chapter 08 *lecture outline copyright © the mcgraw-hill companies, inc. permission required for...

Chapter 08

*Lecture Outline

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint

without notes.

INTRODUCTION

• Genetic variation refers to differences between members of the same species or those of different species– Allelic variations are due to mutations in

particular genes– Chromosomal aberrations are substantial

changes in chromosome structure or number• These typically affect more than one gene• They are quite common, which is surprising

8-2Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

8.1 Variation in Chromosome Structure

• Cytogenetics -The field of genetics that involves the microscopic examination of chromosomes

• A cytogeneticist typically examines the chromosomal composition of a particular cell or organism– This allows the detection of individuals with abnormal

chromosome number or structure– This also provides a way to distinguish between

species• Refer to Figure 8.1a


© Scott Camazine /Photo Researchers

Human Fruit fly Corn

© Michael Abbey/Photo Researchers© Carlos R Carvalho/Universidade

Federal de Viçosa.

Figure 8.1


(a) Micrographs of metaphase chromosomes


Cytogeneticists use three main features to identify and classify chromosomes 1. Location of the centromere 2. Size 3. Banding patterns

These features are all seen in a Karyotype A micrograph in which all of the chromosomes within

a single cell are arranged in a standard fashion The procedure for making a karyotype was discussed

in Chapter 3 (See Figure 3.2)

Cytogenetics

A karyotype of a diploid human cell

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-6Figure 3.2

(c) For a diploid human cell, two complete sets of chromosomes from a single cell constitute a karyotype of that cell.

11 m

© Leonard Lessin/Peter Arnold

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22

XY

Metacentric Submetacentric Acrocentric Telocentric

P

q

P

q

P

q

P

q

(b) A comparison of centromeric locations

8-7

Figure 8.1

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Short arm; For the French, petite

Long arm



Since different chromosomes can be the same size and have the same centromere position, chromosomes are treated with stains to produce characteristic banding patterns Example: G-banding

Chromosomes are exposed to the dye Giemsa Some regions bind the dye heavily

Dark bands Some regions do not bind the dye well

Light bands

In humans 300 G bands are seen in metaphase 800 G bands in prometaphase

Cytogenetics

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-9Figure 8.1

Banding pattern during metaphase

Banding pattern during prometaphase

P

q

P

q

(d) Conventional numbering system of G bands in human chromosomes

XY22212019181716151413

1

12

3

3

2

1

1

2

34

121110987654321

654321

2132112

121234

12345

543216543211234

1234567

1234

76543214321123123456789

654321

12345678

123

12345

54

3211234512312345

54321

1234561234567

21

21543211

12123456

32121123

1

234

4321321123121234

543211

123456

54321123412345

32

1123451234

3211234

1234

12

21

12312345678

1321123

12

1234

3211231234

3211212345

321123

321123

321123

1

2

13211

12

12

21

123

32112345123456

1

1

23

1

1

2

1

1

2

1

1

2

1

1

2

1

1

1

1

112

1

1

1

12

11

2

2

1

1

2

3

2

1

1

2

1

1

2

3

1

1

2

3

2

1

1

2

2

1

12

3

2

1

1

2

2

1123

1

1

2

1

1

2

1

1

2


The banding pattern is useful in several ways:

1. It distinguishes Individual chromosomes from each other

2. It detects changes in chromosome structure 3. It reveals evolutionary relationships among

the chromosomes of closely related species

Cytogenetics

8-10


There are two primary ways in which the structure of chromosomes can be altered 1. The total amount of genetic material in the

chromosome can change Deficiencies/Deletions Duplications

2. The total amount of genetic material remains the same, but is rearranged

Inversions Translocations

8-11

Mutations Can Alter Chromosome Structure

8-12

• Deficiency (or deletion)– The loss of a chromosomal segment

• Duplication– The repetition of a chromosomal segment compared to the

normal parent chromosome• Inversion

– A change in the direction of part of the genetic material along a single chromosome

