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Tutorials » Genetics and Evolution » Crossing Over and Genetic Diversity

Crossing Over and Genetic Diversity
- Genetics and Evolution

Gregor Mendel, an Austrian monk, is most famous in this field for his study of the phenotype of pea plants, including the shape of the peas on the pea plants.

Gregor Mendel's Work

Mendel's goal was to have a firm scientific basis on the relationship of genetic information passed on from parents to offspring. In light of this he focused on how plant offspring acquired the phenotype of their seeds. In this example, there are two choices, round and wrinkled seeds.

The plants that were used in the experiment had to be true breeding, i.e. those plants with round seeds must have had parents with round seeds, who in turn had parents producing round seeds etc. This is done to increase the accuracy of results.

After successfully producing two generations from these true breeding plants, the following was evident

  • The first generation of plants produced all had a round seed phenotype.
  • When these first generation plants were crossed, a ratio of 3 round seeds averaged every 1 wrinkled seed.
  • The ratio of 3:1 was not exact, though this is because of the randomness of the processes that are executed to produce these plants. For example, independent assortment is completely random, as are mutations, therefore variable results occur producing a sampling error.
  • Due to the scale of the experiment done by Gregor Mendel, the sampling error was smaller than that of a smaller scale experiment.

Mendel successfully hypothesised that the reason for this trend in phenotypes from generation to generation was down to the fact that genetic information was being passed on from their parents.

The fact that round seeds appeared more frequently than wrinkled seeds is due to round seeds being the dominant phenotype, which when present effectively 'masks' the phenotype of the recessive (wrinkled seed) gene.

Dominant and Recessive Alleles

  • All plant seeds produced in the first generation were round
  • 3 out of every 4 plant seeds produced in the second generation were round

The parents, one possessing wrinkled seeds the other possessing round were crossed together, for some reason in the first and second generation the presence of the round seed gene in offspring superceded the presence of the wrinkled seed. This is called dominance.

The next page investigates this dominance, and how it can successfully be predicted. The following page also has examples of a monohybrid and dihybrid cross.


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