3.2 inheritance

• Mendel's experiments with pea plants


  1. Crossed a tall pea plant with a dwarf pea plant

  2. Crossed two of the tall offspring together

  
• Hereditary units


  Units that are passed on to descendants unchanged, determine the inheritance of characteristics
• Mendel showed that height in pea plants was the result of separately inherited 'hereditary units' passed down from each parent plant to the offspring plants

  
• The 'unit' for tall plants (T) was dominant over the 'unit' for short plants (t)
• Mendel carried out experiments to show how other characteristics of pea plants are inherited in the same way, e.g. flower colour

  
• Mendel's conclusions about heredity in plants


  • Characteristics are determined by 'hereditary units' and these hereditary units are passed on from parent to offspring unchanged
  • The offspring receives one 'hereditary unit' from each parent
  • Hereditary units can be dominant or recessive (a dominant characteristic is always expressed when present)
• Importance of Mendel's work


  • It provided the foundation for modern genetics
  • The importance of Mendel's discovery was not recognised until after his death
  • His studies were totally new to science in the 19th century
  • There was no knowledge of the mechanisms behind his findings (DNA, genes and chromosomes had not been discovered yet)
• Gene


  A short length of DNA found on a chromosome that codes for a particular characteristic (expressed by the formation of different proteins)
• Alleles


  Variations of the same gene
• As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene
• One of the alleles is inherited from the mother and the other from the father
• Phenotype


  The observable characteristics of an organism (seen just by looking - like eye colour, or found – like blood type)
• Genotype


  The combination of alleles that control each characteristic
• Dominant allele


  Only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
• Recessive allele


  Needs to be inherited from both parents in order for the characteristic to show up in the phenotype
• Homozygous


  When the two alleles of a gene are the same
• Heterozygous


  When the two alleles of a gene are different
• We cannot always tell the genotype of an individual for a particular characteristic just by looking at the phenotype
• If two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents - we describe them as being 'pure breeding' as they will always produce offspring with the same characteristics
• A heterozygous individual can pass on different alleles for the same characteristic each time it is bred with any other individual and can therefore produce offspring with a different genotype and phenotype than the parents - as such, heterozygous individuals are not pure breeding
• Monohybrid inheritance


  The inheritance of characteristics controlled by a single gene
• Using a Punnett square to investigate monohybrid inheritance

  1. Shows the possible combinations of alleles that could be produced in the offspring

  2. Allows the ratio of these combinations to be worked out
• Dominant allele


  Shown using a capital letter
• Recessive allele


  Shown using the same letter but lower case
• Crossing a pure-breeding short pea plant with a pure-breeding tall pea plant


  All offspring will have the tall phenotype

  
• Crossing the offspring from the first cross


  Results in a ratio of 3 tall : 1 short for any offspring
• Crossing a heterozygous plant with a short plant


  The heterozygous plant will be tall with the genotype Tt, the short plant is homozygous recessive - tt

  
• How to construct Punnett squares

  • Determine the parental genotypes
  • Select a letter that has a clearly different lower case
  • Split the alleles for each parent and add them to the Punnett square around the outside
  • Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring
• Family pedigree diagrams


  Used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family
• Females have the sex chromosomes XX, males have the sex chromosomes XY
• Only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
• Using a Punnett square to show the inheritance of sex


  The X and Y chromosomes take the place of the alleles usually written in the boxes
• Codominance


  Both alleles within a genotype are expressed in the phenotype of an individual
• Inheritance of blood group

  • An example of codominance
• Blood group alleles and phenotypes

  • I^A and I^B are codominant, both dominant to I^O
  • I^A results in the production of antigen A in the blood
  • I^B results in the production of antigen B in the blood
  • I^O results in no antigens being produced in the blood
• Using a genetic diagram to predict the outcome of crosses involving codominant alleles


  E.g. showing how a parent with blood group A and a parent with blood group B can produce offspring with blood group O
• Inheritance of blood group

  • Example of codominance
• Alleles I^A and I^B

  Codominant, but both are dominant to I^O
• I^A


  Results in the production of antigen A in the blood
• I^B


  Results in the production of antigen B in the blood