• Translocation– A segment of one chromosome becomes attached to a

different chromosome– Simple translocations

• One way transfer• A piece of a chromosome is attached to another chromosome

– Reciprocal translocations• Two way transfer• Two different types of chromosomes exchange pieces, producing

two abnormal chromosomes with translocations


(a)

(b)

(c)

(d)

(e)

q p

Deletion

Duplication

Inversion

Simple

Reciprocal

4 3 2 1 1 2 3 4 3 1 1 2 3

4 3 2 1 1 2 3 4 3 2 3 2 1 1 2 3

4 3 2 1 1 2 3 4 32 1 1 2 3

1 1 2 3

11 1 2 32

4 3 2 1 1 2 3

4 3 2 2 1112 1

1 1

4 3 2 1

4 3 2 11

1 2 3

translocation

translocation


2

Figure 8.28-13

Human chromosome 1

Human chromosome 21

(a) Terminal deletion (b) Interstitial deletion

Single breakTwo breaks andreattachmentof outer pieces

(Lost and degraded)

+

(Lost and degraded)

+

4 3 2 1 1 2 3

4 3

4 3

2 1 1 2 3

2

3 2

1 1 2 3

4 1 1 2 3


A chromosomal deficiency occurs when a chromosome breaks and a fragment is lost

Deficiencies

Figure 8.3



The phenotypic consequences of deficiencies depends on the 1. Size of the deletion 2. Chromosomal material deleted

Are the lost genes vital to the organism?

When deletions have a phenotypic effect, they are usually detrimental For example, the disease cri-du-chat syndrome in humans

Caused by a deletion in the short arm of chromosome 5 Refer to Figure 8.4

Deficiencies

(a) Chromosome 5 (b) A child with cri-du-chat syndrome

Deletedregion


© Biophoto Assocates/Science Source/Photo Researchers © Jeff Noneley

8-16

Figure 8.4


A chromosomal duplication is usually caused by abnormal events during recombination

Repetitive sequences can cause misalignment between homologous chromosomes.

If a crossover occurs, nonallelic homologous recombination results

Duplications

Figure 8.5

Repetitive sequences

Misalignedcrossover

A

A

B

B

C

C

D

D

A

A

B

B

C

C

D

D

Duplication

Deletion

A B C D

A B C D

A B C C

A B D

D



Like deletions, the phenotypic consequences of duplications tend to be correlated to size Duplications are more likely to have phenotypic effects if

they involve a large piece of the chromosome

However, duplications tend to have less harmful effects than deletions of comparable size

In humans, relatively few well-defined syndromes are caused by small chromosomal duplications

Duplications


The majority of small chromosomal duplications have no phenotypic effect

However, they are vital because they provide the raw material for the addition of genes to a species

This can ultimately lead to the formation of gene families A gene family consists of two or more genes that are

derived from the same ancestral gene

Duplications and Gene Families

Duplications can provide additional genes, forming gene families

• Over time, duplicated genes may accumulate mutations which alter their function– As a result, they may have similar but distinct functions– They are now members of a gene family– Two or more genes derived from a common ancestor are

homologous– Homologous genes within a single species are paralogs

– Refer to figure 8.6



Abnormal genetic event thatcauses a gene duplication

Gene

Over the course of many generations,the 2 genes may differ due to thegradual accumulation of DNAmutations.

Paralogs (homologous genes)

GeneGene

Mutation

Gene Gene


Figure 8.6

Genes derived from a single ancestral gene


The globin genes all encode subunits of proteins that bind oxygen Over 500 million years, the ancestral globin gene has

been duplicated and altered so there are now 14 paralogs in this gene family on three different chromosomes

