Reproduction & Inheritance

• Gametes


  The sex cells of an organism
• Fertilisation


  The fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)

  
• Gametes


  • Formed during meiosis and only have one copy of each chromosome, so they are haploid cells
  • For humans, the sperm and egg cells contain 23 single chromosomes in their nucleus

  
• Sperm cells


  • Have a flagellum (tail) that allows them to swim towards the egg cell
  • Contain many mitochondria that provide energy for movement of the flagellum (swimming)
  • An acrosome that contains digestive enzymes to break down the zona pellucida surrounding the egg cell

  
• Egg cells


  • Are much larger than sperm cells as most of their internal space contains food to nourish a growing embryo
  • Have follicle cells that form a protective coating
  • Have a jelly-like glycoprotein layer (zona pellucida) that forms an impenetrable barrier after fertilisation by a sperm cell has occurred
• Fertilisation in mammals


  1. Semen is ejaculated high up into the vagina of the female, near the cervix

  2. Sperm cells follow a chemical trail released by the egg cell and travel up through the cervix to reach the uterus

  3. If a sperm cell meets the egg cell in the oviduct, fertilisation can occur

  4. The head of a sperm cell releases enzymes that digest a path through the zona pellucida, allowing the sperm to pass through the egg cell membrane

  5. The egg cell immediately releases the contents of cortical granules into the space between the egg cell membrane and the zona pellucida, causing the zona pellucida to rapidly thicken and harden

  6. The nucleus of the sperm cell then enters the egg and fuses with the nucleus of the egg cell, forming a zygote
• Locus


  The location of a gene on a chromosome
• Alleles


  Different forms of a gene that have slightly different nucleotide sequences but occupy the same position (locus) on the chromosome
• Autosomal linkage

  Autosomal linkage only occurs on the autosomes

  Two or more genes on the same autosome do not assort independently during meiosis, instead they stay together in the original parental combination
• Sex linkage


  Genes that are only present on one sex chromosome and not the other, their inheritance is dependent on the sex of the individual

  Males only have one X chromosome, they are much more likely to show sex-linked recessive conditions
• Meiosis


  1. Meiosis I: The nucleus of the original 'parent' cell is diploid (2n) and the homologous pairs of chromosomes are split up, producing two haploid (n) nuclei

  2. Meiosis II: The chromatids that make up each chromosome separate to produce four haploid (n) nuclei
• Meiosis


  • Produces genetically different gametes through crossing over and independent assortment
• Crossing over


  The process by which non-sister chromatids exchange alleles

  This occurs when homologous chromosomes pair up and are in close proximity during meiosis I

  It can result in a new combination of alleles on the two chromosomes
• Independent assortment


  The random alignment of homologous pairs along the equator of the spindle during meiosis I, leading to different combinations of alleles in daughter cells