8.2

• Genotype

  The genetic makeup of an organism
• Phenotype

  Observable characteristics and is influenced by both the genotype and the environment
• Allele

  Different forms of a particular gene, found at the same locus (position) on a chromosome. A single gene could have many alleles
• What is meant by a dominant allele?

  An allele whose characteristic will always appear in the phenotype, whether one or two are present
• What is meant by a recessive allele?

  An allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be present
• What is meant by codominant alleles?

  Two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
• What is meant by homozygous?

  Both alleles are dominant, or both alleles are recessive
• What is meant by heterozygous?

  One allele is dominant, the other is recessive
• What is meant by multiple alleles?

  A gene with more than two alleles
• Genetic cross diagram

  • Parental phenotypes: Brown eyes, Blue eyes

  Parental genotypes: Bb, bb

  Gametes: B, b, b, b

  Offspring genotypes: Bb, Bb, bb, bb

  Offspring phenotypes: 2:2 brown eyes:blue eyes
• Monohybrid

  A phenotypic characteristic is controlled by a single gene
• Dihybrid

  Two phenotypic characteristics are controlled by two different genes present on two different chromosomes
• Autosomal linkage

  Where two or more genes are located on the same (non-sex) chromosome. In this case, only one homologous pair is needed for all four alleles to be present. For genes that aren't linked, two homologous pairs are needed
• Example of autosomal linkage

  • Drosophila (fruit flies); colour and wing length are autosomally linked, meaning they are inherited in pairs
• Sex-linkage

  Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual e.g. haemophilia
• Males are more likely to express a recessive sex-linked allele because most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if it's recessive. Since females get two alleles, this is less likely
• Chi-squared test


  A statistical test to find out whether the difference between observed and expected data is due to chance or a real effect
• Criteria for chi-squared test

  • Data placed in discrete categories
  • Large sample size
  • Only raw count data allowed i.e. not percentages
  • No data values equal zero
• How to perform a chi-squared test

  The formula results in a number, which is then compared to a critical value (for the corresponding degrees of freedom). If the number is greater than or equal to the critical value, we conclude there is no significant difference and the results occured due to chance
• We can use a chi-squared test to compare expected phenotypic ratios with observed ratios to test our understanding of how different genes and alleles are inherited
• • What is a pedigree diagram?

  Can be used to trace the pattern of inheritance of a specific trait, e.g. a genetic disorder, through generations of a family
• What does a pedigree diagram show?

  • Whether a trait is caused by a dominant or recessive allele
  • Whether a trait is more likely to be inherited by males or females
  • The genotypes of individuals in the family
  • The probability that an individual in the family will inherit a trait
• Explain why autosomally linked genes are inherited in pairs, referring to meiosis

  Genes on the same chromosome, particularly those closest together, are unlikely to undergo recombination during meiosis. Therefore they are inherited as if they were the same gene
• Why are males more likely to express a recessive sex-linked allele?

  Males are more likely to express a recessive sex-linked allele because most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if it's recessive. Since females get two alleles, this is less likely