Understanding genetics: Inheritance, variation and evolution: Biology: GCSE (9:1)

• Gregor Mendel


  A monk who studied the inheritance of different characteristics in thousands of pea plants in the mid-19th century

  
• Displayed inheritance in pea plants

  Mendel found that when red-flowered plants were bred with white-flowered plant, all the offspring produced red flowers

  
• Dominant allele


  Red-flowers are expressed by a dominant allele, where the allele is always expressed if present

  
• Recessive allele

  White-flowers are expressed by a recessive allele, where the allele is only expressed if two copies of the allele are present

  
• Alleles


  Different versions or form of the same type of gene, the interaction of two alleles determines an overall displayed characteristic

  
• Gene


  A small section of DNA that acts as a code for a specific characteristic

  
• Mendel's findings

  Mendel noticed that the inheritance of each characteristic is determined by units that are passed on to descendants unchanged
• Importance of Mendel's work


  Mendel didn't know at the time that the units he described were actually genes, his work expanded the knowledge of genetic inheritance before DNA was even discovered
• Mendelian inheritance


  A type of biological inheritance that follows the principles proposed by Mendel, where the expression of a single trait is the result of the interaction of two alleles

  
• Blended inheritance


  At the time of Mendel's work many scientists believed that characteristics were simply blended when they were inherited
• Shape of pod or colour of flowers

  Some of the characteristics that Mendel looked at, he bred many individual pea plants and recorded the outcomes of each cross

  
• Evidence against blended inheritance

  The shape of a pea pod had no effect on the colour of the flowers whatsoever, causing Mendel to suggest that inheritance was controlled by units that do not change when passed on

  
• Masked characteristics


  Mendel also noticed that some characteristics such as white flowers could be masked and could reappear in later generations, we now know this is due to recessive alleles

  
• Reasons Mendel's work was not originally accepted

  Mendel could not explain the science of inheritance, scientists did not understand his work and his work was published in an obscure and inaccessible journal
• Forgotten work


  Mendel's work was thought to be insignificant at the time and was soon forgotten, however advanced microscopes later allowed for observations that supported Mendel's ideas
• Chromosome observation

  In the late 19th century the behaviour of chromosomes during cell division was first observed

  
• Observation of Mendel units on chromosomes

  In the early 20th century it was observed that chromosomes and Mendel's units behaved in similar ways, this led to the idea that the units now called genes were located on chromosomes
• Discovering the structure of DNA

  In the mid-20th century the structure of DNA was determined and the mechanism of gene function was worked out

  
• Scientists who discovered the structure of DNA

  James Watson and Francis Crick used an X-ray image given by Maurice Wilkins and produced by Rosalind Franklin to determine the structure of DNA in 1953

  
• Gene theory


  The scientific work by many scientists led to the gene theory being developed, where genes are considered heritable units that code for a characteristic