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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

4

Part of chromosome 3 is replicated and becomesattached to the Y chromosome

Fruit fly (Drosophila)Chromosomes

Translocation15

Y

ru

ru

b

b

X

2

3

4

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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 6

Gene mutations can occur during DNA replicationprior 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

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