CB3

• Humans start life as a single fertilised egg cell, a zygote. This is formed when two gametes (sex cells) fuse during fertilisation. The zygote then forms a ball of cells using a type of cell division called mitosis (see CB2a).
• The zygote then forms a ball of cells using a type of cell division called mitosis
• Almost all human cells carry exactly the same instructions. These instructions control each individual cell, and also shape, coordinate and control our bodies
• Gametes


  Human reproductive cells (sperm cell and egg cell)
• Mitosis


  Turns the zygote into a ball of cells
• Cells with different numbers of chromosomes

  • Human sperm cell
  • Human liver cell
  • Human zygote
• Gamete


  Contains half the normal number of chromosomes (haploid, 1n)
• Gene


  A section of DNA that contains the code for making a protein
• Humans have about 20 000 genes
• There are 23 different chromosomes in humans and most nuclei contain two of each type, making 46 in total
• Meiosis


  Produces gametes (haploid cells)
• Chromosomes look X-shaped at the start of meiosis because each chromosome has replicated and the copies remain attached
• Zygote is formed by the fusion of a sperm cell nucleus and an egg cell nucleus during fertilisation
• Embryo cells are produced by mitosis
• Sperm cells are produced by meiosis
• Triploid syndrome is a rare human disease where the cells have three sets of chromosomes instead of the usual two
• Mitosis


  Produces diploid cells
• Meiosis


  Produces haploid cells
• DNA is a double helix molecule
• DNA contains four bases: adenine, thymine, cytosine and guanine
• Complementary base pairs


  A pairs with T, C pairs with G
• The order of bases in a gene contains the coded instructions for a protein
• Everyone except identical twins has different DNA
• DNA analysis can be used to identify people and determine relationships
• Hydrogen bond


  A weak force of attraction between slightly charged parts of DNA bases
• The human genome contains about 3 billion base pairs
• Genes can have slightly different versions (alleles) which give rise to genetic variation
• Homozygous


  Having two identical alleles for a gene
• Heterozygous


  Having two different alleles for a gene
• Dominant allele


  An allele that is expressed even when only one copy is present
• Recessive allele


  An allele that is only expressed when two copies are present
• Alleles


  Different versions of the same gene
• Dominant allele


  • Shown by a capital letter (e.g. R for purple)
  • Always written before the recessive allele (e.g. Rr and never rR)
• Recessive allele


  • Shown by the lower case version of the same letter (e.g. r for not purple)
  • Only seen if both alleles are recessive
• Genotype


  The alleles an organism has
• Phenotype


  What the organism looks like
• A recessive characteristic is only seen if both alleles are recessive
• Genetic diagram


  1. Shows the possible combinations of alleles when two organisms breed

  2. Used to explain the inheritance of one gene (monohybrid inheritance)

  3. Used to predict the ratios of the phenotypes
• ADWH is a condition in which people have curly hair that looks like sheep's wool, caused by a dominant allele (D)
• One parent is heterozygous and the other is homozygous for the recessive allele


  Ratio of children with and without the condition