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 and how do they arise?

  Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)
• homologous chromosomes:

  A) homozygous dominant

  B) heterozygous

  C) homozygous recessive

  
• 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) alleles of a single gene in a population
• Describe the different types of alleles

  Dominant allele - Always expressed (shown in the phenotype)

  Recessive allele - Only expressed when 2 copies present (homozygous recessive)

  / NOT expressed when dominant allele present (heterozygous)

  Codominant alleles - Both alleles expressed / contribute to phenotype (if inherited together)
• What is meant by the terms homozygous and heterozygous?

  Homozygous - Alleles at a specific locus (on each homologous chromosome) are the same

  Heterozygous - Alleles at a specific locus (on each homologous chromosome) are different
• 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
• Explain the evidence from a pedigree diagram which would show that the

  allele for [named phenotype] is dominant

  ● [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]
• Explain the evidence from a pedigree diagram which would show that the

  allele for [named phenotype] is recessive

  ● 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 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

  This assumes males are XY and females are XX, as in humans. In some organisms, it is swapped. In these cases,

  females (XY) would be 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
• Explain the evidence from a pedigree diagram which would show that the

  allele for [named phenotype] on the X-chromosome is recessive

  ● Mother [n] WITHOUT [named phenotype] has child [n] WITH [named phenotype]

  ● So mother [n] must be heterozygous / carrier of recessive allele
• Explain the evidence from a pedigree diagram which would suggest that

  [named recessive phenotype] is caused by a gene on the X chromosome

  Only males tend to have [named recessive phenotype]
• Explain the evidence from a pedigree diagram which would show that the

  gene for [named phenotype] is not on the X chromosome

  ● [Named phenotype] father [n] has daughter [n] WITHOUT [named phenotype]

  ● Father [n] would pass on allele for [named phenotype] on X chromosome so

  daughter [n] would have [named phenotype]

  OR

  ● [Named phenotype] mother [n] has son [n] WITHOUT [named phenotype]

  ● Mother [n] would pass on allele for [named phenotype] on X chromosome so

  son [n] would have [named phenotype]
• Explain how autosomal linkage affects inheritance of alleles

  ● 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
• What is epistasis?

  Interaction of (products of) non-linked genes where one masks / suppresses the expression of the other
• Describe when a chi-squared ($x^2$) test can be used


  ● When determining if observed results are significantly different from expected results (frequencies)

  ○ Eg. comparing the goodness of fit of observed phenotypic ratios with expected ratios

  ● Data is categorical (can be divided into groups eg. phenotypes)
• 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 of whole population

  ● Some genotypes may be lethal (cause death)
• Describe how a chi-squared value can be calculated

  $x^2\ =$$\ \sum_{ }^{ }\frac{\left(O-E\right)^2}{E}$ O = frequencies observes

  E = frequencies expected
• Describe how a chi-squared value can be analysed

  1. Number of degrees of freedom = number of categories - 1 (eg. 4 phenotypes = 3 degrees of freedom)

  2. Determine critical value at p = 0.05 (5% probability) from a table

  3. If X squared 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
• a geneticist carried out a cross between fruit flies with grey bodies and long wings and fruit flies with black bodies and short wings. results only showed 2 phenotypes: grey body AND long wings and black body AND short wings. explain these results

  • 2 genes are linked (autosomal linkage)
  • no crossing over
  • no Gl and no gL