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1. Variation 2. Sources of variation. Sources of heritable variation. 2. There are two principal ways in which heritable variations can be acquired. Independent assortment of genes. Mutation. Independent assortment of genes. 3. Independent assortment arises as a result of meiosis

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Variation 2


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slide1

1

Variation 2

Sources of variation

sources of heritable variation
Sources of heritable variation

2

There are two principal ways in which heritable

variations can be acquired

Independent assortment of genes

Mutation

independent assortment of genes
Independent assortment of genes

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)

example 1
Example 1

b

b

B

b

C

c

c

c

b

b

b

B

C

c

c

c

4

Genotype:Bbcc

Genotype:bbCc

Phenotype: brown eyes,straight hair

Phenotype: blue eyes, curly hair

Male

sperm

mother

cell

Female

ovum

mother

cell

X

Meiosis; the homologous chromosomes are separated in the gametes

recombinations
Recombinations

5

bC

bc

Bc

bc

Possible recombinations at fertilisation

ova

sperms

Bb

cc

Bb

Cc

bb

cc

bb

Cc

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

example 2 cattle
Example 2 (cattle)

6

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 areuniformly black

BbUu

BbUu

BbUu

BbUu

F1 offspring

f2 offspring

7

F2 offspring

An F1 bull is mated with an F1 cow

X

BbUu

BbUu

Possible offspring

Parental type

New variety

Parental type

New variety

slide8

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

punnett square
Punnett square

9

BbUu x BbUu

Enter female gametes here

Enter male

gametes here

phenotypes

Red uniform

Black uniform

Black spotted

Red spotted

genotypes

breeding true
Breeding true

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

BBUUBBuu bbUUbbuu

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

linkage

11

Linkage

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.

crossing over
Crossing over

12

A

B

A

B

a

b

a

b

A

B

a

b

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

The chromatids rejoin

but with their opposite

partner

slide13

A

b

B

a

A

B

A

b

a

B

a

b

13

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

mutations

14

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

translocation

15

Translocation

2

3

Y

X

4

Part of chromosome 3 is

replicated and becomes

attached to the Y chromosome

Chromosomes

Fruit fly (Drosophila)

slide16

16

2

Y

X

3

ru

b

4

ru

b

slide17

17

Leading to

mis-shapen

eyes

dark patterned

thorax

imperfect

cross veins

broad wings

incurved

hind legs

radish x cabbage

18

Radish x cabbage

radish

and cabbage

are cross pollinated

X

18

chromosomes

18

chromosomes

Seed pod of radish

Seed pod of cabbage

slide19

19

9 radish

9 cabbage

chromosomes

hybrid seed pod

The hybrid is sterile (i.e. produces no seeds) because

the chromosomes cannot pair up properly at meiosis

polyploidy
Polyploidy

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

The occurrence of multiple sets of chromosomes is called polyploidy

The hybrid is a tetraploid variety

wheat
Wheat

a

b

c

d

e

21

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

chromosome mutations in humans

22

Chromosome mutations in humans

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

gene mutations
Gene mutations

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

sickle cell anaemia

24

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-Val-

This leads to the production of faulty haemoglobin molecules

which become distorted in low oxygen concentrations and

cause sickle cell anaemia

mutations25
Mutations

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:

Albinism

Sickle cell anaemia

Haemophilia

Cystic fibrosis

Dwarfism

Colour blindness

slide26

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

slide27

27

A mutation in a cell at an early stage in flower formation produced

a parti-coloured blossom in this chrysanthemum

©Brookhaven National Laboratory

mutation in bacteria
Mutation in bacteria

28

antibiotic

bacterium

reproducing

these bacteria

are killed

by antibiotic

mutation

(resistance

to antibiotic)

the resistant

mutants survive

mutation rate

29

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

mutagens
Mutagens

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

question 1

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

question 2

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

question 3

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

question 4

34

Question 4

Crossing over results from …

  • translocation of portions of chromosomes

(b) new combinations of genes in gametes

(c ) exchange of portions of homologous chromatids

(d) chromosome mutations

question 5

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

question 6

36

Question 6

Gene mutations can occur during DNA replication

prior to …

  • mitosis

(b) meiosis

question 7

37

Question 7

Sickle cell anaemia results from …

  • 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

question 8

38

Question 8

Which of these conditions result from a gene mutation ?

(a) pneumonia

(b) haemophilia

(c) cystic fibrosis

(d) tuberculosis

answer

39

Answer

Correct

answer40

40

Answer

Incorrect