Genetics, Populations, ecosystem and inheritance

• Topics

  • 7.1 Inheritance
  • 7.2 Populations
  • 7.3 Evolution may lead to speciation
  • 7.4 Populations in ecosystems
  • Required practical 12
• Genotype


  Genetic constitution of an organism
• Phenotype

  The expression of this genetic constitution (genotype) and its interaction with the environment
• Alleles

  Variations of a particular gene (same locus) that arise by mutation (changes in DNA base sequence)
• Diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs), so they can have up to 2 alleles of a gene
• There may be many (more than 2) alleles of a single gene in a population
• 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)
• 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
• 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
• Monohybrid cross (basic)


  1. Parental phenotypes

  2. Parental genotypes

  3. Gamete genotypes

  4. Offspring genotypes

  5. Offspring phenotypes

  6. Ratio
• Monohybrid cross with multiple alleles

  1. Parental phenotypes

  2. Parental genotypes

  3. Gamete genotypes

  4. Offspring genotypes and phenotypes
• Monohybrid cross using a pedigree diagram

  1. Parental phenotypes

  2. Parental genotypes

  3. Gamete genotypes

  4. Offspring genotypes

  5. Offspring phenotypes

  6. Probability
• Sex-linked gene

  A gene with a locus on a sex-chromosome (normally X)
• Males are more likely to express a recessive X-linked allele because they have only one X chromosome
• Monohybrid cross with sex-linkage

  1. Parental phenotypes

  2. Parental genotypes

  3. Gamete genotypes

  4. Offspring genotypes

  5. Offspring phenotypes

  6. Probability
• Monohybrid cross with sex-linkage and codominance

  1. Parental phenotypes

  2. Parental genotypes

  3. Gamete genotypes

  4. Offspring genotypes

  5. Offspring phenotypes

  6. Ratio
• Dihybrid cross with sex linkage

  1. Parental phenotypes

  2. Parental genotypes

  3. Gamete genotypes

  4. Offspring genotypes

  5. Offspring phenotypes

  6. Ratio
• Autosomal linkage

  Two genes located on same autosome (non-sex chromosome), so alleles on same chromosome inherited together
• Crossing over between homologous chromosomes can create new combinations of alleles, but if the genes are closer together on an autosome, they are less likely to be split by crossing over
• Autosomal linkage (example 1)

  Explanation of results
• Autosomal linkage (example 2)

  1. Phenotype of offspring

  2. Number of offspring
• The result of this cross was 225 offspring with a grey body & long wings and 220 with a black body & short wings
• The two genes are linked

  • Autosomal linkage
  • No crossing over occurs
  • Genes are close together
• So only GL and gl gametes produced, no Gl and gL gametes produced, no Ggll and ggLl offspring produced
• Genes for height and type of leaf are on the same homologous pair of chromosomes


  • Crossing over has occurred
  • Few Tm and tM gametes produced
  • Fewer tall, mottled and dwarf, normal offspring produced
• Epistasis


  Interaction of (products of) non-linked genes where one masks / suppresses the expression of the other
• Dihybrid cross with epistasis

  1. Parental genotypes: aabb, AaBb

  2. Gamete genotypes: ab, AB, ab, aB, Ab

  3. Offspring genotypes: AaBb, Aabb, aaBb, aabb

  4. Offspring phenotypes: White (x2), yellow, green

  5. Ratio: 2:1:1
• Chi-squared (X2) test

  • Used to determine if observed results are significantly different from expected results (frequencies)
  • Data is categorical (can be divided into groups eg. phenotypes)
• Observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios
• Calculating chi-squared value

  1. O = frequencies observed

  2. E = frequencies expected (multiply total n with each expected ratio as a fraction)
• Analysing chi-squared value

  1. Number of degrees of freedom = number of categories - 1

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

  3. 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
• A population is a group of organisms of the same species in one area at one time that can interbreed
• Gene pool


  All the alleles of all the genes in a population at any one time
• Allele frequency


  Proportion of an allele of a gene in a gene pool (decimal or percentage)
• Hardy-Weinberg principle


  • Allele frequencies will not change from generation to generation, given: population is large, no immigration/emigration, no mutations, no selection, mating is random
• Hardy-Weinberg equation


  • p2 + 2pq + q2 = 1
  • p = frequency of one (usually dominant) allele
  • q = frequency of the other (usually recessive) allele
  • p2 = frequency of homozygous (usually dominant) genotype
  • 2pq = frequency of heterozygous genotype
  • q2 = frequency of homozygous (usually recessive) genotype