Chapter 13: Inheritance

• Gene


  A sequence of nucleotides, that is part of a DNA molecule, controls the production of one particular polypeptide. It is also a unit of inheritance.
• Allele


  Different forms of the same gene. Alleles occupy the same locus on a pair of homologous chromosomes.
• One chromosome is from father and another is from mother, if they have the exact same genes, they are homologous chromosomes.
• Alleles of "eyelid type" gene

  • Single eyelid - AAA AAA AAA
  • Double eyelid - AAA TAA AAA

  
• Genotype


  The combination of alleles for a particular gene.
• Phenotype


  The expressed trait or outward appearance.
• Homozygous


  Having the two identical alleles of a particular gene.
• Heterozygous


  Having two different alleles of a particular gene.
• Dominant allele


  Expresses itself and gives the same phenotype in both homozygous dominant and heterozygous conditions.
• Recessive allele


  Only expresses itself in homozygous recessive condition.

  
• Genetic diagram


  A model used to explain how alleles are passed on to an offspring and predict the phenotype that will be displayed by an offspring.

  
• Punnett square


  A genetic model used to predict the phenotype that will be displayed by an offspring.

  
• Ratio = 3 tall : 1 dwarf NOT 3:1. Chance = 75% tall and 25% dwarf!
• Genetic diagram for PKU cross between homozygous dominant and homozygous recessive parents


  N/N x n/n

  
• Punnett square for PKU cross between homozygous dominant and homozygous recessive parents


  N/N x n/n
• Genetic diagram for PKU cross between two heterozygous parents


  N/n x N/n
• Punnett square for PKU cross between two heterozygous parents


  N/n x N/n
• Observed ratios different from expected ratios

  Reason: 1. The sample size is too small. 2. Fertilisation is a random process.
• Codominance


  When two alleles controlling a trait both express themselves in the organism.
• Multiple alleles


  Gene that exists in more than two alleles.
• Sex chromosomes


  X and Y chromosomes.
• Sex determination genetic diagram


  Male gametes contain X or Y, female gametes contain X

  
• Mutation


  The sudden random change in the sequence of a gene or chromosome number.

  
• Sickle-cell anaemia


  • Caused by a gene mutation that results in haemoglobin S instead of normal haemoglobin A. This makes red blood cells sickle-shaped and have lower oxygen carrying capacity.

  
• Albinism


  • Caused by a gene mutation that results in loss of pigments in skin, hair and eyes. Sensitive to sunlight.
• Down syndrome


  • Caused by an extra copy of chromosome 21. Results in characteristic facial features and mental/physical difficulties.

  
• Continuous variation


  Traits that show a range that can be expressed in infinite decimal places.
• Discontinuous variation

  Traits that show clear and distinct traits that is either a "yes" or "no".
• Continuous variation is affected by many genes, discontinuous variation is affected by 1 or few genes.

  
• Continuous variation


  • Height, skin colour, weight, intelligence
• Discontinuous variation


  • Eye-lid types, ability to roll tongue, blood group