Inheritance

• Define genotype

  The genetic constitution of an organism
• Define phenotype

  The expression of an organism's genetic constitution, combined with its interaction with the environment
• What is an allele

  ~Different forms of a particular gene, found at the same locus on a chromosome

  ~A single gene could have many alleles
• How many alleles per gene do diploid organisms carry

  Two
• What is meant by a dominant allele

  An allele whose characteristics 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 and heterozygous

  Homozygous = both alleles are dominant, or both alleles are recessive

  Heterozygous = one allele is dominant, the other 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

  Where 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

  ~Therefore males only get one copy of the allele, so will express this characteristic even if its recessive

  ~Since females get two alleles, this is less likely
• Which parent do males inherit sex-linked characteristics from

  ~Their mother, since the Y chromosome can only come from their father

  ~Therefore, if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass on the trait
• Describe the four possible phenotypes of these offspring

  ~Normal female

  ~Carrier female

  ~Normal male

  ~Affected male
• What is meant by autosomal linkage

  ~Where two or more genes are located on the same 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
• What is meant by epistasis

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

  Recessive epistasis = where two homozygous recessive alleles mask expression of another allele

  Dominant epistasis = where one dominant allele masks expression of multiple other alleles
• What is the 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
• What are the criteria for the chi-squared test

  ~Data placed in discrete categories

  ~Large sample sizes

  ~Only raw count data allowed

  ~No data values equal zero
• How is a chi-squared test performed

  ~The formula results in a number, which is then compared to a critical value

  ~If the number is greater than or equal to the critical value, we conclude there is a significant difference between the observed and expected data and that the results did not occur due to chance
• How can we use a chi-squared test in relation to the content of this topic

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