Inheritance, variation and Evolution

• DNA


  Decayribonucleic acid, the chemical that all of the genetic material in a cell is made up from
• DNA


  • It contains coded information-basically all the instructions to put an organism together and make it work
  • It determines what inherited characteristics you have
• Chromosomes


  Really long molecules of DNA found in the nucleus of animal and plant cells
• Chromosomes normally come in pairs
• DNA shape


  A polymer made up of two strands coiled together in the shape of a double helix
• Gene


  A small section of DNA found on a chromosome that codes for a specific sequence of amino acids which are put together to make a specific protein
• Only 20 amino acids are used, but they make up thousands of different proteins
• Genome

  The entire set of genetic material in an organism
• Understanding the human genome

  • Allows scientists to identify genes linked to different diseases
  • Helps understand inherited diseases and develop treatments
  • Allows tracing the migration of human populations
• 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. Gametes (sex cells) are produced

  2. Fertilisation occurs when the egg and sperm cells fuse

  3. The fertilised egg has the full number of chromosomes (half from the father, half from the mother)
• Asexual reproduction


  There is only one parent so the offspring are genetically identical to that parent
• Asexual reproduction

  Occurs by mitosis-an ordinary cell makes a new cell by dividing in two
• Meiosis


  The process that produces gametes with half the normal number of chromosomes
• Meiosis


  1. The cell duplicates its genetic information, forming two armed chromosomes

  2. The chromosome pairs line up in the centre of the cell

  3. The pairs are pulled apart so each new cell only has one copy of each chromosome

  4. The arms of the chromosomes are pulled apart to produce four genetically different gametes
• Mitosis


  The process where the cell produced by gamete fusion divides to make a copy of itself, repeating many times to produce lots of new cells in an embryo
• X and Y chromosomes


  The 23rd pair of chromosomes that determine whether you are male (XY) or female (XX)
• The Y chromosome causes male characteristics, the XX combination allows female characteristics to develop
• Gamete production

  1. In sperm, the X and Y chromosomes are drawn apart in meiosis, with a 50% chance each sperm gets an X or Y

  2. In eggs, the original cell has two X-chromosomes, so all the eggs have one X-chromosome
• Genetic diagrams

  Models used to show all the possible genetic combinations when you cross different genes or chromosomes
• Alleles


  Different versions of the same gene
• Homozygous


  When an organism has two of the same alleles for a particular gene
• Heterozygous


  When an organism has two different alleles for a particular gene
• Dominant allele


  The allele that determines the characteristic that is shown, even if the organism is heterozygous
• Recessive allele


  The allele that is not shown in the organism's characteristics unless it is homozygous
• Genotype


  The combination of alleles an organism has
• Phenotype


  The characteristics an organism displays
• Genetic diagrams


  • Punnett squares showing possible offspring genotypes and phenotypes
• If a homozygous dominant organism is crossed with a homozygous recessive, all the offspring will be normal (heterozygous)
• If a homozygous dominant organism is crossed with a heterozygous organism, all the offspring will be normal (heterozygous)
• When a heterozygous organism is crossed with another heterozygous organism, the offspring will have a 1:1 ratio of the dominant and recessive phenotypes
• Family tree


  A diagram that shows how characteristics are inherited in a family
• From a family tree, you can tell if an allele for a disorder is dominant or recessive
• A family tree can be used to predict the probability of a new baby inheriting a disorder or being a carrier
• Long hair


  Caused by a dominant allele H
• Short hair


  Caused by a recessive allele h
• This is a 1:1 ratio-it's what you'd expect when a parent with only one dominant allele heterozygous-Hh is crossed with a parent with recessive alleles homozygous recessive-hh
• Knowing how inheritance works can help you to interpret a family tree
• From the family tree, you can tell that 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 22% chance that the new baby will have the disorder and a 50% chance that it will be a carrier, as both of its parents are carriers but are unaffected