Chromosome Number
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
- In most cases these haploid cells are called gametes and they are involved in sexual reproduction
- 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
Name of species | Diploid chromosome number (2n) | Haploid chromosome number |
Human (Homo sapiens) | 46 | 23 |
Chimpanzee (Pan troglodytes) | 48 | 24 |
Rice (Oryza sativa) | 24 | 12 |
Horse threadworm (Parascaris equorum) | 4 | 2 |
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
Examiner Tip
Note that you need to know the diploid chromosome numbers of humans (46) and chimpanzees (48), but that you do not need to learn any other specific chromosome numbers