6) Genetic Inheritance

•  Gregor Mendel investigated this by carrying out breeding experiments on pea plants.
• Until the mid-19th century, most people thought that sexual reproduction produced a blend of characteristics, e.g. if a red flowering plant was crossed with a white flowering plant, then pink flowering plants were produced.
•  He found that characteristics are determined by 'units' that are inherited (passed on) and do not blend together.
•  Later in the 19th century, the behaviour of chromosomes during cell division was observed.
•  Then in the early 20th century, scientists realised that chromosomes and Mendel's 'units' behaved in similar ways. They decided that the 'units', now called genes, were located on chromosomes.
•  In the mid-20th century, scientists worked out what the structure of DNA looked like and the mechanism by which genes work.
• The importance of Mendel's discovery was not recognised during his lifetime because:

  • he was a monk working in a monastery, not a scientist at a university
  • he did not publish his work in a well-known book or journal.
• Some characteristics are controlled by a single gene, e.g. fur colour in mice and red-green colour blindness in humans.
• • Each gene may have different forms called alleles, e.g. the gene for the attachment of earlobes has two alleles - attached or free.
• An individual always has two alleles for each gene:

  • One allele comes from the mother.
  • One allele comes from the father.
• The combination of alleles present in a gene is called the genotype, e.g. bb.
•  How the alleles are expressed (what characteristic appears) is called the phenotype, e.g. blue eyes.
•  Alleles can either be dominant or recessive.
•  If the two alleles present are the same, the person is homozygous for that gene, e.g. BB or bb.
• • If the alleles are different, they are heterozygous, e.g. Bb.
• Most characteristics are controlled by several genes working together.
• If only one gene is involved, it is called monohybrid inheritance.
•  Genetic diagrams or Punnett squares can be used to predict the outcome of a monohybrid cross.
•  These diagrams use: capital letters for dominant alleles and lower case letters for recessive alleles.