Inheritance

• Define genotype?

  The genetic constitution of an organism
• Define phenotype?

  Expression of an organisms genotype, and its interaction with the environment
• What is an allele?

  Different versions of a gene found at the same locus on a chromosome
• How many alleles per gene do diploid organisms carry?

  Two.
• What is a dominant allele?

  An allele whose characteristic will always appear in the phenotype, so only one is needed
• What is a recessive allele?

  An allele whose characteristic only appear 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 and heterozygous?

  • Homozygous - both alleles are dominant, or both alleles are recessive
  • Heterozygous - one allele is dominant, one is recessive
• Define monohybrid inheritance?

  Where one phenotypic characteristic is controlled by a single gene
• Define dihybrid inheritance?

  Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time
• What is meant by sex-linkage?

  When an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual
• Why are males more likely to express a recessive sex-linked allele?

  Most sex-linked alleles are located on the X chromosome. Males only get one copy of this allele, so will express this characteristic even if it's recessive. Since females have XX chromosomes this is less likely.
• What is meant by autosomal linkage?

  When two or more genes are located on the same autosome (non-sex chromosome). The genes are unlikely to be separated by crossing over during meiosis so are often inherited together
• What is meant by epistasis?

  When two non-linked genes interact, with one gene either masking or supressing the other
• Define the two types of epistasis?

  • Recessive epistasis - when two homozygous recessive alleles mask expression of another allele
  • Dominant epistasis - when one dominant allele masks expression of multiple other alleles
• How can we use a chi-squared test in relation to genetics?


  We can compare expected phenotypic characteristics with observed ratios to test our understanding of how different genes or alleles are inherited