Biology Mrs Edwards

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Chromosomes and Genes

  • most cells contain nucleus
  • nucleus contain chromosomes
  • 46 chromosomes (23 pairs) made of long strands of DNA
  • small selection of DNA is called a gene
  • each chromosome has lots of genes

genes control inherited characteristics e.g. eye colour, natural hair colour

passed on from parent to young in sex cells called gametes (sperm/egg cells)

  • egg contains X (female XX)
  • sperm contains X or Y (male XY)

genes, like chromosomes are found in pairs - 2 genes for each characteristic

genotype: what gene you have // phenotype: what you look like

  • e.g. genotype = brown gene and blue gene, phenotype = brown eyes

different forms of the same genes are called ALLELES e.g. brown eyes, blue eyes

  • alleles may be dominant (stronger) - only one allele needed e.g. brown eyes
  • alleles may be recessive (weaker) - two alleles would be needed e.g. blue eyes

if alleles are the same it is homozygous e.g. brown brown

if alleles are different it is heterozygous e.g. brown blue



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