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Genetics and Evolution (Speciation (Types of natural selection…
Genetics and Evolution
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Crossing over
Prophase 1 of Meiosis
Each homologous chromosome consists of two sister chromatids because all DNA has been replicated in interphase before the start of meiosis.
Chromatids of the two different chromosomes in a pair are non sister chromatids.
While the chromosomes are paired, sections of chromatid exchange in a process called crossing over
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The process of crossing over:
- At one stage in prophase 1 all the chromatids of two homologous chromosomes become tightly paired up together. This is called synapsis.
- The DNA molecule of one of the chromatids is cut. A second cut is made at the exactly the same point in the DNA of a non sister chromatid.
- The DNA of each chromatid is joined up to the DNA of the non sister chromatid. This has an effect of swapping sections of DNA between chromatids.
- In later stages of prophase 1 the tight pairing of the homologous chromosomes ends, but the sister chromatids remain tightly connected. Where each cross over has occurred there is an X shaped structure called a chiasma.
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Speciation
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Polyploidy in allium:
- Most allium species have a diploid number that is a multiple of 16.
- The ancestral allium probably had this number.
- Allium species with 32 chromosomes evolved by polyploidy.
- In a species with a diploid number of 16 an individual with 32 chromosomes is tetraploid.
- Polyploidy is an instant speciation.
- Tetraploids are re-productively isolated from diploids.
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Usually involves one population not interbreeding with any other populations of its species becoming reproductively isolated.
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Geographically isolated populations often have different allele frequencies from the rest of the species. e.g. cystic fibrosis 0.04 on faroe islands by only 0.03 in northern Europe.