Genes, chromosomes and DNA

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  • Cells in your body have a nucleus.
  • Chromosomes are found in the nucleus
  • Each chromosome is one very long bit of DNA.
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  • DNA is double helix
  • Each of the two DNA strands contain 'bases'
  • DNA has four different bases
  • A= Adelnine
  • C= Cytosine
  • G= Guanine
  • T= Thymine
  • A always pairs up with T
  • C always pairs up with G
  • Called complementary base-pairing
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  • In a nucleus of each cell, there are two copies of every chromosome.
  • This means that each cell has two copies of every gene. For example, you've got two copies of the gene that control your eye colour.
  • You can have different versions of the same gene. These are called alleles.
  • Two different alleles are called heterozygous.
  • Two alleles the same are called homozygous.


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Controlling characteristics

  • We use letters to show the alleles that an organism has.
  • You have two alleles for each gene.
  • Each one can be either dominant or recessive.
  • Big letters= dominant alleles
  • Small letters= recessive alleles.
  • Genotype means what alleles you have, Bb
  • Phenotype means the actual characteristic 'brown eyes'.
  • If both alleles are dominant, you have a dominant characteristic
  • If both alleles are recessive, you have a recessive characteristic.
  • If you have one dominant and one recessive allele, you will only have the dominant characteristic.
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