• Created by: Holly45
  • Created on: 19-03-15 18:33


  • human cells have 46 chromosomes in 23 pairs
  • used when plants and animals want to grow or replace cells that may have been damaged
  • the offspring have exactly the same genes as the plant - no variation

1. DNA is spread in long strips
2. DNA is copied
3. Chromosomes line up at the centre and fibres pull them apart
4. The 2 arms of each chromosome go to opposite ends of the cell. They become the nuclei of the new cells
5. Cytoplasm divides and forms 2 new cells containing exactly the same DNA

1 of 7


  • sexual reproduction where 2 cells called gametes are formed
  • gametes only have one copy of each chromosome so you can combine one sex cell from your mother and father
  • the new individual will have a mixture of 2 sets of chromosomes

1. Duplicates the DNA
2. The chromosome pairs line up in the centre
3. The pairs are pulled apart, each cell has some of the mothers and some of the fathers chromosomes. The cell splits
4. The chromosomes line up again and the cell splits
5. You get 4 gametes each with only a single set of chromosomes

2 of 7

Stem cells

  • some cells are differentiated so they can develop into different types of cell
  • they can be found in early embryos
  • to get cultures of one specific cell they have to change the environment they're growing in
  • some people are against because they feel that human embryos shouldn't be used for experiments
  • others think that patients who already exist and who are suffering is more important than the rights of an embryo
  • campaigners feel that scientists should concentrate on finding other sources or stem cells
3 of 7

X and Y chromosomes

  • there are 22 matched pairs of chromosomes in every human body cell. The 23rd pair is labelled ** or XY
  • all men have an X and a Y chromosome
  • all women have 2 X chromosomes
  • There is a 50/50 ratio of either a boy or a girl for a baby
4 of 7

Work of Mendel

  • he noted how characteristics in plants were passed on
  • Mendal showed that the height of pea plants was determined by separatly inherited 'hereditary units' passed on from each parent
  • the dominant unit will always be expressed
5 of 7

Genetic diagrams

  • alleles are different versions of the same gene
  • if an organism has 2 alleles for a particular gene is the same then it's homozygous
  • if it's 2 alleles are different then it's heterozygous
  • to display a dominent characteristic the organism can either be CC or Cc
  • the dominant allele overrules the recessive
6 of 7

Genetic disorders


  • the allele that causes cystic fibrosis is a recessive allele
  • because it's recessive, people with only one copy of the allele won't have the disorder
  • both parents must be either carriers or sufferes for the child to have it


  • caused by the dominant allele and so can be inherited if just one parent carries it


  • embryos are fertilised in a lab and then implanted in the womb
  • before being implanted it's possible to remove a cell from each embryo to be analysed
  • the rejected embryos are destroyed
  • implies that people with disorders are 'undesired'
  • helps stops people from suffering
  • treating disorders costs the government money
7 of 7


No comments have yet been made

Similar Science resources:

See all Science resources »See all Biology resources »