Biology section 3

• Sexual reproduction


  Genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent
• Sexual reproduction


  1. Father and mother produce gametes (reproductive cells)

  2. Gametes only contain half the number of chromosomes of normal cells (haploid)

  3. Gamete fusion produces a fertilised egg (zygote) with the full set of chromosomes (diploid)

  4. Zygote undergoes cell division (mitosis) and develops into an embryo

  5. Embryo inherits characteristics from both parents
• Meiosis


  A type of cell division that produces genetically different cells, happens in reproductive organs
• Meiosis


  1. DNA is duplicated before cell division

  2. Chromosomes line up in pairs in the centre of the cell

  3. Pairs are pulled apart, each new cell gets one copy of each chromosome

  4. Chromosomes line up again, chromatids are pulled apart

  5. Four haploid daughter cells (gametes) are produced, all genetically different
• DNA


  Molecule that carries instructions for characteristics, made up of nucleotides
• DNA nucleotide


  • Contains one sugar molecule, one phosphate molecule, and one base (A, T, C, or G)
  • Sugar and phosphate molecules form the backbone, bases join in complementary pairs (A-T, C-G)
• Chromosome


  Long, coiled up molecule of DNA found in the nucleus of eukaryotic cells
• Gene


  Section of DNA on a chromosome that codes for a particular protein
• Genome


  All of an organism's DNA
• Extracting DNA from fruit cells

  1. Mash fruit, add detergent and salt to release DNA

  2. Filter to remove large bits

  3. Add cold alcohol to precipitate the DNA
• Allele


  Different versions of the same gene
• Homozygous


  Having two of the same alleles for a gene
• Heterozygous


  Having two different alleles for a gene
• Dominant allele


  Allele that overrules a recessive allele
• Recessive allele


  Allele that is only expressed if both alleles are recessive
• Genotype


  Combination of alleles an organism has
• Phenotype


  Characteristics an organism displays, determined by its genotype
• Monohybrid cross


  Shows inheritance of a single characteristic
• Monohybrid cross example


  • Hamsters with superpowers (recessive) vs normal hamsters (dominant)
• Sex chromosomes


  23rd chromosome pair, XX for females, XY for males
• Sex determination in humans

  1. Eggs have one X chromosome, sperm can have X or Y

  2. Whether sperm with X or Y fertilises egg determines sex of offspring
• Family pedigree


  A family tree showing inheritance of genetic disorders
• Females


  Have two X chromosomes (XXX). The XX combination allows female characteristics to develop.
• There's an equal chance of having either a boy or a girl
• When you plug all the letters into the diagram, it shows that there are two XX results and two XY results, so there's the same probability of getting a boy or a girl
• This 50:50 ratio is only probability. If you had four kids they could all be boys
• Eggs


  Have one X chromosome
• Sperm


  Can have either an X chromosome or a Y chromosome
• Sex determination in humans depends on whether the sperm that fertilises an egg carries an X or a Y
• Cystic fibrosis


  A genetic disorder of the cell membranes
• The allele which causes cystic fibrosis (CF) is a recessive allele, carried by about 1 person in 30
• Because it's recessive, people with only one copy of the allele won't have the disorder-they're known as carriers
• For a child to have a chance of inheriting the disorder, both parents must either have the disorder themselves or be carriers
• There's a 1 in 4 chance of a child having CF if both parents are carriers
• The allele for cystic fibrosis isn't dominant because plenty of the family carry the allele but don't have the disorder
• There is a 1 in 4 (25%) chance that the new baby will have cystic fibrosis and a 1 in 2 (50%) chance that it will be a carrier because both of its parents are carriers but don't have the disorder
• The probability of each outcome can also be expressed as a ratio 1:2:1 for unaffected: carrier: disorder
• Variation


  Differences between organisms of the same species
• Different species look different, but even organisms of the same species will usually look at least slightly different
• Genetic variation


  Caused by organisms having different alleles (versions of genes) which can lead to differences in phenotype (the characteristics an organism displays)