Genetics

• Genotype is the genetic constitution of an organism
• Phenotype is the appearance of a characteristic due to the expression of the genotype and its interaction with the environment
• Alleles are different forms of the same gene
• Locus is the position of a gene on a chromosome

  Alleles may occupy the same locus on each member of a pair of homologous chromosomes
• A dominant allele is always expressed in the phenotype
• A recessive allele is only expressed when the genotype is homozygous recessive
• Homozygous refers to both alleles being the same
• BB = homozygous dominant
• bb = homozygous recessive
• Heterozygous refers to both alleles differing e.g. Bb
• Codominance is where both alleles are expressed in phenotype
• Multiple allelism is where there are more than 2 alleles of a particular gene however only 2 alleles can be present in a genotype
• Sex linked gene is where the gene is carried on only one type of sex chromosome usually the X chromosome
• Autosomal gene linkage is where genes are present on the same chromosome at different loci and as a result are usually inherited togther
• Meiosis

  • 2 homologous chromosomes come together and segregate into separate gametes
  • each gamete contains only one of the original pair of alleles
• Monohybrid inheritance

  • inheritance of one particular characteristic
  • 3:1 ratio is Mendel's monohybrid ratio
• Test cross

  • used to determine the genotype of an organism with a dominant phenotype
  • always involved crossing the organism with the homozygous recessive
• Sampling error

  • the larger the sample the more likely that the observed ratio will be similar to the expected ratio
  • if only a small number of offspring are produced then the greater is the sampling error and less likely that the observed ratio is the same as the expected ratio
• Codominance

  • where both alleles are expressed in the phenotype
  • neither allele is dominant with both contributing to the phenotype of the characteristic
• Multiple allelism

  • where there are more than 2 alleles of a particular gene
  • only 2 alleles can be present in a genotype
  • e.g. ABO blood groups
• ABO blood groups

  • 4 phenotypes
  • blood groups A, B, AB and O
  • 3 alleles where only 2 can be present in one genotype
• Sex determination

  • diploid number in humans is 46, 23 homologous pairs of chromosomes
  • consists of 22 pairs of non sex chromosomes and 1 pair of sex chromosomes
  • XX is female
  • XY is male
  • probability is 50%
• Sex linked genes

  • gene is carried on only one type of sex chromosome, usually the X chromosome
  • males require 1 recessive allele to have the condition as they only have one X chromosome
  • females require 2 recessive alleles to have the condition as they have 2 X chromosomes
  • females can be heterozygous
  • e.g. colour blindness, haemophilia
• Allele: recessive

  • Evidence: 2 unaffected parents who have an affected child
  • Explanation: parents must be heterozygous as they are unaffected, the condition must be caused by a recessive allele
• Allele: dominant

  • Evidence: 2 affected parents who have unaffected children
  • Explanation: both parents must be heterozygous and pass on their recessive alleles as they are affected the condition must be caused by a dominant allele
• Allele: not sex-linked (where known to be dominant)

  • Evidence: an affected father who has an unaffected daughter
  • Explanation: the father would pass on the dominant X allele chromosome so all daughters would be affected
• Allele: not sex-linked (when known to be dominant)

  • Evidence: an unaffected mother who has an affected son
  • Explanation: the unaffected mother would only be able to pass on the recessive allele to her sons so (if it were sex-linked) they should also be unaffected
• Allele: not sex-linked (when known to be recessive)

  • Evidence: an unaffected father who has an affected daughter
  • Explanation: the father would pass on the dominant allele on the X chromosome (if sex-linked) so all daughter would be unaffected
• Allele: not sex-linked (when known to be recessive)

  • Evidence: an affected mother who has an unaffected son
  • Explanation: the affected mother would only be able to pass on the recessive allele to her sons so they should also be affected
• Dihybrid inheritance

  • refers to the inheritance of 2 different characteristics each controlled by a different gene
  • a genetic cross is carried out between two pure breeding (i.e. homozygous for each allele)
  • due to independent assortment of homologous chromosomes during meiosis each offspring plant would produce 4 different types of gamete with respect to the genes
  • 2 genes are on separate pairs of homologous chromosomes and each pair align independently on the spindle during meiosis I ~
  • 9:3:3:1 ratio is expected
• Dihybrid cross observed and expected ratio may not be similar due to:

  • small sample size- sampling error would be greater
  • fusion of gametes at fertilisation is random
  • selection against a particular phenotype
• Chi-squared test

  • used to determine if there is a statistical difference between the expected and observed ratios from the results of a genetic cross
• Epistasis

  • occurs when 2 or more genes contribute to a phenotype
  • the allele of one gene affected or masks the expression of another gene at another locus
  • often occurs in metabolic pathways controlled by enzymes coded for by different genes
  • each enzyme is dependent on the previous enzyme for its substrate
  • in epistasis the expression of one gene can mask the expression of another
• Autosomal linked genes

  • autosomal chromosomes are non-sex chromosomes
  • genes are present on the same chromosome at different loci
  • linked genes are inherited together so fewer genetic combination of their alleles are possible in the gametes (no independent assortment)
  • leads to reduced variety of gametes produced which reduces the variety of offspring produced
  • linked ratio is 3:1
• Hardy-Weinberg principle

  • method used to calculate the frequencies of the alleles and genotypes for a particular gene in a population
  • p= the frequency of dominant allele
  • q= frequency of the recessive allele
  • in a population where there are only 2 alleles: p + q = 1.0
• Hardy-Weinberg principle

  • p2 + 2pq + q2 = 1.0 (1.0 is the percentage of the population)
  • p2 = the frequency of the homozygous dominant genotype
  • 2pq = the frequency of the heterozygous genotype
  • q2 = the frequency of the homozygous recessive genotype
• Hardy-Weinberg equilibrium is based upon the principle that states the frequency of dominant and recessive alleles in a population will remain constant from generation to generation provided that:

  1. the population is large
  2. mating within the population is random
  3. no mutations of the gene occur
  4. there's no selection i.e. all alleles are likely to be passed to the next generation
  5. the population is isolated; there is no flow of alleles into or out of the population