There are two main ways heritable variations can arise: independent assortment of genes during meiosis and fertilization, which introduces new combinations of parental genes, and spontaneous gene and chromosome mutations. Independent assortment occurs when parental chromosomes separate during meiosis and recombine in new configurations during fertilization. Gene mutations happen due to errors in DNA replication, while chromosome mutations result from changes like gains or losses of chromosomes. Both types of mutations can lead to new traits.

1. 1

2. There are two principal ways in which heritable
variations can be acquired
Independent assortment of genes
Mutation
Sources of heritable variation
2

3. Independent assortment arises as a result of meiosis
and fertilisation
The separation of parental chromosomes at meiosis and
their recombination at fertilisation introduces the
possibility of new combinations of genes
In the example which follows, the parental types have
each inherited one set of chromosomes from the father
(blue) and one set from the mother (red)
Independent assortment of genes
3

4. Phenotype: brown eyes,straight hair
Genotype: Bbcc
Male
sperm
mother
cell
Phenotype: blue eyes, curly hair
Female
ovum
mother
cell
Genotype: bbCc
Meiosis; the homologous chromosomes are separated in the gametes
B b
cc
B
c
b
c
c
b b
C
b
C
c
b
X
4

5. Bc
bc
bC bc
Bb
Cc
bb
Cc
Bb
cc
bb
cc
ova
sperms
Possible recombinations at fertilisation
Bbcc Brown eyes, straight hair – like father
bbCc Blue eyes, curly hair – like mother
BbCc Brown eyes, curly hair – new variation
Bbcc Blue eyes, straight hair – new variation
5

6. BBUU bbuu
X
BbUu BbUu BbUu BbUu
A black (B) uniformly coloured (U) bull is crossed with a red (b) spotted (u) cow
The alleles B and U are dominant so all the calves are uniformly black
F1 offspring
6

7. An F1 bull is mated with an F1 cow
BbUu BbUu
Parental type Parental typeNew variety New variety
X
Possible offspring
7

8. The next slide offers a blank Punnett square which can
be printed out and filled in to show the genotypes and
expected number of phenotypes from a cross between
the F1 black, uniform, cattle
8

9. Enter female gametes here
Enter male
gametes here
BbUu x BbUu
genotypes
phenotypes Black uniform Black spotted Red spotted Red uniform
9

10. If you worked out the genotypes from the Punnett square,
you will realise that the only cattle which will breed true
are the ones homozygous for both characteristics
BBUU BBuu bbUU bbuu
Other crosses are likely to result in some offspring which
do not resemble either parent
For this reason, all possible variations will keep cropping
up in a breeding population
If, however, either of the homozygotes BBUU or bbuu
were to be more successful* than the others, they might
become established as a stable population
10

11. B b
C c
If the genes for eye colour and hair
curliness occurred on the same
chromosome, they would be said
to be linked
At meiosis you would expect the linked genes to
remain together in the gametes.
In this case you would expect that most people with
brown eyes would also have curly hair, and most people
with blue eyes would have straight hair.
11

12. In fact, linked genes may be separated at meiosis by a
process known as crossing over
The genes AB and ab
are linked
but when the homologous
chromosomes paired up at
meiosis, breaks occurred
in adjacent chromatids
A B
a b
The chromatids rejoin
but with their opposite
partner
A B
a b
A B
a b
12

13. A
b
a
B
The chromosomes
separate, carrying the
exchanged portions
with them
The linkage is broken
When gametes
are
formed during
meiosis, some will
have Ab and aB
and some will
have AB and ab
Ba
A b
A B
a b
13

14. A mutation is a spontaneous change in a gene or
chromosome
Gene mutations may arise when a gene fails to make an exact
copy of itself during replication prior to meiosis or mitosis
Chromosome mutations can result from:
Damage to, or loss of a chromosome
Incomplete separation of chromosomes at meiosis leading to
extra chromosomes in one gamete
Part of a chromosome becoming attached to another chromosome
(translocation)
Doubling the whole set of chromosomes
Mutations
14

15. XY
2
3
4
Part of chromosome 3 is
replicated and becomes
attached to the Y chromosome
Fruit fly (Drosophila)Chromosomes
Translocation
15

16. Y
ru
ru
b
b
X
2
3
4
16

17. mis-shapen
eyes
dark patterned
thorax
imperfect
cross veins
broad wings
incurved
hind legs
Leading to
17

18. Seed pod of radish Seed pod of cabbage
are cross pollinated
18
chromosomes
18
chromosomes
radish and cabbage
18

19. hybrid seed pod
9 radish
9 cabbage
chromosomes
The hybrid is sterile (i.e. produces no seeds) because
the chromosomes cannot pair up properly at meiosis
19

20. 18 radish
18 cabbage
chromosomes
If, however, the chromosome set doubles
the hybrid is fertile and produces seeds because the
homologous chromosomes can pair up at meiosis
20
The occurrence of multiple sets of chromosomes is called polyploidy
The hybrid is a tetraploid variety

