NZQA Biology AS 91605: Ploidy

Normally, human cells have distinct haploid (n) and diploid (2n) cells. The normal process of meiosis turns diploid body cells into haploid sex cells.

Sometimes, chromosomes get stuck during meiosis. This is called chromosomal nondisjunction.

  • Nondisjunction leaves messed up gametes; instead of having 23 chromosomes, they may have 22, 24, or even another number
  • The body will often purge these germ cells before they ever get the chance to fertilise

In the case of these germ cells not being purged, it will stem into either aneuploidy or polypoloidy.

Aneuploidy: a few too many or too few chromosomes

Aneuploidy can lead into various birth defects such as Down’s Syndrome, Turner Syndrome, and does lead into polyploidy.

  • Down’s Syndrome occurs on trisomy 21 and the total number of chromosomes comes to 47
    • either the sperm or egg cell has an extra copy of chromosomes 23 due to nondisjunction during meiosis
    • Down’s syndrome affects any sex
    • symptoms include;
    • short stature
    • weak muscles
    • distinct facial expressions
    • intellectual disability
    • heart defects
    • eye conditions
    • hearing problems
    • dental problems
    • ADHD
  • Turner Syndrome occurs on monosomy X and the total number of chromosomes comes to 45
    • either the sperm was missing an X or Y chromosome, or the egg was missing the X chromosome and the sperm carried the Y chromosome
    • the baby is always female
    • symptoms include;
    • wide neck
    • small jaw
    • distinct facial features
    • small stature
    • learning disabilities
    • autism
    • lack of puberty
    • early menopause
    • infertility

Polyploidy: full sets of extra chromosomes. Polyploidy occurs when meiosis goes catastrophically wrong and the gametes end up with a complete double set of chromosomes (or a complete missing set).

This is called complete nondisjunction and the body usually identifies and purges these gametes.

There are two main types of polyploidy:

  1. autopolyploidy (genome doubling) — the multiplication of one basic set of chromosomes

    1. they are derived from within a single species

    2. may be common in plants, although its prevalence may be underestimated in the taxonomic literature

      1. potatos (Solanum tuberosum) are undoubtedly autopolyploids

    3. an advantage of autopolyploidy is the larger effective population sizes than diploids allowing selective processes to be much more effective relative to random genetic drift

    4. a disadvantage of autopolyploidy is the interference with sexual differentiation when the sex of the organisms is determined by either the ratio between the number of sex chromosomes and the number of autosomes (as in Drosophila), or by a degree of ploidy (as in Hymenoptera)

  2. allopolyploidy — the combination of genetically distinct, but similar chromosome sets

    1. derived from hybridisation between two species

    2. much more common in nature than autopolyploidy; about 80% of all land plants may be allopolyploids

    3. allopolyploidy is rare in animals; it is commonly found in insects, fish, reptiles, and amphibians

      1. the African clawed frog (Xenopus laevis) is an allotetraploid
      2. the red vizcacha rat (Tympanoctomys barrerae) and the golden vizcacha rat (Pipanacoctomys aureus) are suspected tetraploids

    4. consequences of polyploidy at the phenotypic level are often mild. In many cases, polyploidy seem to have almost no effect on the phenotype

      1. diploid, autopolyploid, and allopolyploid Chrysanthemum species vary in chromosome number from 18 to 198, yet they are almost indistinguishable rom one another
      2. roses (Rosa), leptodactylid toads (Odontophyrnus), and goldfish (Carasius) have been found to have the same observations as Chrysanthemum

New hybrids are normally infertile. Mitotic nondisjunction can double the chromosome number and self-fertilisation may produce fertile hybrids.

However, in humans, if a 2n gamete soon goes on to fertilize another gamete, this will cause a number of syndromes:

  • triploid syndrome — total chromosome number comes to 69
    • either the sperm or the egg has two full sets of chromosomes, leading to triploid (3n) baby
    • triploid babies will eithe rmiscarry or die within their first year of life

Polyploidy in non-human species is not always fatal. Plants, especially, can survive and even thrive with extra sets of chromosomes. One reason for this is self-fertilisation: plants can combine with their own gametes, even after nondisjunction.

  • For example, two messed up diploid gametes can combine to make a tetraploid (4n) plant, which will continue to have diploid gametes in the next generation
  • Organisms with an odd number of autosomes (e.g the domestic banana plant Musa acuminata) cannot undergo meiosis or reproduce sexually
  • Monoploid (n) and triploid (3n) plant lines are usually sterile, and sometimes seedless
    • most plants we eat are either a result of ancient, accidental polyploidy or intentional polyploidy due to modern agricultural techniques