Revision - GCSE Biology - Genetics - Variation Mitosis and Meiosis

Variation in plants and animals

Variation within a species is the way that two animals of the same species display different characteristics and/or behaviour. There are two types of variation; genetic and environmental.

  • Genetic difference - All animals of the same species will look slightly different because their genes will be different, apart from identical twins, where the genes are identical.
  • Environmental variation is variation due to the environment, e.g. scars, tattoos. Twins demonstrate this because their genes are identical, but they do not always look the same.
  • In plants the environmental factor plays a much bigger role. Plants are affected by temperature, sunlight, moisture levels and soil composition. A plant in a good soil area might be twice as strong in one where the soil quality is poor

If we take most variables (height, hair colour, ear size) and take a random sample and make a chart out of it (e.g. height) it will always come out as a normal distribution curve, which is shaped a bit like a bell. However, with things like blood group or tongue rolling, you can’t be a mixture, you must be one. This is called discontinuous variation. These different variations have been caused by mutation.

Variation is due to the way meiosis happens. When gametes are formed, half the chromosomes go into on gamete, and half into the other. However, which chromosome from the homologous pair goes into which gamete is random, and this random process is called free assortment. Couple with the fact that crossing over occurs there are millions of different variations that could come about.

Four variations which are only genetic

  • Eye colour
  • Hair colour (as long as it hasn’t been dyed)
  • Inherited diseases
  • Blood group

A combination of the two is; weight, height, skin colour, condition of teeth, academic ability, athletic ability etc. However, the difficulty is deciding which is more important in each case (environment or genes). The common view on education, is that you maximum intelligence is predetermined by your genes, but that nobody reaches their full potential, therefore education is also very important.


The human genome project

  • Positive things
    • Doctors would know if someone was likely to get a disease, and they might be able to cure some genetic disease such as cystic fibrosis and sickle cell anaemia
    • Testing will be more accurate as some diseases are hard to test for at the moment (like Alzheimer’s which you cannot test for while the person is alive, so it’s a bit late afterwards really…)
    • Better medicine will be developed because knowing exactly what causes a disease makes it easier to treat
    • it will encourage people to take further medication to prevent the diseasse from spreading.


  • Negative things
    • If someone is told at an early age that they are susceptible to a dangerous type of brain eating slug they will co crazy worrying, meaning stress will be very high, which is bad for your health. They may never even get it. Damn brain slug.
    • If everyone’s genetic code becomes common knowledge then there is a risk of the “genetically healthy” looking down on the, “genetically inferior” and pressure on people with poor genes not to have children
    • Life insurance for the genetically inferior might become very expensive
    • Doctors might create designer babies, which will stop people looking different as everyone strives towards their children being as perfect as possible

The big words of genetics

  • DNA – Deoxyribonucleic acid - The molecules which contain genes
  • Chromosomes – Are X shaped things found in nucleus. Humans have 23 pairs, and they contain DNA, and also genes
  • Gene – A section of a DNA molecule, or part of a chromosome
  • Allele – One a gene. When you have two different versions of the same gene, each version is called an allele
  • Dominant – An allele will determine the characteristic, as it is dominant over the recessive allele
  • Recessive – An allele which will not usually determine the characteristic as it is dominated over by the dominant allele
  • Homozygous – is an individual with two alleles of the same variation (e.g. hh or HH)
  • Heterozygous – An individual with two alleles of different types (e.g. Hh)
  • Genotype – Are you homozygous or heterozygous
  • Phenotype – What your genotype means, e.g. if you are homozygous what characteristics does that have
  • Mitosis – The type of cell division where one cell splits up into two new identical cells
  • Meiosis – The type of cell division where two gamete cells combine to form four genetically different cells
  • Diploid – (in context – a diploid cell) a cell which contains all 46 chromosomes (e.g. all 23 pairs)
  • Haploid – (in context – a haploid cell) a cell which contains just half the normal number of chromosomes, 23. Found in gamete cells.
  • Gamete – Either a sperm or egg cell. All gametes are haploid
  • Zygote – After the gametes have fused together they form a zygote.
  • Homologous – (in context – a homologous pair) A pair of chromosomes which go together (i.e. there are 23 pairs; one of these pairs is a homologous pair).
  • Sex-determining chromosomes - A pair of chromosomes determining the sex of the human (and most of mammals and some insects), a female has XX chromosomes and a male has XY chromosomes