21. a b c d e
Primitive wheat (a) crossed with
wild grass (b) to produce an
infertile hybrid.
Chromosome doubling produces
a fertile hybrid (c)
which is crossed with wild grass
(d) to produce an infertile hybrid
Chromosome doubling results
in fertile hybrid (e)
This hybrid is a cultivated wheat used for flour production
Many of our crop plants result from doubling of chromosome number
21

22. Chromosome mutations in humans usually result in spontaneous
abortion of the foetus
But a proportion survive e.g.
Downs syndrome. The affected person has one extra chromosome
in their genome (i.e. 47 instead of 46 chromosomes)
This results in characteristic facial features, varying degrees of
mental impairment and, usually, a very cheerful disposition
Klinefelters syndrome. The affected male has an extra
X chromosome (XXY)
The person appears to be a normal male but he is infertile
22

23. Gene mutations often arise as a result of faulty replication of DNA
If a nucleotide is not copied accurately, the triplet which contains
the fault will not code for the correct amino acid
A protein with an incorrect amino acid will not function properly
If the protein is an enzyme, this means that the enzyme will not work
The cell chemistry will be disrupted if an essential enzyme fails
to function normally
This means that most gene mutations have a damaging effect
on the cells and the whole organism
Gene mutations
23

24. The amino acid sequence in part of the haemoglobin molecule is
The triplet code for Glutamic acid is either CTT or CTC+
Sometimes (rarely) in the course of replication, the middle
thymine (T) is replaced by adenine (A)
So the triplet becomes CAT or CAC
But CAT or CAC code for Valine, not glutamic acid
The amino acid sequence therefore becomes
This leads to the production of faulty haemoglobin molecules
which become distorted in low oxygen concentrations and
cause sickle cell anaemia
Val- His- Leu- Thr- Pro- Glu-*
Sickle cell anaemia
Val - His-Leu-Thr- Pro-Val-
24

25. A mutated gene which is dominant, or inherited as a homozygous
recessive, will affect the whole organism
A mutated gene which does not result in abortion or early death
of an organism will be inherited by the offspring
Very rarely, a mutation will be beneficial and will be inherited by
the offspring
Gene mutations in humans may result in:
Haemophilia Cystic fibrosis
Albinism
Colour blindness
Sickle cell anaemia
Dwarfism
25

26. If a gene mutation takes place in a gamete which contributes to
a zygote, the mutation will affect the whole organism
A gene mutation in a body cell will affect only that cell and
any cells derived from it (See next slide)
It is mutations in body cells which give rise to cancers
These mutations usually affect the genes which control cell division
The control on cell division is reduced so that the cell divides
repeatedly, giving rise to a tumour
26

27. A mutation in a cell at an early stage in flower formation produced
a parti-coloured blossom in this chrysanthemum
©Brookhaven National Laboratory
27

28. bacterium
reproducing
mutation
(resistance
to antibiotic)
antibiotic
these bacteria
are killed
by antibiotic
the resistant
mutants survive
Mutation in bacteria
28

29. Mutations may occur as infrequently as once in 100,000 replications
Nevertheless there are many replications involved in gamete
production
The human ejaculate of about 500 million sperms is bound to carry
some sperms with mutations
Mutation rate
29

30. Some environmental effects may increase the mutation rate
These include radiation (X-rays, ultra-violet, radioactive materials)
and carcinogenic chemicals such as those in tobacco smoke
These are all known as
mutagens
Mutagens which affect body cells may produce cancers.
Mutagens affecting the reproductive organs may result in
defective offspring
Mutagens
30

31. Question 1
Independent assortment of genes arises from the
processes of …
(a) meiosis and fertilisation
(b) mitosis and cell division
(c) replication of DNA
(d) pairing of homologous chromosomes
31

32. Question 2
From the genotype AaBb which of the following
combinations could occur in the gametes ?
(a) AB ab Ab aB
(b) Aa Bb AB ab
(c) Ab aB Bb Aa
(d) Ab aB AB Ab
32

33. Question 3
Genes are said to be linked if …
(a) they occur together in a genome
(b) they appear in the same gamete
(c) they occur on the same chromosome
(d) they are carried on homologous chromosomes
33

34. Question 4
Crossing over results from …
(a) translocation of portions of chromosomes
(b) new combinations of genes in gametes
(c ) exchange of portions of homologous chromatids
(d) chromosome mutations
34

35. Question 5
Chromosome mutations can occur as a result of…
(a) gain of an extra chromosome
(b) doubling of the entire set of chromosomes
(c) faulty replication of DNA
(d) breaking of a chromosome
35

36. Question 6
Gene mutations can occur during DNA replication
prior to …
(a) mitosis
(b) meiosis
36

37. Question 7
Sickle cell anaemia results from …
(a) a change in a nucleotide triplet
(b) a change in a single nucleotide
(c) a change in a single amino acid
(d) a chromosome mutation
37

38. Question 8
Which of these conditions result from a gene mutation ?
(a) pneumonia
(b) haemophilia
(c) cystic fibrosis
(d) tuberculosis
38

39. Answer
Correct
39

40. Answer
Incorrect
40