Different paralogs carry out similar but distinct functions All bind oxygen Myoglobin stores oxygen in muscle cells Hemoglobins bind and transport oxygen via red blood cells Different globins are expressed in the red blood cells during

different developmental stages provide different characteristics corresponding to the oxygen

needs of the embryo, fetus and adult

Refer to figure 8.7

Myoglobins

chains chains

Hemoglobins

Mill

ions

of y

ears

ago

1,000

800

600

400

200

0

Mb


ζ ψζ ψα2ψα1 α2 α1

ε G A ψβ δ β

Ancestral globin


Figure 8.7

DuplicationBetter at binding

and storing oxygen in muscle

cells

Better at binding and transporting oxygen via red

blood cells

Expressed very early in embryonic life

Expressed maximally during the second and third trimesters

Expressed after birth


Copy Number Variation (CNV) A segment of DNA that varies in copy number among

members of same species May be missing a particular gene May be a duplication

Surprisingly common in animals and plants 1-10% of a genome may show CNV Associated with some human diseases

schizophrenia autism susceptibility to infectious disease cancer

Copy Number Variation is Relatively Common


Chromosomal deletions and duplications have been associated with human cancers May be difficult to detect with karyotype analysis Comparative genomic hybridization can be used

Developed by Anne Kallioniemi and Daniel Pinkel in 1992 Largely used to detect changes in cancer cell chromosomes

Experiment 8A-Comparitive Genomic Hybridization to detect

deletions and duplications



Experimental level Conceptual level

Isolate DNA from human breast cancercells and normal cells. This involvedbreaking open the cells and isolating theDNA by chromatography. (SeeAppendix for description ofchromatography.)

1.

Label the breast cancer DNA with agreen fluorescent molecule and thenormal DNA with a red fluorescentmolecule. This was done by using theDNA from step 1 as a template, andincorporating fluorescently labelednucleotides into newly made DNAstrands.

2.

The DNA strands were then denatured by heat treatment. Mix together equalamounts of fluorescently labeled DNAand add it to a preparation of metaphasechromosomes from white blood cells.The procedure for preparing metaphasechromosomes is described in Figure 3.2.The metaphase chromosomes were alsodenatured.

3.

Allow the fluorescently labeled DNA tohybridize to the metaphasechromosomes.

4.

Visualize the chromosomes with afluorescence microscope. Analyze theamount of green and red fluorescencealong each chromosome with acomputer.

5.

DNA

From breastcancer cells

From normalcells

Metaphasechromosomes

Slide

Metaphasechromosome

Deletions in the chromosomes of cancer cells show a green to redratio of less than 1, whereaschromosome duplications showa ratio greater than 1.

Rat

io o

f g

reen

an

d r

ed f

luo

resc

ence

inte

nsi

ties

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

2.0

2.5

Deletion

DeletionDeletion

Deletion

DuplicationChr. 1 – 20 Mb

Chr. 9

Chr. 11 Chr. 17

Chr. 16


THE DATA


Interpreting the Data

The data shows the ratio of green fluorescence (cancer DNA) to red (normal DNA). Chromosome 1 shows a Duplication (ratio of 2) Chromosome 9, 11, 16, 17 show Deletions (ratio of 0.5)

Allows the detection of large chromosomal changes

Newer techniques use microarrays (see Chapter 20)

8-28

A chromosomal with an inversion has a segment that has been flipped to the opposite orientation

Inversions

Figure 8.10

Inverted region Inverted region

A

(a) Normal chromosome

B C D E FG H I

A

(b) Pericentric inversion

B C GF E D H I A

(c) Paracentric inversion

E D C B FG H I


Centromere lies within inverted

region

Centromere lies outside inverted

region


In an inversion, the total amount of genetic information stays the same

Therefore, the great majority of inversions have no phenotypic consequences

In rare cases, inversions can alter the phenotype of an individual

Break point effect An inversion break point occurs in a vital gene

Position effect A gene is repositioned in a way that alters its gene expression

About 2% of the human population carries inversions that are detectable with a light microscope

Most of these individuals are phenotypically normal However, some individuals with inversions may produce offspring

with phenotypic abnormalities


Individuals with one copy of a normal chromosome and one copy of an inverted chromosome

Inversion Heterozygotes

Such individuals may be phenotypically normal They also may have a high probability of producing gametes that are

abnormal in their total genetic content The abnormality is due to crossing over within the inverted segment

During meiosis I, pairs of homologous sister chromatids synapse with each other

For the normal and inversion chromosome to synapse properly, an inversion loop must form

If a crossover occurs within the inversion loop, highly abnormal chromosomes are produced