A human nucleus contains 23 pairs of chromosomes, 23 of which we got from our mother and 23 of which we got from our father. Note that the X is a single chromosome, and two Xs would be a pair. Each double arm of the chromosome is called a chromatid and is a copy of the other arm. The centre of the chromosome is the centromere. A small part or “slice” of a chromosome is a gene. Each gene codes of a specific thing, e.g. hair colour. The DNA is coiled up to form the arms of the Chromosome, and is shaped as a double helix. A DNA molecule is a fairly long length of DNA


Mitosis occurs where new cells need to be formed, and in asexual reproduction.

The DNA starts off as long strands in a soup-like mush in the nucleus.

mitpsis 1
Mitosis 2

The chromosomes become clear as the DNA twists up. Each double arm (chromatid) is a copy of each other. 

Chromosomes line up along the centre line and begin to be pulled apart by fibres.

Mitosis 3

Membranes form about the separated chromosomes. These become nuclei of daughter cells.

Mitosis 4

The chromosomes unwind back into loose threads within the daughter cells, where they reproduce the other arm. Once this has been done they reform the X shape.

This creates two genetically identical cells, and is how plants and animals grow and repair damaged skin and tissue. It is also how some plants reproduce, and is called asexual reproduction.

Asexual Reproduction

In asexual reproduction there is only one parent, so the offspring must have identical genes to the parent.

A gene is a long length of DNA, and DNA is a list of instructions of how to put the organism together and make it work. Each gene is a separate instruction to a particular cell, on how to arrange amino acids (of which there are around 20) to form one of the thousands of proteins. Genes tell a cell what order to put the amino acids together, which subsequently determines what the cell makes, and therefore determines what type of cell it is.

The bases of DNA are Guanine Adenine, Thymine and Cytosine. These are always paired together Thymine with Adenine and Guanine with Cytosine (Curly letters together, straight letters together). These bases tell the cell what order to put the amino acids. Each unit of DNA has a base, a sugar and a phosphate (the sugar and phosphate is the “backbone”)

For making proteins the amino acids order must be taken to the ribosome where proteins are made. However, DNA can’t leave the nucleus. This is where RNA (Ribonucleic acid) comes in. It is much smaller than DNA, and can copy the order of amino acids from a certain part of the DNA strand (by unzipping it), then floats over to the ribosome where it sticks and the ribosome produces the protein. This is called proteins synthesis


Meiosis is similar to mitosis but creates four genetically different cells (They are genetically different due to DNA crossover which will be explained)

  • First there is a pair of chromosomes.
  • Then there is DNA crossover, tiny random bits of the chromosomes “fall” from one onto the other, or homologous chromosomes wrap around each other and bits break off
  • Now the two pairs split up.
  • Note that these two are genetically different, and also the arms are genetically different. In short there are 4 genetically different arms.
  • The X chromosome splits in half now, so each of the four genetically different arms is now a cell, and in this case this arm is now a chromosome. Therefore, we end up with having four genetically different cells with half the number of chromosomes of the original cell (21)

In the human there are 23 homologous pairs of chromosomes (homologous means that the number 12 chromosome from your mum pairs with the number 12 chromosome from you dad). Fertilization is the fusion of haploid male and female gametes, restoring the diploid number of chromosomes in a zygote. Each of the 23 chromosomes from your dad pair of with the 23 chromosomes from your mum, meaning you get a mixture of their characteristics.

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: