Inheritence

• 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 and its interaction with the environment
• What is a gene?

  A base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule
• What are alleles and how do they arise?

  • Variations of a particular gene (same locus)
  • They arise by mutations (changes in DNA base sequence)
• How many alleles of a gene can be found in diploid organisms?

  • 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
  • But there may be many (more than 2) variations of a single gene in a population
• Describe a dominant allele:

  Always expressed (shown in the phenotype)
• Describe a recessive allele:

  • Only expressed when 2 copies present (homozygous recessive)
  • Not expressed when dominant allele present (heterozygous)
• Describe codominant alleles:

  Both alleles expressed/contribute to phenotype (if inherited together)
• What is meant by the term homozygous?

  Alleles at a specific locus (on each homologous chromosome) are the same
• What is meant by the term heterozygous?

  Alleles at a specific locus (on each homologous chromosome) are different
• What does a monohybrid cross show?

  Inheritance of one phenotypic characteristic coded for by a single gene
• What does a dihybrid cross show?

  Inheritance of two phenotypic characteristics coded for by two different genes
• Explain how autosomal linkage affects inheritance of alleles:

  • Two genes located on same autosome (non sex chromosome)
  • So alleles on same chromosomes are 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
• What is a sex-linked gene?

  A gene with a locus on a sex chromosome (normally X)
• Explain why males are more likely to express a recessive X linked allele?

  • Females (XX) have 2 alleles -> only express recessive allele if homozygous recessive but can be carriers
  • Males (XY) have 1 allele (inherited from mother) -> recessive allele always expressed
• What is epistasis?

  Interaction of non linked genes where one masks / suppresses the expression of the other
• Describe when a Chi-squared test can be used:

  • when determining if observed results are significantly different from expected results
  • Data is categorical (can be divided into groups)
• Suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios:

  • Fusion / fertilisation of gametes is random
  • Autosomal linkage / epistasis / sex-linkage
  • Small sample size = not representative
  • Some genotypes maybe be lethal
• What is the formula for Chi-squared?

  
• Describe how a chi- squared value can be analysed:

  1. Number of degrees of freedom = number of categories - 1
  2. Determine critical value at p = 0.05/5%
  3. If chi squared 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 a 5% chance the difference is due to chance