Reproduction

• sexual reproduction 


  • fertilisation: fusion (joining) of male + female gametes in meiosis
  • mixing of genetic info
  • variation - offspring receives genetic info
• asexual reproduction

  • one parent only
  • doesn’t involve gametes
  • no variation - no mixing of genetic info
  • clones: genetically identical offspring
  • mitosis
• gamete


  • male - sperm
  • female - egg
  • single, not paired chromosomes
• stages of meiosis


  • all chromosomes are copied
  • cell divides in two
  • cells divide again, forming gametes
• meiosis

  • halves the number of chromosomes
  • four gametes
• fertilisation

  • full number of chromosomes is restored
• what happens after fertilisation?

  • cells divide by mitosis, produce embryo
  • embryo develops, cells differentiate
• DNA

  • made of two strands
  • each strand is a polymer
  • double helix
  • found in chromosomes
• gene encodes for sequence of amino acids to make a specific protein
• genome


  entire genetic material of an organism
• advantages of human genome

  • help search for genes that are linked to disease
  • understand and treat inherited disorders
  • ancestry
• allele


  version of a gene
• what does a gene control?


  characteristic
• genotype 


  alleles present
• phenotype


  characteristics caused by person’s alleles
• homozygote


  two identical alleles e.g. BB or bb
• heterozygote


  two different alleles e.g. Bb
• dominant allele


  only one copy present to be expressed in phenotype
• recessive allele


  two copies are present to be expressed in the phenotype
• inherited disorders


  • cystic fibrosis
  • polydactyly
  • Huntington’s disease
  • sickle cell anaemia
• what is cystic fibrosis?

  • cell membrane disorder
  • thick mucus in airways and pancreas
• what causes cystic fibrosis?

  • recessive allele
  • carrier has one recessive allele
• what is polydactyly?


  • having extra fingers or toes
• what causes polydactyly?


  • dominant allele
• screening for genetic disorders

  1. collect cells from embryo/fetus
  2. screening embryo
  3. DNA isolated from embryo and tested
  4. parents decide to keep baby or terminate pregnancy
• how to collect cells for screening

  • amniocentesis
  • chorionic villus sampling
• amniocentesis

  • 15-16 weeks of pregnancy
  • use needle to extract amniotic fluid around fetus
  • fetal cells used for screening
• chorionic villus sampling

  • 10-12 weeks of pregnancy
  • take small sample of tissue from placenta