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. Crossing-Over Crossing-over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes.
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.
The result is an exchange of genetic material between homologous chromosomes. This occurs when homologous chromosomes align. Chromatids from each chromosome can cross over and recombine swap sections. This results in two recombinant chromosomes and two non-recombinant chromosomes. Independent Assortment and Random Fertilization During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles.
Review What is crossing-over and when does it occur? Jun 22, Each factor contributes to a different combination of alleles in a haploid gamete. Humans have 23 chromosomes so this gives rise to 8,, genetically unique gametes through independent assortment alone Random fertilization refers to the fact that if two individuals mate, and each is capable of producing over 8million potential gametes, the random chance of any one sperm and egg coming together is a product of these two probabilities - some 70 trillion different combinations of chromosomes in a potential offspring.
Related questions How is artificial selection different from genetic engineering? How can a mutation lead to evolution? One significant advantage for genetic variation produced by sexual reproduction over the consistency of asexual reproduction is seen with viral disease. In many species that produce asexually, such as the sea anemone, a single virus may have devastating effects on a population Figure In sexually reproducing organisms with a varied gene pool, a virus will likely have a less detrimental effect because some of the genetic variations that arise may provide some degree of resistance to the virus.
In humans, for example, gene variants that confer resistance to viruses have probably been favored by natural selection. For example, a study in from the University of Milan analyzed 52 populations worldwide and identified human genes that modulate susceptibility to viral infections. Independent assortment produces new combinations of alleles.
How is genetic variation generated? There are several points during sexual reproduction at which genetic variation can increase. In meiosis I, crossing over during prophase and independent assortment during anaphase creates sets of chromosomes with new combinations of alleles. Genetic variation is also introduced by random fertilization of the gametes produced by meiosis. Any of the genetically unique sperm generated by a male may fertilize the genetically unique egg produced by a female.
During metaphase I , the homologous pairs of chromosomes are aligned along the metaphase plate. Cell Division 2: Molecular Biology 1. Metabolic Molecules 2. Water 3. Protein 5.
Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4.
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