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Chromosomes and meiosis

Name: __________________________
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IEB requirements from the Subject Assessment Guidelines

Meiosis

AIM 3:
AIM 2: APPRECIATING AND
INVESTIGATING AIM 1:
UNDERSTANDING THE
PHENOMENA IN LIFE KNOWING LIFE SCIENCES
HISTORY, IMPORTANCE
SCIENCES
AND APPLICATIONS
OF LIFE SCIENCES IN
Learners should know: SOCIETY
Attitudes towards those with
genetic abnormalities.
Model replication and 1. the location of chromosomes in cells and their
meiosis and comment structure. Consider case studies of
on accuracy chromosomal abnormalities –
Look at microscope slides 2. the significance of chromosomes in cell division. range of XY conditions
of phases of division (Klinefelter's syndrome, etc.).
Draw other phases from a 3. the difference between haploid and diploid Debate: What is male/female.
reference diagram number and understand the significance of each
Use karyotypes Distinguish cause from effect,
to deduce 4. where, when and why meiosis takes diagnosis, prognosis.
information place in animals and flowering plants
Meiosis in flow
diagrams and life 5. the process of DNA replication as part of
cycles interphase. (suggest covering here to avoid
confusion with protein synthesis)

6. the process and terminology of meiosis.

Only names of basic phases (e.g. Prophase
I) are required.

7. about chromosomal mutations that can arise as a

result of abnormal meiosis,(use Down syndrome
as an example).

8. how to work with karyotypes.

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Chromosomes:

A chromosome is made up of a DNA molecule wrapped around histone proteins.

Chromosomes are found in the nucleus of a cell.

A short piece of DNA in a chromosome comprises a gene. A gene can be defined as a

hereditary unit consisting of a sequence of DNA that occupies a specific location on a
chromosome and determines a particular characteristic in an organism. Genes control
hereditary characteristics.

When a cell divides, the DNA needs to replicate to make two identical sets of information that
needs to be passed on to new cells.

A cell that is not actively dividing is said to be in interphase.

If a cell is going to divide, then the DNA replicates during late interphase of a cell’s life cycle.
The result is two strands of DNA molecules of exactly the same genetic material. Each strand
is called a chromatid. After DNA replication, each chromosome is made up of two chromatids
joined to each other at a point called the centromere.

Structure of a chromosome after DNA replication (late interphase)

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The total number of chromosomes per cell varies from species to species and is not related to
the size or complexity of the species. For example, butterflies have 380+ chromosomes per
cell, humans have 46 chromosomes per cell, Drosophila (fruit flies) have 8 chromosomes per
cell and potatoes have 48 chromosomes per cell. This number is constant in all organisms of
a species.

When an organism is produced by sexual reproduction, a male gamete fuses with a female
gamete. If the number of chromosomes in each gamete was the normal number for a cell of
that species, the number of chromosomes would double with each generation. This does not
happen. The gametes are formed by a special type of cell division – meiosis – whereby the
chromosome number is halved. Thus, when two gametes fuse, the normal chromosome
number for the species is restored.

All chromosomes occur in pairs called homologous chromosome pairs. One chromosome of
the homologous pair will have been contributed from the male parent and the other will have
been contributed from the female parent. The letter n is used to represent the number of
chromosome pairs in a species. This is also known as the haploid number of chromosomes.
As each chromosome pair consists of two chromosomes, the total number of chromosomes in
a species is 2n. This total number of chromosomes is known as the diploid number of
chromosomes.

All normal human body cells contain 46 chromosomes (23 pairs of homologous chromosomes).
This is the diploid (2n) number of chromosomes. When gametes are produced (ova or sperm
cells) the chromosome number is halved to 23 in humans. This is the haploid (n) number of
chromosomes in humans.

When two haploid gametes fuse, the diploid number is restored. As the chromosome number
is halved in meiosis, meiosis is known as a reduction division.

Meiosis only takes place in the ovaries and testes of animals and in the anthers and the ovules
in the ovaries of flowering plants.
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For interest:
 Simpler organisms, such as algae, sometimes have meiosis occurring in different stages
in their life cycles and in other simple organisms, the normal chromosome number in a
cell is haploid.
 Some plants sometimes form diploid gametes that fuse to form tetraploid plants. There
are other plants that form polyploid generations.

