Topic 7 - Genetics, Populations, Evolution and Ecosystems

• Genotype


  The genetic constitution of an organism
• Phenotype


  The expression of the genes but also the interaction with the environment
• Homozygous


  When you have a pair of homologous chromosomes carrying the same alleles for a single gene
• Heterozygous


  When you've got the homologous chromosomes carrying two different alleles for a single gene
• Recessive allele


  Only expressed if there's no dominant allele present
• Dominant allele


  Always expressed
• Codominant


  Both alleles are equally dominant and expressed in the phenotype
• Multiple alleles


  More than two alleles for a single gene
• Sex linkage


  The gene whose locus is on the x chromosome
• Autosomal linkage


  Genes located on the same chromosome not the sex chromosomes
• Epistasis


  One gene modifies or masks the expression of a different gene at a locus
• Monohybrid


  Inheritance of just one gene
• Dihybrid


  Inheritance of two genes at a time
• A big part of the inheritance topic is being able to determine the outcome of genetic crosses
• For the different types of inheritance this is how you would represent it in a genetic cross
• Monohybrid is what you would have done at gcse
• For codominant, the base letter represents the gene and the superscript letter represents the allele
• For multiple alleles, you can't use the upper and lower case because you have more than two alleles
• For sex linkage, the gene is only found on the x chromosome, not the y chromosome
• Autosomal linkage is always linked to dihybrid because it's looking at the inheritance of two genes found on the same chromosome
• Epistasis is complicated enough that it often doesn't link to any of the other types of inheritance
• Dihybrid crosses are when two genes are considered at the same time
• Parental genotypes are homozygous dominant


  All offspring will be heterozygous for both traits
• If both parents are heterozygous for both traits


  Offspring ratio is 9:3:3:1
• Autosomal linkage is when two genes are located on the same chromosome
• Crossing over can result in new combinations of alleles in the gametes, affecting the predicted offspring ratio
• Crossing over


  Results in new combinations of alleles in the gametes
• Autosomal linkage


  Two genes are located on the same chromosome, but not the X or Y chromosome
• Autosomal linkage


  1. Alleles for each gene are linked on the same chromosome

  2. Have to be inherited together

  3. Whole chromosome pulled to create one gamete

  4. Other chromosome pulled to create other gamete
• Autosomal linkage


  Only two types of gametes possible: dominant alleles or recessive alleles
• Autosomal linkage


  Results in a 3:1 ratio instead of 9:3:3:1
• Crossing over


  Creates new combinations of gametes
• Chi-squared


  Statistic used to investigate differences between expected and observed frequencies
• Using chi-squared


  1. State null hypothesis

  2. Convert ratio to expected frequency

  3. Calculate chi-squared value

  4. Compare to critical value

  5. Determine if significant difference
• Hardy-Weinberg principle


  Mathematical model to predict allele frequencies within a population
• Gene pool


  All the alleles of all the genes within a population at one time
• Population


  All the individuals of one species in one area at one time
• Adult frequency


  Proportion of an allele within a gene pool
• p


  Frequency of dominant allele
• q


  Frequency of recessive allele