topic 7

• Chi-squared is one of the three statistics that need to be known for A-level biology
• Chi-squared


  Used to investigate whether there is a difference between frequency data
• If you are not confident on chi-squared, there is a first video that goes through all the details
• Using chi-squared for inheritance questions

  1. Work out expected frequency using Punnett square

  2. Record observed frequency

  3. Calculate chi-squared statistic

  4. Compare chi-squared value to critical value

  5. Accept or reject null hypothesis
• Null hypothesis


  • There is no significant difference between the expected and observed frequency
• You don't need to calculate chi-squared in the exam, you would just be given the value or p-value
• Inheritance example - corn kernels

  • Expected 75% purple, 25% yellow
  • Observed 21 purple, 13 yellow
• Steps to use chi-squared for corn kernel example

  1. Calculate expected frequencies

  2. Calculate chi-squared statistic

  3. Determine degrees of freedom

  4. Compare chi-squared value to critical value

  5. Accept or reject null hypothesis
• If chi-squared value does not exceed critical value, there is more than 5% probability the results are due to chance
• Accepting the null hypothesis means there is no significant difference between expected and observed frequencies
• Using chi-squared shows the corn kernels followed the expected 3:1 ratio, proving Mendelian genetics
• Epistasis


  When one gene influences the expression of another one
• Epistasis questions will always be dihybrid examples because it's to do with two genes and one gene masking the expression of the other
• Examples of epistasis


  • Cake color in mice
  • Color in labradors
  • Color of certain squash fruits
• Inheritance of labrador fur color

  1. Gene 1 controls whether pigment is expressed

  2. Gene 2 controls which pigment is expressed

  3. Possible gametes

  4. Punnett square to determine genotypes and phenotypes
• If a labrador has two copies of the recessive e allele


  They will be yellow in color
• If a labrador has a dominant b allele


  They will be black in color
• If a labrador has a recessive b allele


  They will be brown in color
• The ratio of phenotypes in the labrador example is 9:4:3
• Pedigree diagram


  • Circles represent females
  • Squares represent males
  • Colors indicate fur color
  • Genotypes of some individuals are provided
• Determining genotypes from a pedigree diagram


  1. Start by filling in what you can identify straight away

  2. Look at parents/offspring to determine missing alleles
• Gene 1 in squash fruit color

  Dominant allele inhibits an enzyme needed to make squash green or yellow, resulting in white fruit
• Gene 2 in squash fruit color

  Dominant allele codes for an enzyme that functions normally to make the squash yellow, recessive allele results in green fruit
• Determining squash fruit color phenotype from genotypes

  Use the flow diagram to determine the effect of the two genes
• The squash fruit color example is another example of epistasis
• Dominant


  An allele that will always be expressed
• Recessive


  Alleles which are only expressed if there isn't a dominant one present
• Genotype


  The genetic composition of the organism
• Phenotype


  The characteristic of an organism due to expression of the genotype and the environment
• Gene


  A small section of DNA which could code for a polypeptide
• Alleles


  Alternative forms of one gene
• Dihybrid


  Looking at the inheritance of two genes within the same Punnett square
• Mendelian genetics


  Genetics based on the work of Gregor Mendel
• Gene 1 (shape)


  Round (dominant) or wrinkled (recessive)
• Gene 2 (colour)


  Yellow (dominant) or green (recessive)
• Dihybrid cross


  1. Label parent phenotypes

  2. Determine parent genotypes

  3. Identify possible gametes

  4. Combine gametes to get offspring genotypes

  5. Determine offspring phenotypes

  6. Calculate phenotype ratios
• When crossing a homozygous dominant parent with a homozygous recessive parent, all the F1 offspring will be heterozygous
• Crossing F1 heterozygous offspring

  1. Identify all possible gamete combinations

  2. Use a Punnett square to determine offspring genotypes

  3. Determine offspring phenotypes

  4. Calculate phenotype ratios (9:3:3:1)
• Autosomal linkage


  Two genes are located on the same chromosome (not a sex chromosome)
• Autosomal linkage


  Reduces the number of possible gamete combinations, leading to a different phenotype ratio (e.g. 3:1)