Each cell of an animal has a diploid number of chromosomes (2n) because each can be traced
back to the zygote which was formed by the fusion of the two haploid (n) gametes. Because
each gamete supplies a complete set of chromosomes (23 in humans), the zygote has a double
set (23 pairs) or 46 chromosomes. Then, as the zygote divides by mitosis - a division in which
the daughter cells formed are identical to each other and to the original parent cell – every body
(somatic) cell contains the diploid number of chromosomes again i.e. 46, or 23 pairs, in a
human.

Every body (somatic) cell has a set of chromosomes (23 in humans) from the maternal parent
and a set (23 in humans) from the paternal parent. Each maternal chromosome has a partner
paternal chromosome and these two form a homologous pair of chromosomes. The partners,
called homologues, are identical in size, and shape and carry genes for the same hereditary
characters. Homologues are the same length and also have the centromeres at the same
position. The genes are arranged in exactly the same order in each homologue but may carry
a different form of the characteristic e.g. the maternal gene may code for blond hairs and the
paternal code for brown hair. These alternative forms of the gene are called alleles.

From Ayerst, P., Langley, R., Majozi. P., Metherell, A. and Smith, D. (2007). Shuters Life Sciences Grade 12.
Pietermaritzburg; Shuter and Shooter.

In humans there are 22 pairs of chromosomes (called autosomes) plus one pair of sex
chromosomes (called gonosomes) per cell. There are two types of sex chromosomes: X and
Y. Females have two X chromosomes, while males have an X and Y chromosome.
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Homologous chromosomes can be clearly seen if a karyotype is created.

A karyotype represents a complete set of chromosomes from a single cell in an organism. It

shows the number, size and shape of the chromosomes during metaphase of mitosis. The
chromosomes are shown in their pairs, the pairs are numbered, the chromosomes are arranged
from longest to shortest (gonosomes last normally), with the centromeres in a line and the short
arms of the chromosomes pointing upwards. The banding pattern does not represent individual
genes, but regions of the chromosome that contain many hundreds of genes.

Human Female Karyotype

(from:http://www.biology.iupui.edu/biocourses/N100/2k2humancsomaldisorders.html)
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Human Male Karyotype

(from:http://www.biology.iupui.edu/biocourses/N100/2k2humancsomaldisorders.html)
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MITOSIS

Mitosis is the process that takes place when exact copies of cells are made. During mitosis
the chromosome are duplicated exactly and the resulting cells are identical to the original cells
in every aspect. Before mitosis takes place the DNA in the nucleus is replicated and each
chromosome has a double set of identical DNA at the start of mitosis.

Mitosis was covered in Grade 10 and the diagram below is a brief summary of the process.

The different stages of mitosis (from: de Wet,H., Dugard,J., Freedman, R. and Webb, J.
2010. Life Sciences for All. Grade 12. Macmillan, South Africa.
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MEOISIS

Where and when meiosis occurs

In animals, meiosis takes place during the formation of gametes (ova and sperm cells), and in
plants it occurs during formation of spores that contain gametes.

Meiosis in animals, including humans, occurs in the gonads – the ovaries and the testes.

Site of meiosis in flowering plants

In seed bearing plants meiosis takes place in the pollen grains in the pollen sacs in the male
anthers and in the ovules in the female ovaries.

The purpose of meiosis is to separate the homologous chromosome partners, so that each
gamete will possess one chromosome of each homologous pair. This means that the haploid
condition (n) as found in the gametes will be re-established. Thus, after a meiotic division, each
resulting set of daughter cells will have a random haploid mixture of maternally- and paternally-
derived chromosomes.

Meiosis involves 2 nuclear divisions which usually follows closely one after the other. The 2
divisions are known as the first and second meiotic divisions.
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A. FIRST MEIOTIC DIVISION (MEIOSIS l)

The number of chromosomes is halved.

Segments of chromatids of homologous pairs of chromosomes are exchanged.
Double stranded chromosomes migrate to the poles (i.e. the two chromatids of a chromosome
do not separate).

