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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 the most common source of genetic variation in sexually reproducing organisms is
If you have questions that need to be answered about the topic the most common source of genetic variation in sexually reproducing organisms is, then this section may help you solve it.
What is the main reason behind genetic variation?
Germline mutations are important to genetic variation because they occur in gametes and may be passed on to offspring, and gene flow and sexual reproduction are the most frequent sources of genetic variation.
Which is the most frequent cause of variation in a population capable of sexual reproduction?
What causes genetic variation in organisms that reproduce sexually?
Crossing over (in prophase I), random chromosome distribution (in metaphase I), and random fusion of gametes from various parents are the three main causes of genetic variation in sexual reproduction.
What are the main factors that cause genetic variation in living things?
Mutation is the primary cause of genetic variation and evolution, with recombination and gene immigration serving as secondary sources of genetic diversity.
What type of genetic variation is most prevalent in humans?
Scientists refer to these variations as SNPs, or single-nucleotide polymorphisms, and they occur, on average, every 1,000 bases in two different haploid genomes, making them the most prevalent genetic variations in the human genome.
Does independent assortment contribute to genetic diversity?
There is a huge potential for genetic variation due to this independent assortment, which allows the chromosomes inherited from either the mother or the father to sort into any gamete.
Quiz: What are the two primary sources of genetic variation?
Mutation and recombination are the two main processes that cause genetic variation.
Test your knowledge of the genetic variation’s sources.
Genetic recombination during sexual reproduction, mutation, and lateral gene transfer are the three main causes of genetic variation.
What are the main two variations’ causes?
Variation, which results from recombination and mutation, is the difference in traits among individuals within a population.
What are the two primary genetic sources in meiosis?
In meiosis, two key mechanisms—independent assortment of gametes and genetic recombination through crossing over—create genetic variation.
What is the only factor that varies?
Answer and Explanation: Natural selection and genetic drift are evolutionary processes that work with the random variation produced by mutation. Mutation is the primary source of all variation.
What influences meiosis genetic variation the most?
Meiosis increases genetic variation because each gamete contains a different set of DNA, resulting in a different combination of genes in the resulting zygote. Recombination or crossing over occurs during prophase I.
What are the four genetic variation sources?
Mutation, which can produce completely new alleles in a population, random mating, random fertilization, and recombination between homologous chromosomes during meiosis, which reorders alleles within an organism’s progeny, are all possible sources of genetic variation.
Which three sources of variation are there?
Mutation, genetic recombination, and population gene flow are the three main causes of genetic variation.