Refer to figure 8.11

Replicated chromosomes

A B C D E F G H I

A

A

B

B

C

C

D

D

E

E

A

B

C D

E

F

F

G

G

H

H

I

A B C D E F G H I

F e d h iGHI

A B C D E f g c b a

I F G H I

a b c g f e d h i

a

a

b

b

c

c

g

g g

f

ff

e

ee

d

dd

h

h

i

a b c g f e d h i

ia

bc

gf

ed

h i

Replicated chromosomes

Withinversion:

Homologous pairingduring prophase

Crossover site

Products after crossing over

Normal:

Withinversion:

Acentricfragment

Duplicated/deleted

Dicentricchromosome

Dicentric bridge

Normal:

(a) Pericentric inversion (b) Paracentric inversion

Crossover site

A B C D E F G H I

A B C D E F

A B C d e a

G H I

I H G F E D c b f g h i

a de c b f g h i

A B C D E F G H I

a e d c b f g h i

a e d c b f g h i

Homologous pairingduring prophase

Products after crossing over


Figure 8.118-31

No centromere, chromosome is

lost

Chromosome will break if

centromeres move to opposite

poles


A chromosomal translocation occurs when a segment of one chromosome becomes attached to another

In reciprocal translocations two non-homologous chromosomes exchange genetic material Reciprocal translocations arise from two different

mechanisms 1. Chromosomal breakage and DNA repair 2. Abnormal crossovers Refer to Figure 8.12

Translocations

8-33Figure 8.12

22

Environmentalagent causes 2chromosomesto break.

Reactive ends

Nonhomologouschromosomes

Reciprocaltranslocation

1 1 7 7

22

2 2

DNA repairenzymes recognizebroken ends andincorrectly connectthem.

(a) Chromosomal breakage and DNA repair

(b) Nonhomologous crossover

71

Reciprocaltranslocation

Crossoverbetweennonhomologouschromosomes


Telomeres prevent chromosomal DNA from sticking to each other


Reciprocal translocations lead to a rearrangement of the genetic material, not a change in the total amount Thus, they are also called balanced translocations

Reciprocal translocations, like inversions, are usually without phenotypic consequences In a few cases, they can result in position effects

Translocations


In simple translocations the transfer of genetic material occurs in only one direction These are also called unbalanced translocations

Unbalanced translocations are associated with phenotypic abnormalities or even lethality

Example: Familial Down Syndrome In this condition, the majority of chromosome 21 is

attached to chromosome 14 (Figure 8.13a) The individual would have three copies of genes found

on a large segment of chromosome 21 Therefore, they exhibit the characteristics of Down syndrome Refer to Figure 8.13b

Person with a normal phenotype whocarries a translocated chromosome

Translocated chromosomecontaining long arms ofchromosome 14 and 21

Fertilizationwith a normalgamete

Gamete formation

14 1421

21

Possible gametes:

Possible offspring:

Normal Balancedcarrier

Familial Downsyndrome(unbalanced) Unbalanced, lethal

(a) Possible transmission patterns

(b) Karyotype of a male with familial Down syndrome

(c) Child with Down syndrome


© Will Hart/PhotoEdit

© Paul Benke/University of Miami School of Medicine

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 X Y

46, XY,214,1t(14q21q)

Figure 8.13

8-36


Familial Down Syndrome is an example of Robertsonian translocation

This translocation occurs as such Breaks occur near the centromeres of two non-

homologous acrocentric chromosomes The small acentric fragments are lost The larger fragments fuse at their centromeric regions to

form a single chromosome which is metacentric or submetacentric

This type of translocation is the most common chromosomal rearrangement in humans

Approximately one in 900 births


Individuals carrying balanced translocations have a greater risk of producing gametes with unbalanced combinations of chromosomes This depends on the segregation pattern during meiosis I