Each of the two daughter cells formed at the end of the first meiotic division then undergoes a
second meiotic division.

B. SECOND MEIOTIC DIVISION (MEIOSIS ll)

This is similar to ordinary mitotic division.

There is a separation of the chromatids (i.e. single-stranded daughter chromosomes migrate
to the poles).

As with mitosis, meiosis can also be considered as occurring in series of stages.

Meiosis is preceded by interphase. In a cell that is about to divide, the DNA of each
chromosome replicates, so that each chromosome is composed of two chromatids that have
identical DNA. It is possible that mutations can occur during DNA replication.
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FIRST MEIOTIC DIVISION (MEIOSIS l):

PROPHASE I:

1. The chromatin network begins to contract and each chromosome shortens and becomes
visible as two chromatids joined by a centromere.

2. The chromosomes arrange themselves in their homologous pairs. The chromosomes of

each homologous pair come to lie side by side. This arrangement of chromosomes is
described as a bivalent (each bivalent consists of 4 chromatids.)

3. The centrioles of the centrosome of animal cells move to opposite poles. This determines
the direction of the nuclear division. A spindle made up of protein threads, develops
between the two centrioles.

4. Crossing over takes place. During crossing over there is an exchange of pieces of
chromatids. Whole groups of genes are swapped between homologous chromosomes. The
point on the chromosome where crossing over takes place is called a chiasma (plural
chiasmata). The process of crossing over introduces genetic variation. This is one of the
reasons why the offspring of parents do not look identical to each other or to their parents.

During this process, mutations can occur, some being harmful and others beneficial as
they introduce new forms of genes into the genetic make-up of a species.

5. The nucleolus and nuclear membrane disappear.

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Crossing over of chromatids

from www.biologie.uni-hamburg.de/b-online/library/cat-removed/xover.gif

Crossing over can be very complicated, as is shown in the photomicrograph below.

A is a photomicrograph of a bivalent at late prophase

l in the testis of a grasshopper.

B is an interpretative diagram. Chiasmata are

numbered according to which chromatids are in
contact. Note that chiasmata can be formed between
any two non-sister chromatids.

From Roberts, M., Reiss, M., &

Monger, G. (2000). Advanced
Biology. Walton-on-Thames;
Nelson.
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METAPHASE I:

1. The bivalents (pairs of chromosomes) arrange themselves on the equatorial plane of the
spindle with one of each pair on either side of the equator. The arrangement of maternally
and paternally derived chromosomes on either side of the equator is completely random.
This is known as independent assortment and the chromosomes of paternal origin and
those of maternal origin are randomly segregated to opposite poles.

It is possible for chromosomal mutations to occur at this point. These will be discussed
further on in these notes.

2. One of the two centromeres of each bivalent lie on either side of the equatorial plane i.e.
there is a double row of chromosomes.

ANAPHASE I:

1. One chromosome (consisting of

two chromatids) of each pair moves to
one pole, while the other chromosome
of the pair moves to the opposite pole.
Although one chromosome of each
pair moves away to a pole, the two
chromatids of each chromosome
found at the poles at the end of the
anaphase are not identical because of
crossing over.
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TELOPHASE I

1. The chromosomes regroup themselves at the poles.

2. At each pole there is now half the number of chromosomes present in the original cell i.e.
haploid number. Each of these chromosomes consists of 2 chromatids joined by a
centromere.

CYTOKINESIS

The cytoplasm divides to form two haploid cells, each cell with only one of each homologous
pairs of chromosomes.

THE SECOND MEIOTIC DIVISION (MEIOSIS ll):

This is similar to an ordinary mitotic division but, as a rule, the prophase is omitted and the
chromosomes take up positions characteristic of the metaphase of mitosis.

PROPHASE II:

1. A new spindle made of fibres develops, in the opposite plane to the one formed in meiosis
l, and the nuclear membrane disintegrates.

METAPHASE II:

1. Double stranded chromosomes line up in a single line along the equator.

2. Centromeres attach to the spindle threads.

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ANAPHASE II:

1. Each centromere divides and the two chromatids of each chromosome separate and move
to opposite poles — i.e. single stranded chromosomes (daughter chromosomes) are
pulled to opposite poles.

