Chromosome Number (SL IB Biology)

Diploid & haploid

• A diploid cell is a cell that contains two complete sets of chromosomes (2n)
  • Adult body cells are usually diploid cells
• Haploid cells contain one complete set of chromosomes (n), meaning that they have half the number of chromosomes compared to normal body cells
  • In most cases these haploid cells are called gametes and they are involved in sexual reproduction
    • In animals, they are the female egg cell and the male sperm cell
  • There are some unusual species which have haploid cells for other parts of their life cycle
• During fertilisation the nuclei of haploid gametes fuse together to form the nucleus of a diploid zygote 
• Both gametes must contain the same number of chromosomes in order for the zygote to be viable.
  • For a diploid zygote this means that the gametes must be haploid
• Every body cell that arises from the zygote will contain the same number of chromosomes
  • Exceptions to this include red blood cells, which have no nucleus and so contain no chromosomes

Chromosome number is halved in gametes, and restored to diploid after fertilisation

Chromosome number

• The number of chromosomes possessed by different species varies and is dependent upon changes that have occurred during that species' evolution
• Each individual in a species always has the same number of chromosomes
  • There are a few rare instances where a chromosome mutation has occurred, giving a different chromosome number
• Differences in chromosome number is one reason why organisms from different species are unable to breed together successfully
• When stating the chromosome number of an organism, it needs to be clear whether you are giving
  • The number of chromosomes found in a diploid cell
  • The number of pairs of chromosomes found in a diploid cell
    • This will be the same as the haploid chromosome number
  • The number of chromosomes found in a haploid cell

Species chromosome number table

Different species have different numbers of chromosomes

• The diploid number must always be an even number
  • This is because the diploid number (2n) must always be divisible by two to produce a whole haploid number (n)
• Note that the number of chromosomes a species possesses is not linked to how 'advanced' a species is in evolutionary terms