IB Biology

1. 4.2 & 10.1 Meiosis
Topic 4 & 10 Genetics

2. 4.2 Meiosis
 4.2.1 State that meiosis is a reduction division of a diploid
nucleus to form haploid nuclei.
 4.2.2 Define homologous chromosomes.
 4.2.3 Outline the process of meiosis, including pairing of
homologous chromosomes and crossing over, followed by
two divisions, which results in four haploid cells.

3. 4.2 Meiosis
 Limit crossing over to the exchange of genetic material
between non-sister chromatids during prophase I. Names
of the stages are required.
 4.2.4 Explain that non-disjunction can lead to changes in
chromosome number, illustrated by reference to Down
syndrome (trisomy 21).
 The characteristics of Down syndrome are not required.
 4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and structure.

4. 4.2 Meiosis
 Limit crossing over to the exchange of genetic material
between non-sister chromatids during prophase I. Names
of the stages are required.
 4.2.4 Explain that non-disjunction can lead to changes in
chromosome number, illustrated by reference to Down
syndrome (trisomy 21).
 The characteristics of Down syndrome are not required.
 4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and structure.

5. 4.2 Meiosis
 4.2.6 State that karyotyping is performed using cells
collected by chorionic villus sampling or amniocentesis,
for pre-natal diagnosis of chromosome abnormalities.
 4.2.7 Analyse a human karyotype to determine gender
and whether nondisjunction has occurred.

6. 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.
 10.1.5 Explain the relationship between Mendel’s law of
independent assortment and meiosis.

7. Fertilisation
Male germ cell in
testis (diploid)
Female germ cell in
ovaries (diploid)
46 46
Sperm-Gamete
(haploid)
Egg-Gamete
(haploid)
23 23
Fertilisation
zygote
46
Embryo
46
46
Foetus
Mitosis
Mitosis
MeiosisMeiosis
(diploid)

8. Haploid and Diploid
 The nucleus of normal human body cells consist of 46
chromosomes or 23 pairs of chromosomes (2 of each
chromosome).
 This is referred to as the diploid number for humans (2n).
 Gametes, sex cells, only have one set of chromosomes (23).
 This is referred to as the haploid number for humans (n).
 In diploid cells, each pair of chromosomes have the same
genes, arranged in the same sequence (loci), but they do not
necessarily have the same alleles of all of the genes.
 They are therefore not identical but instead are homologous,
Homologous pairs.

9. Reductive Division
 The number of chromosomes in a cell can be reduced
from diploid to haploid by the process of Meiosis.
 Meiosis is described as Reductive Division.
 Organisms that reproduce sexually have to halve their
chromosome number at some stage in their life cycle
because the fusion of gametes during fertilisation double
it again (restores the diploid number).

10. Meiosis
 Meiosis is a type of cell division which:
 Results in the production of gametes (sex cells)
 Occurs in germ cells in the gonads - diploid
 Four gametes are produced from every germ cell
 Each gamete has half the number of chromosomes as the
original parent cell – haploid
 Meiosis involves TWO divisions:
 Meiosis I
 Meiosis II
 In sexual reproducing species, haploid cells must be
formed by meiosis before fertilisation to ensure the
diploid number of chromosomes in offspring is obtained.

11. Meiosis I
Ref: Advanced Biology, Kent

12. Meiosis II
Ref: Advanced Biology, Kent

13. Meiosis
Meiosis I
 Homologous chromosomes pair up.
They are called a bivalent.
 Non-sister chromatids cross over at
points called chiasmata.
 They may exchange genetic
material – crossing over.
 Homologous pairs line up at
equator.
 Maternal and paternal
chromosomes of each pair line up
independently of other pairs –
independent assortment.
 Homologous chromosomes separate
and move towards opposite poles.
 Two new cells form, each with half
the original chromosome number.
Meiosis II
 New spindle apparatus forms.
 Chromosomes line up at the
equator in a single line.
 Centromeres divide and sister
chromatids move towards
opposite poles.
 Each cells divides, resulting in a
total of four haploid cells.
 Each cell formed is genetically
unique due to crossing over and
independent assortment.

14. Outline of meiosis.

15. Meiosis – Gamete Production
 Meiosis is the name given to a specialised for of cell
division which produces the gametes.
 In animals this process occurs in organs called the
Gonads.
 In mature human females, eggs are produced by a process
called Oogenesis in the Ovaries.
 In mature human males, millions of sperm are produced
daily by a process called spermatogenesis in the testes.

