1. IB2.24.4 The work of Mendel © Oxford University Press 2011 The work of Mendel

2. IB2.24.4 The work of Mendel © Oxford University Press 2011 Gregor Mendel was an Austrian monk, born in 1822. He used his monastery garden to study variation and inheritance in pea plants, noting how characteristics in plants were passed on from one generation to the next. Although his work did not achieve the recognition it deserved in his lifetime, the results of his research did become the foundation of modern genetics.

3. IB2.24.4 The work of Mendel © Oxford University Press 2011 Mendel grew several varieties of pea plants in his monastery garden. He selected varieties with different characteristics. He made sure, by sowing seeds from them and looking at the offspring, that each variety he used was ‘true-breeding’. For example, white-flowered plants always produced white-flowered offspring.

4. IB2.24.4 The work of Mendel © Oxford University Press 2011 In his experiments, Mendel cross-pollinated two different varieties of pea plant. He took pollen (male gametes) from the flowers of one variety of pea plant and placed it onto the female part of the flowers of the other variety of pea plant to fertilize the egg cell and produce seeds. He then collected the seeds and grew them.

5. IB2.24.4 The work of Mendel © Oxford University Press 2011 In one of his experiments, Mendel crossed tall-stemmed plants with short- stemmed plants, noting the characteristics of the offspring. Mendel described the tall-stem characteristic as dominant and the short-stem characteristic as recessive. Mendel noted that all the F 1 generation were tall- stemmed. The short- stem characteristic seemed to have disappeared. Having allowed the F 1 generation to interbreed, Mendel noted that some of the F 2 generation were tall-stemmed and some were short-stemmed. He realised that there were three times as many tall plants as short plants.

6. IB2.24.4 The work of Mendel © Oxford University Press 2011 Mendel’s conclusions  Inheritance was not by a blending mechanism but was due to inheritance factors.  Factors for different characteristics are passed from parent to offspring separately.  The cells in the pea plant each have two inheritance factors for every characteristic, such as stem length.  Each pollen grain and each egg (female gamete) carries only one inheritance factor for a characteristic.  Any pollen grain could fertilise any egg.  If the offspring inherited one inheritance factor for tall stem and one inheritance factor for short stem, it would show the dominant characteristic and be tall.  If the offspring inherited two short-stem inheritance factors, it would show the recessive characteristic; it would have a short stem.