of genetic variation and evolution, and it also contributes to genetic diversity along with recombination and gene immigration.
Does sexual reproduction lead to a reduction in genetic diversity?
The paradigm has been that sex increases additive genetic variance, which provides the fuel for natural selection to improve population fitness, since Weismann (1891).
How does sexual reproduction contribute to the maintenance of genetic diversity?
Asexual reproduction, on the other hand, does not require sperm and eggs because one organism splits into two organisms that have the same combination of genes, whereas sexual reproduction contributes genetic diversity because the sperm and eggs that are produced contain different combinations of genes than the parent organisms.