During meiosis I, homologous chromosomes synapse with each other For the translocated chromosomes to synapse properly,

a translocation cross must form Refer to Figure 8.14

Balanced Translocations and Gamete Production

Figure 8.14

8-39

Translocation cross

Two normal haploid cells+ 2 cells withbalanced translocations

Possible segregation during anaphase of meiosis I

Normalchromosome 1

Chromosome 1plus a piece ofchromosome 2

Normalchromosome 2

Chromosome 2plus a piece ofchromosome 1

All 4 haploid cellsunbalanced

All 4 haploid cellsunbalanced

1

1

1

12

2

2

1

1

2

2

1

1

2

2

1

1

2

2

1

2

1

1 1

2

2 2

21 1

21 1

2 22

2

2

1

1


(a) Alternate segregation (c) Adjacent-2 segregation (very rare)(b) Adjacent-1 segregation


Meiotic segregation can occur in one of three ways 1. Alternate segregation

Chromosomes diagonal to each other within the translocation cross segregate into the same cell following meiosis I

One cell receives 2 normal chromosomes and the other receives 2 translocated chromosomes

Leads to viable gametes 2. Adjacent-1 segregation

Adjacent non-homologous chromosomes segregate into the same cell after meiosis I

Both cells have one normal and one translocated chromosome Leads to 4 genetically unbalanced gametes

3. Adjacent-2 segregation Centromeres do not segregate properly during meiosis I One cell receives both copies of the centromere on chromosome

1 and the other both copies of the centromere on chromosome 2 Leads to 4 genetically unbalanced gametes

8-41 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Alternate and adjacent-1 segregations are the likely outcomes when an individual carries a reciprocal translocation Indeed, these occur at about the same frequency

Moreover, adjacent-2 segregation is very rare

Therefore, an individual with a reciprocal translocation usually produces four types of gametes Two of which are viable and the other two, nonviable This condition is termed semisterility

8.2 VARIATION IN CHROMOSOME NUMBER

• Chromosome numbers can vary in two main ways – Euploidy

• Variation in the number of complete sets of chromosome

– Aneuploidy• Variation in the number of particular chromosomes within a set

– Euploid variations occur occasionally in animals and frequently in plants

– Aneuploid variations, on the other hand, are regarded as abnormal conditions

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-42

1(X)

Normalfemalefruit fly:

(a)Diploid; 2n (2 sets)

Triploid; 3n (3 sets)

Tetraploid; 4n (4 sets)

Chromosome composition

Polyploidfruit flies:

(b) Variations in euploidy

Trisomy 2 (2n + 1)

Monosomy 1 (2n – 1)

Aneuploidfruit flies:

(c) Variations in aneuploidy

2 3 4


Figure 8.158-43

Polyploid organisms have three or more sets of chromosomes

Individual is said to be trisomic

Individual is said to be monosomic


The phenotype of every eukaryotic species is influenced by thousands of different genes The expression of these genes has to be intricately

coordinated to produce a phenotypically normal individual Aneuploidy commonly causes an abnormal

phenotype It leads to an imbalance in the amount of gene products Three copies of a gene will lead to 150% production A single chromosome can have hundreds or even

thousands of genes Refer to Figure 8.16

Aneuploidy

100%

1

Normal individual

Trisomy 2 individual

Monosomy 2 individual

2 3


100% 100%

100% 150% 100%

100%

50% 100%

8-45Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or displayFigure 8.16

In most cases, these effects are

detrimentalThey produce

individuals that are less likely to survive

than a euploid individual


Alterations in chromosome number occur frequently during gamete formation About 5-10% of embryos have an abnormal chromosome

number Indeed, ~ 50% of spontaneous abortions are due to such

abnormalities

In some cases, an abnormality in chromosome number produces an offspring that can survive Refer to Table 8.1

Aneuploidy


The autosomal aneuploidies compatible with survival are trisomies 13, 18 and 21 These involve chromosomes that are relatively small Carrie fewer genes than larger chromosomes

Aneuploidies involving sex chromosomes generally have less severe effects than those of autosomes This is explained by X inactivation

In an individual with more than one X chromosome, all additional X chromosomes are converted into Barr bodies

The phenotypic effects listed in Table 8.1 may be due to 1. The expression of X-linked genes prior to embryonic X-

inactivation 2. An imbalance in the expression of pseudoautosomal genes

8-49

Some human aneuploidies are influenced by the age of the parents Older parents more likely to produce abnormal offspring Example: Down syndrome (Trisomy 21)

