Inheritance, Variation and Evolution

• Sexual reproduction

  • Type of reproduction
  • Involves the production of gametes by meiosis
  • A gamete from each parent fuses to form a zygote
  • Genetic information from each gamete is mixed so the resulting zygote is unique
• Gametes

  • Sex cells (sperm cells and egg cells in animals, pollen and egg cells in flowering plants)
  • Haploid (half the number of chromosomes)
• Meiosis

  1. Form of cell division involved in the formation of gametes (non-identical haploid cells) in reproductive organs

  2. Chromosome number is halved

  3. Involves two divisions
• Interphase

  Copies of genetic information are made during this process
• First stage of meiosis

  1. Chromosome pairs line up along the cell equator

  2. The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation)

  3. Chromosome number is halved
• Second stage of meiosis

  1. Chromosomes line up along the cell equator

  2. The chromatids are separated and move to opposite poles of the cell

  3. Four unique haploid gametes are produced
• Importance of meiosis for sexual reproduction

  • It increases genetic variation
  • It ensures that the zygote formed at fertilisation is diploid
• Fertilisation


  1. Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells)

  2. As the embryo develops, cells differentiate
• Advantage of sexual reproduction

  • It creates genetic variation in offspring, increasing the probability of a species adapting to and surviving environmental changes
  • Natural selection can be speeded up by humans in selective breeding to increase food production
• Disadvantages of sexual reproduction

  • Two parents are required. This makes reproduction difficult in endangered populations or in species which exhibit solitary lifestyles
  • More time and energy is required so fewer offspring are produced
• Asexual reproduction


  • Type of reproduction
  • Involves mitosis only
  • Produces genetically identical offspring known as daughter cells
• Advantages of asexual reproduction

  • Only one parent is required
  • Lots of offspring can be produced in a short period of time, enabling the rapid colonisation of an area and reducing competition from other species
  • Requires less energy and time as do not need a mate
• Disadvantage of asexual reproduction

  • No genetic variation (except from spontaneous mutations) reducing the probability of a species being able to adapt to environmental change
• Plant reproduction


  1. Sexual reproduction to produce seeds

  2. Asexual reproduction by runners (e.g. strawberry plants) or bulb division (e.g. daffodils)
• DNA


  • A double-stranded polymer of nucleotides, wound to form a double helix
  • The genetic material of the cell found in its nucleus
• Genome


  The entire genetic material of an organism
• Importance of understanding the human genome

  • Searching for genes linked to different types of disease
  • Understanding and treating inherited disorders
  • Tracing human migration patterns from the past
• Chromosome


  A long, coiled molecule of DNA that carries genetic information in the form of genes
• Human body cells have 46 chromosomes (23 pairs)
• Human gametes have 23 chromosomes
• Gene


  A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
• Four bases found in nucleotides

  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• DNA nucleotides


  • Common sugar
  • Phosphate group
  • One of four bases: A, T, C or G
• How nucleotides interact to form a molecule of DNA

  1. Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand

  2. Base connected to each sugar

  3. Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds
• How a gene codes for a protein

  1. A sequence of three bases in a gene forms a triplet

  2. Each triplet codes for an amino acid

  3. The order of amino acids determines the structure (i.e. how it will fold) and function of protein formed
• Protein folding


  The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate
• Mutation


  • A random change in the base sequence of DNA which results mostly in no change to the protein coded for, or genetic variants of the protein (slight alteration but appearance and function remain)
  • Mutations occur continuously
• Effect of a gene mutation in coding DNA

  • If a mutation changes the amino acid sequence, protein structure and function may change (an enzyme may no longer fit its substrate binding site or a structural protein may lose its strength)
  • If a mutation does not change amino acid sequence, there is no effect on protein structure or function
• Non-coding DNA


  DNA which does not code for a protein but instead controls gene expression
• Effect of a gene mutation in non-coding DNA

  Gene expression may be altered, affecting protein production and the resulting phenotype
• Alleles


  Different versions of the same gene
• Dominant allele


  A version of a gene where only one copy is needed for it to be expressed
• Recessive allele


  A version of a gene where two copies are needed for it to be expressed
• Homozygous


  When an organism has two copies of the same allele (two recessive or two dominant)
• Heterozygous


  When an organism has two different versions of the same gene (one dominant and one recessive)
• Genotype


  The genes present for a trait
• Phenotype


  The visible characteristic
• Dominant alleles in a Punnett square


  They are represented using uppercase letters
• Recessive alleles in a Punnett square

  They use the lowercase version of the same letter as the dominant allele
• Recessive alleles


  Represented in a Punnett square using the lowercase version of the same letter as the dominant allele