Biology 2 reproduction and inheritance in humans

• Variation


  Differences between organisms of the same species (intraspecific variation) or between different species (interspecific variation)
• Phenotype


  The observable characteristics of an organism
• Differences between organisms


  Can be caused by what they have inherited from their parents (their genetic material), or from the environment, or often from a combination of both
• Continuous variation


  Phenotypes that can be given a numerical value e.g. height or arm length
• Discontinuous variation


  Phenotypes that are not numerical, but can be given a name or a category e.g. blood groups and eye colour
• Alleles


  Different genes which carry the genetic information for different versions of the same thing
• Dominant allele


  An allele that will stop other alleles (called recessive alleles) from working
• Homozygous


  Having two of the same allele
• Heterozygous


  Having two different alleles
• Genotype


  The combination of alleles that a person has for each gene
• Phenotype


  The characteristics that a person's alleles cause
• Our body is made of cells
• Each cell contains alleles, which control the cell
• Each nucleus contains lots of thin threads called chromosomes. Chromosomes carry information which controls how your body works and what you look like
• There are 46 chromosomes in a human cell, except for the egg and sperm cells that contain 23 each
• The chromosomes in our cells can be arranged in pairs because one comes from our mother and the other from our Father
• Chromosomes are made of a very long molecule called DNA
• A gene is a length of DNA. Each gene gives instructions for a different characteristic e.g. eye colour
• Gamete


  Sex cell
• Fertilisation


  When the male sex cell the sperm fuses with the female sex cell the egg
• Zygote


  A fertilised egg
• Sperm cell


  • Contains only one copy of each chromosome (haploid)
  • Doesn't contain much cytoplasm so the cell can be streamlined
  • The head contains enzymes that allow the sperm to penetrate the ovum for fertilisation
  • The middle piece contains mitochondria that release energy for the sperm to swim
• Egg cell


  • Contains only one copy of each chromosome (haploid)
  • Has a large food store in its cytoplasm to provide energy for the zygote
  • Has a mechanism to prevent more than one sperm fertilising the egg
• Genotype


  The letters that represent the allele
• Monohybrid cross


  Describes the inheritance of one characteristic, caused by one gene
• Genetic conditions


  • Cystic fibrosis (caused by a recessive allele)
  • Huntington's disease (caused by a dominant allele)
• Genetic engineering


  A technique that involves taking a gene from one organism and putting it into another organism
• Advantages of genetic engineering

  • Faster and more efficient than selective breeding
  • Can improve crop yields or quality, which is important in developing countries
• Disadvantages of genetic engineering

  • Transfer of the selected gene into other species may cause harm
  • Some people believe it is unethical to interfere with nature in this way
  • GM crop seeds are often more expensive and so people in developing countries cannot afford them
  • It's very new and not much is known about it