CIE A Level Biology (9700) 2019-2021

Revision Notes

16.1.1 Homologous Chromosomes

Homologous Chromosomes

  • In diploid cells there are two complete sets of chromosomes in the nucleus
  • Chromosones have a characteristic shape
    • They have a fixed length
    • The position of the centromere is in a particular location
  • These characteristic features allow for each chromosome to be identified in a photomicrograph
  • In photomicrographs chromosomes are often grouped into their homologous pairs
  • Homologous chromosomes:
    • Carry the same genes in the same positions
    • Are the same shape
  • During fertilization a diploid zygote is formed
    • In a zygote one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete
  • Having the same genes in the same order helps homologous chromosomes line up alongside each other during meiosis

Exam Tip

Although homologous pairs of chromosomes contain the same genes in the same order they don’t necessarily carry the same alleles (form) of each gene!

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