chromosomal aberrations in number

Numerical chromosomal aberrations.

Atika Noor 1

ASSIGNMENT ON GENETICS

NUMERICAL CHROMOSOMAL ABBERATIONS

Submitted to:

Sir IMRAN BASHIR.

Submitted By:

ATIKA NOOR.

Swot Degree College Liaquat Pur.


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ACKNOWLEDGEMENT

Presentation, Inspiration and motivation have

always played an important role in the success of venture.

The success and final outcome of my assignment required a

lot of guidance and assistance .I am acknowledged to indebted all my

success to my Botany teacher

“Sir IMRAN BASHIR”, who encouraged

appealed and enlighten my abilities to help me to prepare such an

excellent assignment.

At last but not the least I pay humble thanks to my college

principal, “Sir NAEEM AKHTAR.”

Atika Noor


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Dedication:

Dedicated to my teacher,

With words,

Across galaxies of time &space;

Travelling just to see my goal.

Lost amidst the countless stars;

To bring me back to where you are.

And my parents who say to inspire me,

“Though you can love you cannot master, you cannot

master what you don’t love.”

Atika Noor


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Table of contents:

Numerical Chromosomal Aberrations

Euploidy

Aneuploidy

Types of euploidy.

Monoploidy (1n)

Diploidy.(2n)

Polyploidy.(3n,4n,5n…)

Types of polyploidy

Autopolyploidy.(AAA) or (AAAA)

Allopolyploidy.(AABB) or (AAB)

Types of Aneuploidy.

Hypoploidy.(2n-x)

Hyperploidy.(2n+x)

Monosomy.(2n-1)

Nullisomy.(2n-2)

Trisomy.(2n+1)

Tetrasomy.(2n+2)


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Numerical Chromosomal Aberrations

Introduction:

All animals and plants have a complete genome consisting of two sets of

chromosomes in somatic cell and one set in gametes .Sometimes

aberrations occur in number or structure which causes abnormality.

Change in number of chromosomes is called numerical chromosomal

aberrations. It is as well termed as ploidy. There are two kinds of ploidy.

They are as follow:

Euploidy.

Aneuploidy.

Euploidy:

The state or condition of having a variation in

chromosome number that is an exact multiple of the

characteristic haploid number.

Types: Euploidy has three types .

1) Monoploidy.

2) Diploidy.

3) Polyploidy.


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Monoploidy:

The condition where each chromosome is represented only once is

called Monoploidy and the organisms having this condition are called

monoploids or haploids.

The haploids have only one set of chromosomes.

e.g. algae and fungi.

A distinction should be made between monoploids and haploid.

e.g.

In Barley=2n=x=7

Corn=2n=x=10

X= basic number.

Origin and production of monoploids:

Monoploids I some cases are found naturally and are produced

due to parthenogenesis as in male insects, bees, wasps, ants etc.

In angiosperms monoploids may also originate spontaneously due

to parthenogenetic development of eggs.

Rarely monoploids plants may originate from the pollen tube and

antipodal of the embryo sac called androgenic monoploids.

e.g. tomato and cotton under cultivation.

Artificial means of production:

1) X–ray treatment.

2) Delayed pollination.

3) Temperature shock (cold treatment)

4) Colchicines treatment.

5) Distant hybridization.

6) Anther pollen culture.


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Morphology of monoploids:

Monoploids plants have reduced size of;

All vegetative parts

Flowers, seeds and stomata.

Diploidy:

The condition in which organism is called has two sets of

chromosomes is called diploidy and these organisms are called

diploid.

Explanation:

Each sexually reproducing organism has two sets of chromosomes

called homologous sets.

Thus diploid has 2n number of chromosomes.

e.g. Maize, wheat most of animals.

Polyploidy:

It is a condition in which more than two sets of chromosomes are

present, is called polyploidy and individuals are called polyploids. i.e. 3n,

4n, 5n, 6n and so on.

Three sets of chromosomes are triploids.

Four sets are tetraploids.

Six sets are called hexaploids.

Some animals such as lizards, amphibians and fish are polyploids.

Types:

a) Autopolyploidy.

b) Allopolyploidy.


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Autopolyploidy:

By the addition of one or more sets of

chromosomes identical to normal haploid complement of the

same species is called autopolyploidy.

If a diploid specie has two sets of chromosomes (2n) or genome

(AA)an auto triploid has three similar sets or genome.

Tetraploids have four sets of genome.

Origin and production of autopolyploidy:

Polyploidy may arise naturally by following means;

o Due to interference with cytokinesis once replication has occurred.

o It may occur during meiosis when unreduced gametes are

produced.

o All these may occur during chilling.

Artificial methods to induce autopolyploidy are follows

o Chloral hydride.

o Mercury chloride etc.

o Radioactive substances and temperature shocks.

o Colchicines’ method.

Autotriploids arise in several ways;

1) A failure of all gametes during meiotic division may produce diploid

gametes. If such gametes are fertilized by a haploid gamete a

triploid organism is produced.

2) Two sperms may fertilize an ovum resulting triploid zygote.

3) Triploids may be produced under experimental conditions by

crossing diploid with tetraploids.

Examples:

Tomatoes.

Grapes.

Watermelon etc.


