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Sources of heritable variation
There are two principal ways in which heritable
variations can be acquired
Independent assortment of genes
Mutation
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Independent assortment of genes
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)
Genotype: Bbcc
Phenotype: brown eyes,straight hair
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Genotype: bbCc
Phenotype: blue eyes, curly hair
Male
Female
sperm
mother
cell
B
b
X
b
c
b
C
c
ovum
mother
cell
c
Meiosis; the homologous chromosomes are separated in the gametes
b
B
c
b
b
c
c
C
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Possible recombinations at fertilisation
ova
bC
bc
Bc
Bb
Cc
Bb
cc
bc
bb
Cc
bb
cc
sperms
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
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A black (B) uniformly coloured (U) bull is crossed with a red (b) spotted (u) cow
X
bbuu
BBUU
The alleles B and U are dominant so all the calves are uniformly black
BbUu
BbUu
F1 offspring
BbUu
BbUu
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An F1 bull is mated with an F1 cow
X
BbUu
BbUu
Possible offspring
Parental type
New variety
Parental type
New variety
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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
BbUu x BbUu
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Enter female gametes here
Enter male
gametes here
phenotypes Black uniform
genotypes
Black spotted Red spotted
Red uniform
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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
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If the genes for eye colour and hair
curliness occurred on the same
chromosome, they would be said
to be linked
B
b
C
c
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.
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In fact, linked genes may be separated at meiosis by a
process known as crossing over
A
B
A
B
a
b
a
b
A
B
a
b
The genes AB and ab
are linked
but when the homologous
chromosomes paired up at
meiosis, breaks occurred
in adjacent chromatids
The chromatids rejoin
but with their opposite
partner
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A
The chromosomes
separate, carrying the
exchanged portions
with them
b
a
B
The linkage is broken
A
B
A
b
a
B
a
b
When gametes are
formed during
meiosis, some will
have Ab and aB
and some will
have AB and ab
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Mutations
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
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Translocation
Part of chromosome 3 is
replicated and becomes
attached to the Y chromosome
Y
X
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3
4
Chromosomes
Fruit fly (Drosophila)
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2
Y
X
3
ru
b
4
ru
b
Leading to
mis-shapen
eyes
dark patterned
thorax
imperfect
cross veins
broad wings
incurved
hind legs
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18
radish
and cabbage
are cross pollinated
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chromosomes
Seed pod of radish
18
chromosomes
Seed pod of cabbage
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hybrid seed pod
9 radish
9 cabbage
The hybrid is sterile (i.e. produces no seeds) because
the chromosomes cannot pair up properly at meiosis
chromosomes
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If, however, the chromosome set doubles
18 radish
chromosomes
18 cabbage
the hybrid is fertile and produces seeds because the
homologous chromosomes can pair up at meiosis
The occurrence of multiple sets of chromosomes is called polyploidy
The hybrid is a tetraploid variety
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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)
a
b
c
d
e
This hybrid is a cultivated wheat used for flour production
Many of our crop plants result from doubling of chromosome number
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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
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Gene mutations
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
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Sickle cell anaemia
The amino acid sequence in part of the haemoglobin molecule is
Val- His- Leu- Thr- Pro- Glu-*
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
Val - His-Leu-Thr- Pro-ValThis leads to the production of faulty haemoglobin molecules
which become distorted in low oxygen concentrations and
cause sickle cell anaemia
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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:
Albinism
Haemophilia
Sickle cell anaemia
Cystic fibrosis
Colour blindness
Dwarfism
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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
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A mutation in a cell at an early stage in flower formation produced
a parti-coloured blossom in this chrysanthemum
©Brookhaven National Laboratory
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Mutation in bacteria
bacterium
reproducing
antibiotic
these bacteria
are killed
by antibiotic
mutation
(resistance
to antibiotic)
the resistant
mutants survive
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Mutation rate
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
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Mutagens
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
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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
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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
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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
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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
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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
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Question 6
Gene mutations can occur during DNA replication
prior to …
(a) mitosis
(b) meiosis
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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
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Question 8
Which of these conditions result from a gene mutation ?
(a) pneumonia
(b) haemophilia
(c) cystic fibrosis
(d) tuberculosis
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Answer
Correct
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Answer
Incorrect