Incidence rises with the age of either parent, especially mothers

Figure 8.17

Infa

nts

wit

h D

ow

n s

yn

dro

me

(pe

r 1

00

0 b

irth

s)

Age of mother

80

50

100

20 25 5045403530

6070

90

203040

1/1925


1/1205 1/8851/365

1/110

1/32

1/12


Down syndrome is caused by the failure of chromosome 21 to segregate properly This nondisjunction most commonly occurs during meiosis

I in the oocyte

The correlation between maternal age and Down syndrome could be due to the age of oocytes Human primary oocytes are produced in the ovary of the

female fetus prior to birth They are arrested in prophase of meiosis I until the time of ovulation

As a woman ages, her primary oocytes have been arrested in prophase I for a progressively longer period of time

This added length of time may contribute to an increased frequency of nondisjunction

Paternal non-disjunction causes Down syndrome 5% of the time


Most species of animals are diploid In many cases, changes in euploidy are not tolerated

Polyploidy in animals is generally a lethal condition Some euploidy variations are naturally occurring

Female bees are diploid Male bees (drones) are monoploid

Contain a single set of chromosomes

A few examples of vertebrate polyploid animals have been discovered Refer to Figure 8.18

Euploidy

(b) Hyla versicolor(a) Hyla chrysoscelis


© A.B. Sheldon © A.B. Sheldon

8-52Figure 8.18

Diploid Tetraploid


In many animals, certain body tissues display normal variations in the number of sets of chromosomes

Diploid animals sometimes produce tissues that are polyploid This phenomenon is termed endopolyploidy

Liver cells, for example, can be triploid, tetraploid or even octaploid (8n)

May enhance ability of cell to produce specific gene products

Polytene chromosomes of insects provide an unusual example of natural variation in ploidy

Euploidy


Occur in the salivary glands of Drosophila and a few other insects

Chromosomes undergo repeated rounds of chromosome replication without cellular division In Drosophila, pairs of chromosomes double approximately

nine times (29 = 512) These doublings produce a bundle of chromosomes

that lie together in a parallel fashion This bundle is termed a polytene chromosome

Polytene Chromosomes


Figure 8.19

(a) Repeated chromosome replication producespolytene chromosome.

(c) Relationship between a polytene chromosome and regular Drosophila chromosomes

L

R

Chromocenter

Each polytene armis composed ofhundreds ofchromosomesaligned side by side.

432

x

L

R


Each chromosome attaches to the chromocenter near its centromere

Central point where chromosomes aggregate


Because of their size, polytene chromosomes lend themselves to an easy microscopic examination They are so large, they can even be seen in interphase

Polytene chromosomes exhibit a characteristic banding pattern (Figure 8.19b) Each dark band is known as a chromomere

The DNA within the dark band is more compact than that in the interband region

Cytogeneticists have identified about 5,000 bands

Polytene chromosomes have facilitated the study of the organization and functioning of interphase chromosomes

(b) A polytene chromosome

Figure 8.198-57


In contrast to animals, plants commonly exhibit polyploidy 30-35% of ferns and flowering plants are polyploid Many of the fruits and grain we eat come from polyploid

plants Refer to Figure 8.20a

In many instances, polyploid strains of plants display outstanding agricultural characteristics They are often larger in size and more robust

Euploidy

(a) Cultivated wheat, a hexaploid species

Figure 8.20 8-59Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

8-60

Polyploids having an odd number of chromosome sets are usually sterile These plants produce highly aneuploid gametes

Example: In a triploid organism there is an unequal separation of homologous chromosomes (three each) during anaphase I

Figure 8.21

Each cell receives one copy of some

chromosomes

and two copies of other chromosomes


Sterility is generally a detrimental trait However, it can be agriculturally desirable because it

may result in 1. Seedless fruit

Seedless watermelons and bananas Triploid varieties

Asexually propagated by human via cuttings 2. Seedless flowers

Marigold flowering plants Triploid varieties

Developed by Burpee (Seed producers) Energy goes into flower production instead of making seeds (competitors can’t sell seeds grown from their plants)

