Post on 10-Aug-2015
There are two principal ways in which heritable variations can be acquired
Independent assortment of genes
Mutation
Sources of heritable variation2
Independent assortment arises as a result of meiosisand 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 haveeach inherited one set of chromosomes from the father(blue) and one set from the mother (red)
Independent assortment of genes
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Phenotype: brown eyes,straight hair
Genotype: Bbcc
Malesperm mother cell
Phenotype: blue eyes, curly hair
Femaleovummother 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
Example 1
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Bc
bc
bC bc
Bb Cc
bbCc
Bbcc
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 variationRecombinations
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BBUU bbuuX
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
Example 2 (cattle)
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An F1 bull is mated with an F1 cow
BbUu BbUu
Parental type Parental typeNew variety New variety
X
Possible offspring
F2 offspring
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The next slide offers a blank Punnett square which canbe printed out and filled in to show the genotypes and expected number of phenotypes from a cross between the F1 black, uniform, cattle
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Enter female gametes here
Enter male gametes here
BbUu x BbUu
genotypes
phenotypes Black uniform Black spotted Red spotted Red uniform
Punnett square
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If you worked out the genotypes from the Punnett square, you will realise that the only cattle which will breed trueare the ones homozygous for both characteristics BBUU BBuu bbUU bbuu
Other crosses are likely to result in some offspring whichdo not resemble either parent
For this reason, all possible variations will keep croppingup 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
Breeding true
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B b
C c
If the genes for eye colour and haircurliness occurred on the same chromosome, they would be saidto 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 withbrown eyes would also have curly hair, and most people with blue eyes would have straight hair.
Linkage
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In fact, linked genes may be separated at meiosis by a process known as crossing over
The genes AB and ab are linkedbut when the homologous chromosomes paired up atmeiosis, 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
Crossing over
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A
b
aB
The chromosomes separate, carrying the exchanged portionswith them
The linkage is broken
When gametes areformed during meiosis, some willhave Ab and aBand some willhave AB and ab
Ba
A b
A B
a b
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A mutation is a spontaneous change in a gene or chromosome
Gene mutations may arise when a gene fails to make an exactcopy 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
Mutations14
XY2
3
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Part of chromosome 3 is replicated and becomesattached to the Y chromosome
Fruit fly (Drosophila)Chromosomes
Translocation15
mis-shapen eyes
dark patterned thorax
imperfect cross veins
broad wings
incurved hind legs
Leading to17
Seed pod of radish Seed pod of cabbage
are cross pollinated
18 chromosomes
18 chromosomes
radish and cabbage
Radish x cabbage
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hybrid seed pod
9 radish9 cabbage
chromosomes
The hybrid is sterile (i.e. produces no seeds) because the chromosomes cannot pair up properly at meiosis
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18 radish18 cabbage
chromosomes
If, however, the chromosome set doubles
the hybrid is fertile and produces seeds because thehomologous chromosomes can pair up at meiosis
Polyploidy
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The occurrence of multiple sets of chromosomes is called polyploidyThe hybrid is a tetraploid variety
a b c d e
Primitive wheat (a) crossed withwild 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
Wheat
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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
Chromosome mutations in humans
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Gene mutations often arise as a result of faulty replication of DNA
If a nucleotide is not copied accurately, the triplet which containsthe 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 failsto function normally
This means that most gene mutations have a damaging effecton the cells and the whole organism
Gene mutations23
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 middlethymine (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 moleculeswhich 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-
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A mutated gene which is dominant, or inherited as a homozygousrecessive, will affect the whole organism
A mutated gene which does not result in abortion or early deathof an organism will be inherited by the offspring
Very rarely, a mutation will be beneficial and will be inherited bythe offspring
Gene mutations in humans may result in:
Haemophilia Cystic fibrosis
Albinism
Colour blindness
Sickle cell anaemia
Dwarfism
Mutations
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If a gene mutation takes place in a gamete which contributes toa 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 dividesrepeatedly, giving rise to a tumour
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A mutation in a cell at an early stage in flower formation produceda parti-coloured blossom in this chrysanthemum
©Brookhaven National Laboratory
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bacterium reproducing
mutation(resistanceto antibiotic)
antibiotic
these bacteriaare killedby antibiotic
the resistantmutants survive
Mutation in bacteria28
Mutations may occur as infrequently as once in 100,000 replications
Nevertheless there are many replications involved in gameteproduction
The human ejaculate of about 500 million sperms is bound to carry some sperms with mutations
Mutation rate29
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
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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 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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