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Chapter 12
Meiosis
Sexual reproduction is a mechanism to ensure variation within a species.
During sexual reproduction, a zygote is formed as a result of the fusion of the
nuclei of 2 gametes.
In humans, how many chromosomes are normally found in one chromosome set?
(Remember, you received one set of chromosomes from your natural mother in
her egg, and one set from your natural father in his sperm).
Cells which contain one set of chromosomes are known as haploid cells
e.g.
Cells which contain two sets of chromosomes are known as ……………… cells
e.g.
The chromosome complement of a haploid cell is represented by the letter n and,
therefore, that of a diploid cell by 2n.
Since the members of each pair of chromosomes in a diploid cell match one
another gene for gene, they are said to be homologous. They may, however,
possess different alleles (forms) of the same gene.
Mass of DNA
Human red blood cells have no DNA.
The mass of DNA in a haploid cell (sperm, egg) is half that found in a diploid body
cell (liver, muscle, brain)
The mass of DNA in a diploid cell will be greatest after DNA has replicated and
chromatids formed just before cell division.
Sites of meiosis
Where does meiosis occur

In animals?
Chapter 12

In plants?
The process of meiosis
Using the diagram of meiosis on pp80-81 Torrance or meiosis handout, read
through p 82 Torrance and carefully consider what is happening throughout the
process of meiosis. You may wish to make notes but the important thing is that
you understand what is going on and have a mental image you can refer to later
when you are considering genetics problem solving.
Significance of meiosis
Meiosis enables new combinations of existing alleles of genes to pass to the
gamete as a result of crossing over at chiasmata resulting in new combinations
of alleles for genes on the same chromosome. This can give 4 genetically
different chromatids each of which ends up in a different gamete.
Also, during the first meiotic division, the homologous pairs of chromosomes line
up at the equator in a way which is random relative to any other pair. This
independent assortment of chromosomes gives rise to different genetic
combinations in the gametes. A human mother cell with 23 homologous pairs of
chromosomes has the potential to produce 223 different combinations from
independent assortment alone (that’s 8 388 608 genetically different eggs) before
we even consider the effects of crossing over. Look at Fig 12.3 p83 Torrance.
To summarise, genetic variation is increased during meiosis due to


Independent assortment of chromosomes
Crossing over
Importance of variation
Using information from p84 Torrance, explain why variation within a species is
important to a species.