16. Meiosis – Gamete Production
Ref: Biology Key Ideas

17. Sources of Variation in meiosis

18. 1. Crossing-Over (Recombination)

19. First meiotic division
 Homologous chromosomes come together
to form a bivalent
 The bivalent has four chromatids (2 from
each chromosome)
 These can intertwine and exchange
segments
 This is called crossing over

20. Methods of genetic recombination:
1. crossing over

21. Homologous
chromosomes
align.

25. Crossing over
 The point at which two chromatids cross is
called a chiasma
 Crossing over changes the mixture of
genetic information carried on a chromatid
 It produces genetic recombination
 This makes an important contribution to
the genetic variability that sexual
reproduction introduces to a species

26. 2. Independent Assortment

27. Independent Assortment
 Meiosis gives rise to genetic variation.
 Variety in gametes is produced by how the bivalents line
up on the equator during Metaphase I.
Ref: Biology Key Ideas

28. Independent assortment
 Refers to the random separation and assortment
of non-homologous chromosomes during
meiosis
 2 chromosomes produces 4 combinations – 2n
.
 Humans have 23 pairs of chromosomes – 223
.
 8 388 608 combinations
 If you double that, because of each gamete, the
total possible combinations is over 64 trillion.
 Independent assortment alone is the source of
significant variation

29. 3. Random fusion of gametes
 8 388 608 combinations of different sperm
(or eggs) due to independent assortment
 Fertilisation is a random fusion of gametes.
(random as to which egg fuses with which
sperm).
 The total possible combinations is over 64
trillion.

30. Mendel’s Law of Segregation
 When gametes are produced, each gamete must receive a
full complement of genes.
 For this reason, the factors/alleles must separate so that
only one factor/allele is present in each gamete.
 Mendel’s Law of Segregation states:
“The characteristics of a diploid organism are determined
by alleles which occur in pairs. Of a pair of such alleles,
only one can be carried in a single gamete”
 Thus each gamete receives one complete set of alleles,
and hence chromosomes:
 ie: 23 chromosomes.
 The two alleles of a gene are located on homologous
chromosomes which move to opposite poles, causing
segregation.

31. Non-disjunctions
 Non-disjunctions are a form of chromosome mutation.
 They occur when homologous chromosomes fail to
separate properly during meiosis.
 An extra chromosome is drawn to on pole, producing
gametes with an extra chromosome and gametes with one
less chromosome.
 This is referred to as Trisomy.
 Down’s syndrome is an example of trisomy 21.
 Down’s syndrome people have an extra chromosome 21.

32. A Non-disjunction Leading to
Down’s Syndrome
Ref: Advanced Biology, Kent

33. Down’s Syndrome
Ref: Advanced Biology, Kent
A Down’s syndrome boy.
A Karyotype of a
Down’s Syndrome boy

34. Turner’s Syndrome
Ref: Advanced Biology, Kent

35. 4.2 Meiosis
 4.2.1 State that meiosis is a reduction division of a diploid
nucleus to form haploid nuclei.
 4.2.2 Define homologous chromosomes.
 4.2.3 Outline the process of meiosis, including pairing of
homologous chromosomes and crossing over, followed by
two divisions, which results in four haploid cells.

36. 4.2 Meiosis
 Limit crossing over to the exchange of genetic material
between non-sister chromatids during prophase I. Names
of the stages are required.
 4.2.4 Explain that non-disjunction can lead to changes in
chromosome number, illustrated by reference to Down
syndrome (trisomy 21).
 The characteristics of Down syndrome are not required.
 4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and structure.

37. 4.2 Meiosis
 Limit crossing over to the exchange of genetic material
between non-sister chromatids during prophase I. Names
of the stages are required.
 4.2.4 Explain that non-disjunction can lead to changes in
chromosome number, illustrated by reference to Down
syndrome (trisomy 21).
 The characteristics of Down syndrome are not required.
 4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and structure.

38. 4.2 Meiosis
 4.2.6 State that karyotyping is performed using cells
collected by chorionic villus sampling or amniocentesis,
for pre-natal diagnosis of chromosome abnormalities.
 4.2.7 Analyse a human karyotype to determine gender
and whether nondisjunction has occurred.

39. 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.
 10.1.5 Explain the relationship between Mendel’s law of
independent assortment and meiosis.