8.3 NATURAL AND EXPERIMENTAL WAYS TO PRODUCE VARIATIONS

IN CHROMOSOME NUMBER

• There are three natural mechanisms by which the chromosome number of a species can vary– 1. Meiotic nondisjunction– 2. Mitotic abnormalities– 3. Interspecies crosses



Meiotic Nondisjunction

Nondisjunction refers to the failure of chromosomes to segregate properly during anaphase

Meiotic nondisjunction can produce haploid cells that have too many or too few chromosomes If such a gamete participates in fertilization

The resulting individual will have an abnormal chromosomal composition in all of its cells

Refer to Figure 8.22

8-56


Figure 8.22

All four gametes are abnormal

During fertilization,

these gametes produce an

individual that is trisomic

During fertilization,

these gametes produce an

individual that is monosomic

for the missing

chromosomen + 1

(a) Nondisjunction in meiosis I

Nondisjunctionin meiosis I

Normal meiosis II

n – 1n + 1 n – 1

Nondisjunction in Meiosis I

Nondisjunctionin meiosis II

Normalmeiosis I

(b) Nondisjunction in meiosis II

n nn + 1 n – 1

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-65Figure 8.22

50% Abnormal gametes

50% Normal gametes

Nondisjunction in Meiosis II


Meiotic Nondisjunction

In rare cases, all the chromosomes can undergo nondisjunction and migrate to one daughter cell

This is termed complete nondisjunction It results in one diploid cell and one without

chromosomes The chromosome-less cell is nonviable The diploid cell can participate in fertilization with a

normal gamete This yields a triploid individual

8-66


Mitotic Abnormalities Abnormalities in chromosome number often occur

after fertilization In this case, the abnormality occurs during mitosis not

meiosis

1. Mitotic disjunction (Figure 8.23a) Sister chromatids separate improperly

This leads to trisomic and monosomic daughter cells

2. Chromosome loss (Figure 8.23b) One of the sister chromatids does not migrate to a pole

This leads to normal and monosomic daughter cells

8-67

8-68Figure 8.23

This cell will be monosomic

This cell will be trisomic

Will be degraded if left outside of the

nucleus when nuclear envelope reforms

This cell will be monosomic

This cell will be normal

(a) Mitotic nondisjunction

(b) Chromosome loss

Not attachedto spindle



Mitotic Abnormalities

Genetic abnormalities that occur after fertilization lead to mosaicism The organism contains a subset of cells that are

genetically different from the rest f the organism

The size and location of the mosaic region depends on the timing and location of the original abnormality In the most extreme case, an abnormality could take place

during the first mitotic division Refer to Figure 8.24 for a bizarre example

8-69


Consider a fertilized Drosophila egg that is XX One of the X’s is lost during the first mitotic division

This produces an XX cell and an X0 cell

8-70

The XX cell is the precursor for this side of the fly, which developed

as a female

The X0 cell is the precursor for this side of the fly, which developed

as a male

This peculiar and rare individual is termed a bilateral gynandromorph

Figure 8.24


Complete nondisjunction can produce an individual with one or more sets of chromosomes This condition is termed autopolyploidy

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Figure 8.25

Diploid species

(a) Autopolyploidy (tetraploid)

Polyploid species (tetraploid)



Interspecies Crosses A much more common mechanism for changes in

the number of sets of chromosomes is alloploidy It is the result of interspecies crosses Most likely occurs between closely related species

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Figure 8.25

Species 1 Species 2

(b) Alloploidy (allodiploid)Alloploid


(c) Allopolyploidy (allotetraploid)

Allopolyploid

Species 1 Species 2


An allodiploid has one set of chromosomes from two different species

An allopolyploid contains a combination of both autopolyploidy and alloploidy

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Figure 8.25

An allotetraploid: Contains two

complete sets of chromosomes

from two different species



In two very closely related species, the number and types of chromosomes might be very similar

