Genetics Picture

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Crossover

Sometimes the genes for two different characteristics are placed on the same chromosome. If the gene for flower color and the gene for stem length would be on the same chromosome, one might not expect to get both red flowering and white flowering pea plants with both long and short stems. Instead, one would expect the allele for long stem to follow the allele for red flowers, so that one would only be able to get red flowering high plants or white flowering small plants. Well, now this is not the case, for during the reduction division the two paired homolog chromosomes cross over each other and exchange parts. This is called crossover or recombination. As a result, the chromosome that an individual gets from its mother contains parts of the chromosome from the mothers mother, and parts of the same chromosome of the mothers father. Though, still it is so that alleles placed on the same chromosome tend to follow each other. The closer they are the less is the likelihood that a crossover will take place in between them, and the greater the chance that the alleles will follow each other. The genes are said to be linked.

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