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Autotetraploids:

Organisms having four sets of chromosomes are called auto

tetraploids and are produced by applying colchicines technique to cells

undergoing mitosis.

They can be produced applying heat shock proteins.

Allopolyploidy:

When the polyploidy results due to doubling of chromosome

number in a F1 hybrid which is derived from two distinctly varying

species, it is called allopolyploids. Organisms are called allopolyploids.

Explanation:

Allopolyploid has two or more genomes from two different

species. It is achieved by multiplication of chromosome sets of a hybrid

of two diploid species.

Examples:

1) A classical example of allotetraploidy in plants is the cultivated

species of American cotton (Gossipium hirsutum).this specie has

26 chromosomes, 13 large and 13 small. It was noted that old

world cotton has only 13 pairs of large chromosomes and wild

American cotton has only 13 pairs of small chromosomes.

New American cotton is tetraploid and is hybrid between old world

cotton and wild American cotton.

2) Wheat and rye example is the best example to explain the

Allopolyploidy.

Tetraploid Triticum durum (wheat),(4n=28) is crossed with diploid

Scale cereal ( rye plant),(2n=14).Triticale (6n=42) hexaploids is

obtained.

The hybrid demonstrates the characteristics of both wheat and rye.


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Effects of polyploidy:

1. Morphological effects:

The polyploidy is invariably related with gigantism. The

polyploid plant has large size of;

Pollen grains.

Cells.

Leaves.

Stomata.

Xylem etc.

2. Physiological effets:

The ascorbic acid content is reported higher in tetraploid tomatoes

and cabbage.

Tetraploid maize seed has 40 %more vitamin A as compared to

diploid corn.

3. Effects on fertility:

The most important effect of polyploidy is that it produces infertility

in plants, as a result no seed formation.

4. Evolution through polyploidy:

Interspecific hybridization combined with polyploidy offers a

mechanism whereby new species may arise.

Aneuploidy:

‘’Aneuploidy may be due to loss of one or more

chromosomes or due to addition of one or more

chromosomes to the complete set of chromosomes and not

the complete set of chromosomes is involved.’’

The nuclei of aneuploids contain chromosomes whose number is not

true multiple of the basic number (n).

Types:

1) Hypoploidy.

2) Hyperploidy.


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Hypoploidy:

Hypoploidy is a state in which cells contain one or more, fewer

chromosomes than what is normal.

Kinds:

a) Monosomy.

b) Nullisomy.

Monosomy:

The loss of only one chromosomes from the whole set of

chromosome is called monosomy.organisms are called monosomic

organisms.(2n-1) condition is present.

Explanation:

In animals Monosomy for one sex is common i.e. XO mechanism

in Drosophila melanogester have appearance; but is sterile male.

Loss of 4 chromosomes results in development of individual with

reduced body.

Loss of chromosome 2 or 3 is lethal.

Monosomy has been observed in maize, tobacco and datura.

A polyploid has many copies of same chromosomes so the loss of

one chromosome is easily tolerated.

Nullisomy:

Loss of a chromosome pair from the whole genome is called Nullisomy.

(2n-2) condition is present.

A nullisomy diploid often don’t survive.

In polyploids nullisomic condition survive as they have many

copies of genome.

Hyperploidy:

Having a chromosome number greater than but not an exact multiple of

the normal euploid number is called Hyperploidy.


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Kinds:

a) Trisomy.

b) Tetrasomy.

Trisomy:

Trisomics are those diploid organisms which have an extra

chromosome. This condition is called Trisomy denoted by (2n+1).

Explanation:

Since the extra chromosome belongs to any of the different chromosome

of a haploid complement the number of possible Trisomics will be equal

to the haploid chromosome number, e.g. haploid chromosome number

of barley is 7 so there may 7 trisomics are possible .

Examples:

Datura trisomics ;

Haploid number of chromosome in datura is 12 and there is

12 possible trisomics.i.e.A plant with 24 chromosomes+ chromosome

no. 1 or plant with 24+an extra chromosome no. 2.

These trisomics have different shapes of capsules than normal diploids.

Drosophila trisomics:

Trisomy was discovered by Bridges in 1921.He found that

there are some female drosophila with an extra X chromosome (XXX) .

The normal drosophila male and female have XY and XX pairs of sex

chromosomes respectively.

Chromosome no. 4 of drosophila can also exist in trisomics form.

Trisomy in man:

Man has also trisomics conditions in several different chromosomes;

Down’s syndrome at chromosomes no. 21.

Patue syndrome at chromosomes no. 13.


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Edward syndrome at chromosomes no. 18.

Kline filter’s syndrome is rare and is present at sex chromosomes.

Indicated by (XXY) or (XXX) females are also observed.

Tetrasomy:

The diploid organisms having two extra chromosomes are

known as tetrasomic.

Genomic formula is (2n+2).

All the 21 possible tetrasomics are available in wheat.


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Referances:

Text Book of Botany.

Notes for B.Sc. by “Sir Imran Bashir.”

Notes for B.Sc. by “Sir Azad Ali Mirza.”

Genetics search on “Google”.

Genetics: Analysis and Principles by Robert Brooker.


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Remarks:

chromosomal aberrations in number

Science

n diploidy

n polyploidy

nx hyperploidy

n x monosomy

monoploids plants

production of monoploids

androgenic monoploids

angiosperms monoploids