7.1 Inheritance

• What is meant by the term genotype?

  Genetic constitution of an organism
• What is meant by the term phenotype?

  The expression of this genetic constitution (genotype) and its interaction with the environment
• What are alleles?

  Variations of a particular gene
• How do alleles arise?

  By mutation in DNA base sequence
• How many alleles of a gene can be found in diploid organisms?

  2 alleles as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
• Why can there be more than 2 alleles of a single gene in a population?

  Many alleles can exist within a population
• What is a dominant allele?

  Always expressed in the phenotype
• What is a recessive allele?

  Only expressed when 2 copies present (homozygous recessive)

  / NOT expressed when dominant allele present (heterozygous
• What are codominant alleles?

  Both alleles expressed / contribute to phenotype (if inherited together)
• What does homozygous mean?

  Same alleles at a specific locus
• What does heterozygous mean?

  Different alleles at a specific locus
• What do monohybrid and dihybrid crosses show?

  ● Monohybrid cross- inheritance of one phenotypic characteristic coded for by a single gene

  ● Dihybrid cross- inheritance of two phenotypic characteristics coded for by two different genes
• How can you determine if a disorder is caused by a dominant allele using a pedigree diagram?

  Parents with disorder have a child without it. ● [Named phenotype] parents [n & n] have child [n] WITHOUT [named phenotype]

  ● So both parents [n & n] must be heterozygous / carriers of recessive allele

  ○ If it were recessive, all offspring would have [named phenotype]
• How can you determine if a disorder is caused by a recessive allele using a pedigree diagram?

  Parents without disorder have a child with it. ● Parents [n & n] WITHOUT [named phenotype] have child [n] WITH [named phenotype]

  ● So both parents [n & n] must be heterozygous / carriers of recessive allele
• What is independent segregation in dihybrid crosses?

  • Alleles from one gene segregate independently of another
  • Results in various combinations of alleles in gametes
• What is a sex-linked gene?

  A gene with a locus on a sex-chromosome
• Why are males more likely to express a recessive X-linked allele?

  ● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers

  ● Males (XY) have 1 allele (inherited from mother)→ recessive allele always expressed
• What evidence suggests that an allele on the X-chromosome is recessive?

  • Mother without phenotype has child with phenotype
  • Mother must be heterozygous/carrier of recessive allele
• What evidence suggests that a recessive phenotype is caused by a gene on the X chromosome?

  • Only males tend to have the phenotype
• What evidence shows that a gene for a phenotype is not on the X chromosome?

  • Named phenotype father has daughter without phenotype
  • Father would pass on allele for phenotype on X chromosome so daughter would have phenotype. same with mother and son.
• What is autosomal linkage?

  ● Two genes located on same autosome (non-sex chromosome)

  ● So alleles on same chromosome inherited together

  ○ Stay together during independent segregation of homologous chromosomes during meiosis

  ● But crossing over between homologous chromosomes can create new combinations of alleles

  ○ If the genes are closer together on an autosome, they are less likely to be split by crossing over
• How does crossing over affect autosomal linkage?

  • Creates new combinations of alleles
  • Closer genes are less likely to be split
• What does a chi-squared test determine?

  If observed results differ from expected results
• What type of data is used in a chi-squared test?

  Categorical data
• Why might observed phenotypic ratios differ from expected ratios in genetic crosses?

  • Random fusion of gametes
  • Autosomal linkage, epistasis, sex-linkage
  • Small sample size
  • Some genotypes may be lethal
• How is a chi-squared value calculated?

  O = observed, E = expected frequencies
• How is a chi-squared value analysed?

  1. Degrees of freedom = categories - 1
  2. Determine critical value at p = 0.05
  3. Compare X² value to critical value. If X2 value is [greater / less] than critical value at p < 0.05

  ○ Difference [is / is not] significant so [reject / accept] null hypothesis

  ○ So there is [less / more] than 5% probability that difference is due to chance
• What is epistasis?

  • Interaction of non-linked genes
  • One gene masks expression of another
• What is the expected phenotype ratio if genes are not linked?

  • 9:3:3:1 ratio