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Figure 8.26 shows the karyotype of an interspecies hybrid between the roan antelope (Hippotragus equinus) and the sable antelope (Hippotragus niger)

These two closely related species have the same number of chromosomes that are

similar in size have similar banding patterns

Evolutionary related chromosomes from two diferrent species are termed homeologous chromosomes

The allodiploid is fertile because the homeologous chromosomes can properly synapse during meiosis


Robinson, T.J. & Harley, E.H. "Absence of geographic chromosomal variation in the roan and sable antelope and the cytogenetics of a naturally occurring hybrid." Cytogenet Cell Genet. 1995; 71(4): 363-9. Permission granted by S. Karger AG, Basel. Reprinted with permission.

1 2 3 4 5 6

7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 23 24

25 26 27 28 29

XX

8-75Figure 8.26


In 1928, the Russian cytogeneticist G. Karpechenko conducted an interspecies cross between raddish (Raphanus) and cabbage (Brassica) Both are diploid and contain 18 chromosomes

Therefore the interspecies hybrid contains 18 chromosomes too However, the radish and cabbage are not closely related

species Their chromosomes are distinctly different from one another and

cannot synapse Thus, the radish/cabbage hybrid is sterile

However, an allotetraploid would be fertile It contains 36 chromosomes which undergo proper synapsis

Refer to Figure 8.27

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(b) Allotetraploid with a diploid set from each species

Metaphase I

(a) Allodiploid with a monoploid set from each species

Metaphase I

Radishchromosome

Cabbagechromosome

8-77Figure 8.27


No synapsis between the 9 radish and 9

cabbage chromosomes

Proper synapsis between the 18

radish chromosomes

and the 18 cabbage

chromosomes



Experimental Treatments Can Promote Polyploidy

Polyploid and allopolyploid plants often exhibit desirable traits Thus, the development of polyploids is of considerable

interest among plant breeders Can be induced by abrupt temperature changes and drugs

The drug colchicine is commonly used to promote polyploidy It binds to tubulin (a protein found in the spindle apparatus)

Thus, it promotes nondisjunction

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Diploid plant

A tetraploid plant

Treat with colchicine.

Allow to grow.

Take a cutting ofthe tetraploidportion.

Root the cuttingin soil.

Tetraploid portionof plant (note thelarger leaves)


8-79Figure 8.28

Caused by complete

nondisjunction


Cell Fusion Techniques Can Be Used to Make Hybrid Plants

Researchers have recently developed techniques to produce hybrids with altered chromosome composition

In cell fusion, individual cells are mixed together and made to fuse It can create new strains of plants It allows the crossing of two species that cannot interbreed

naturally Refer to Figure 8.29

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Festuca arundinacea

Lolium multiflorum

Cells without cell walls

Cells with two separate nuclei

Figure 8.29


Tall fescuegrass

Italian ryegrass

Protoplasts

Cell wall

Heterokaryon

Hybrid cell

Allotetraploid

Add agent that digests cell walls.

Treat protoplasts with agents to promotecellular fusion.

Grow on laboratory media to generate ahybrid plant.

Spontaneous nuclear fusion produces hybridcell with single nucleus.Phenotypic

characteristics are intermediate between

the “parents”


Experimental Production of Monoploids

The production of monoploids can be used to develop homozygous diploid strains of plants

In 1964, Sipra Guha-Mukherjee and Satish Maheswari developed a method to produce monoploid plants from pollen grains This experimental technique is called anther culture It is described in Figure 8.30

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Treat section with colchicine.

Colchicine treated

Diploid plant

Propagate treated section.

Anthers

Plantlets

Transplant and grow.

Grow several weeks.

Cold shock


Parental plant is diploid but not

homozygous for all its genes

Figure 8.30

Is homozygous for all its genes


Induces pollen grains to begin development


Experimental Production of Monoploids

In certain animal species, monoploids can be produced by treatments that induce the eggs to develop without sperm fertilization This is know as parthenogenesis In many cases, the haploid zygote is short-lived

Example: Zebrafish (Danio rerio) Haploid egg is induced to begin development by exposure

to UV-irradiated (inactivated) sperm

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