1. 10.1 Meiosis
Topic 10 Genetics

2. 10.1 Meiosis
10.1.1 Describe the behaviour of the chromosomes in the phases of
meiosis.
10.1.2 Outline the formation of chiasmata in the process of crossing
over.
10.1.3 Explain how meiosis results in an effectively infinite genetic
variety in gametes through crossing over in prophase I and random
orientation in metaphase I.
10.1.4 State Mendel’s law of independent assortment.

3. 10.1 Meiosis
TOK: There are some interesting aspects of Mendel’s work,
including those mentioned in 4.3.11.
The law of independent assortment was soon found to have
exceptions when pairs of genes are linked on a chromosome, but the
law that Mendel discovered in the 19th century does operate for the
majority of pairs of genes.
10.1.5 Explain the relationship between Mendel’s law of
independent assortment and meiosis.

4. Meiosis
 Summary of Meiosis:
 Meiosis involves two divisions. One cell or nucleus
divides to for four cells or nuclei.
 The chromosome number is halved, from diploid to
haploid.
 An almost infinite amount of genetic variety is
produced as a result of crossing over in Prophase I and
the random orientation of bivalents in Metaphase I.

5. Genetic Variation in Meiosis
 Meiosis results in almost infinite genetic variety of
gametes.
 This comes about because of:
 Crossing over in Prophase I.
 Random Orientation in Metaphase I.

6. Crossing Over
 In Prophase I, homologous chromosomes, each consisting
of two identical chromatids, lie adjacent to each other –
they pair up. This is called a synapsis.
 The pair of chromosomes is referred to as a bivalent.
 At this stage corresponding sections of non-sister
chromatids may touch (cross over). This point is called a
chiasma (chiasmata – plural).
 Sections of the chromosomes are swapped between the
non-sister chromatids.
 This produces recombinant chromosomes.
 This process is called Crossing-over.
 Crossing over increases the genetic variability of the
offspring by altering the combination of genes on the
gametes formed.

7. Crossing Over
Ref: Year 12 Biology Biozone

8. Crossing Over
Ref: Year 12 Biology Biozone

9. Crossing Over
Ref: Biology Key Ideas

10. Random Orientation
 During meiosis, the homologous chromosomes line up
along the centre of the cell.
 Each member of each pair will be arranged towards the
centre of the cell in random order.
 Each member arranges independently of the other
chromosomes.
 This is called Random Orientation of Chromosomes.
 Random orientation increases variation.

11. Random Orientation
Ref: Biology Key Ideas

12. Random Orientation in Humans
 In human cells there are 23 pairs of homologous
chromosomes.
 The possible number of combinations is 223 or about 8
million.
 This is for one of you parents and the figure is about the
same for the other parent.
 Multiplying these two together gives about 64 trillion
different arrangements of chromosomes in the offspring.

13. Recombination
 Recombination is the reassortment of genes or characters
into different combinations from those of the parents.
 Recombination occurs for:
 Linked genes:
 Genes that occur on the same chromosome.
 Occurs by crossing over.
 Unlinked Genes:
 Genes that occur on separate chromosomes.
 Occurs by random orientation (Independent Assortment)

14. Independent Assortment
 Mendel devised a number of laws of genetics.
 His second law was the law of Independent Assortment.
 This means that when gametes are formed, each allele of a
gene is selected independently of any other gene.
 This is the result of Random Orientation during
Metaphase I of Meiosis.
 Thus independent assortment increases variation in
meiosis.

15. Independent Assortment
 Mendel’s second law, his Law of Independent Assortment
can be stated as:
“Alleles of genes located on different chromosomes
assort independently of one another.”
or
“Either pair of alleles of a gene is equally likely to be
inherited with either of another pair of alleles
of a different gene.”

16. 10.1 Meiosis
10.1.1 Describe the behaviour of the chromosomes in the phases of
meiosis.
10.1.2 Outline the formation of chiasmata in the process of crossing
over.
10.1.3 Explain how meiosis results in an effectively infinite genetic
variety in gametes through crossing over in prophase I and random
orientation in metaphase I.
10.1.4 State Mendel’s law of independent assortment.

17. 10.1 Meiosis
TOK: There are some interesting aspects of Mendel’s work,
including those mentioned in 4.3.11.
The law of independent assortment was soon found to have
exceptions when pairs of genes are linked on a chromosome, but the
law that Mendel discovered in the 19th century does operate for the
majority of pairs of genes.
10.1.5 Explain the relationship between Mendel’s law of
independent assortment and meiosis.