topic 3: reproduction and inheritance

• Advantages of sexual reproduction

  • Produces variation in offspring
  • Decreases chance of whole species becoming extinct
• Advantages of asexual reproduction

  • Only one parent is needed
  • Uses less energy and is faster as organisms do not need to find a mate
  • In favorable conditions lots of identical offspring can be produced
• Fertilisation


  The fusion of a male and female gamete to produce a zygote that undergoes cell division and develops into an embryo
• Adaptations of insect-pollinated plants

  • Large and bright petals to attract insects
  • Scented with nectar to attract insects
  • Sticky and moderate amounts of pollen grains
  • Anthers inside flower, stiff and attached so that insects can brush past
  • Stigma inside flower, sticky so pollen grains stick to it when an insect brushes past
• Adaptations of wind-pollinated plants

  • Small and dull petals - usually green or brown
  • No scent or nectar
  • Smooth and light pollen grains so they can be easily carried in the wind and in large amounts
  • Anthers outside flower, loose on long filaments so that pollen can be released easily
  • Stigma outside flower, feather so forms network to catch pollen grains drifting in the wind
• Seed and fruit formation

  1. Pollen grains land on stigma

  2. Pollen tube grows out of pollen grain and down style into ovary and then to ovule

  3. Male nucleus travels down pollen tube to fuse with female egg nucleus in ovule, forming zygote

  4. Zygote undergoes mitosis to form seed

  5. Ovule becomes seed and ovule wall becomes seed coat

  6. Ovary becomes fruit
• Conditions needed for seed germination

  • Water to activate enzymes to break down starch food reserves
  • Oxygen for aerobic respiration to release energy for growth
  • Warmth to increase growth rate
• Structures of a germinating seed

  • Embryo (young root and shoot)
  • Food store (starch)
  • Seed coat (protective covering)
• Asexual reproduction


  Produces clones as it only involves one parent, unlike sexual reproduction
• Natural asexual reproduction


  • Runners (e.g. strawberry plants)
• Artificial asexual reproduction


  Cuttings (tissue samples scraped from parent plant and placed in agar growth medium)
• Male reproductive system structures


  • Sex gland (produces semen with sperm)
  • Sperm duct
  • Testis (produces sperm and testosterone)
  • Penis (passes urine and semen out of body)
  • Urethra (tube inside penis to carry urine or semen)
• Female reproductive system structures

  • Ovary (contains ova)
  • Oviduct (connects ovary to uterus)
  • Uterus (has thick lining for implantation)
  • Cervix (ring of muscle at lower end of uterus)
  • Vagina (muscular tube)
• Secondary sexual characteristics in females

  • Breast development
  • Menstrual cycle begins
  • Growth of body hair
  • Widening of hips
  • Increased height
• Secondary sexual characteristics in males

  • Growth of penis and testes
  • Production of sperm
  • Growth of facial and body hair
  • Muscle development
  • Voice lowering and breaking
• Menstrual cycle


  1. Oestrogen causes thickening of uterus

  2. Progesterone maintains thick uterus lining
• Placenta


  Allows diffusion of nutrients and gases between mother's and foetus's blood
• Amniotic fluid


  Liquid in bag surrounding foetus that protects and cushions it
• Gamete


  An organism's reproductive cell (egg or sperm) with half the normal number of chromosomes
• Genome


  The entire DNA of an organism
• Chromosome


  A structure in the nucleus made up of a long strand of DNA
• Gene


  A short section of DNA that codes for a protein and contributes to a characteristic
• Allele/variant


  The different forms of a gene - humans have two alleles for each gene
• Dominant allele


  Only one copy is needed to be expressed and observed
• Recessive allele


  Two copies are needed to be expressed and observed
• Homozygous


  When both inherited alleles are the same
• Heterozygous


  When one inherited allele is dominant and the other is recessive
• Genotype


  The combination of alleles an individual has
• Phenotype


  The physical characteristics that are observed in the individual
• Punnett square


  Used to describe monohybrid inheritance, looking at the probability of offspring having certain genotypes and phenotypes
• Dominant characteristics


  Represented by uppercase letters
• Recessive characteristics


  Represented by lowercase letters
• Human body cells


  • 23 pairs of chromosomes
  • 22 control characteristics
  • 23rd pair carries sex determining genes
• X and Y chromosomes

  Determine the sex of an organism
• Mitosis


  1. Cell grows and organelles increase

  2. Chromosomes replicate

  3. Chromosomes line up at the equator

  4. Spindle fibres pull chromosomes to the poles

  5. Cytoplasm and cell membranes divide to form two identical daughter cells
• Cell division by mitosis


  Occurs during growth, development, replacing damaged cells, and cloning
• Meiosis


  1. Cell makes copies of its chromosomes

  2. Cell divides into two cells with half the normal amount of chromosomes

  3. Each cell divides into two again to produce four genetically different gamete cells
• Genetic variation


  Difference in DNA sequences of individuals within the same species
• Sources of variation


  • Genetic
  • Environmental
  • Interaction of genetics and environment
• Mutations


  Rare, random changes in genetic material that can be inherited