TELOPHASE II:

1. At the poles the single stranded daughter chromosomes lengthen again by unspiralling, to
form a chromatin network.

2. A nuclear membrane and nucleolus again appear.

CYTOKINESIS

The cytoplasm divides to form 4 new daughter cells.

In plant cells a cross-wall develops along the equator of the cell, while in animal cells
membrane constriction occurs.

The original diploid parent cell has now given rise to

 four cells
 that are non- identical
 and that are haploid
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The diagram below shows the process of meiosis in colour to enable you to track the effect of
crossing over and independent assortment more clearly.
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Micrographs showing the process of meiosis

It is important that you are able to recognise each of the stages of meiosis in micrographs and
that you are able to justify your choice for your identification of a stage, giving evidence visible
on a micrograph.

Go through the animation of the stages of meiosis by opening the following link on
your computer

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
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THE SIGNIFICANCE OF MEIOSIS:

1. The number of chromosomes is halved (during the first division).

2. Four daughter cells are formed which often, especially in animals, may develop into gametes.
Each of these gametes has the haploid number of chromosomes. A term that may be
used to refer to the group of four haploid cells formed from one cell that has undergone
meiosis is tetrad, although many books only use that term in plants.

The formation of pollen in a lily plant shows the

tetrads still enclosed together before being released
as individual pollen grains.

3. Each cell / gamete / spore formed is genetically different. This genetic variety is introduced
as a result of the crossing over that occurs during prophase I and the random assortment
and independent segregation of the homologous pairs of chromosomes during anaphase
I.

For your interest:

A mathematical formula shows how many possible combinations of maternal and paternal
chromosomes may be found in a gamete produced by meiosis from a diploid cell.

The number of possible combinations in a gamete = 2n (where n = haploid number of the

species).

E.g. If a diploid cell with a diploid chromosome number of 6 divides by meiosis, then the number
of combinations is 23 = 8 different ways in which the chromosomes can align differently on the
equator.

How many possible combinations are there for a human?

4. It makes the phenomenon of alternation of generations possible. Alternation of

generations is the regular alternation of a diploid generation (2n number of chromosomes)
with a haploid generation (n number of chromosomes) in the life-cycle of a plant.
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Comparison of Mitosis and Meiosis

from http://ghr.nlm.nih.gov/handbook/illustrations/mitosismeiosis.jpg
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, ----
: Mitosis Meiosis
Occurs in somatic cells
. "

Occurs in reproductive organs - testes, ovaries, anthers, '.

sporangia of mosses and ferns
Occurs in haploid, diploid and polyploid cells Occurs in diploid and polyploid cells, not in haploid cells
Produces somatic cells, spores in bread mould, and gametes in Produces gametes in animals, and spores in mosses and ferns
mosses, ferns and flowering plants
One nuclear division Two nuclear divisions
Homologous chromosomes never pair and crossing over never In meiosis 1, homologous chromosomes pair and crossing over
occurs occurs
Chromosomes line up separately at the equator of the spindle In meiosis 1, chromosomes line up in their homologous pairs at
the equator of the spindle
Centromeres divide and chromatids move as daughter In meiosis 1, centromeres do not divide and chromosomes,
chromosomes to opposite poles consisting of two chromatids, move to opposite poles
Daughter cells that form have the same number of Daughter cells that form have half the number of chromosomes
chromosomes as the parent cell as the parent cell
Two daughter cells are produced, genetically identical to one Four daughter cells produced are genetically different from
another and to the parent cell each other and from the parent cell

Meiosis can only occur in a diploid cell and gives rise to four daughter cells that are haploid
and non-identical.

Mitosis can occur in haploid or diploid cells and gives rise to two identical daughter cells
that are also identical to the parent cell from which they were formed.

What is chance that any two human sex cells will be identical? 1 in 2 raised to the 23rd
power = 1 in 8,388,608. So if someone tells you "you're 1 a million", you should answer
"No, I'm 1 in 8 million" (more or less) :-) . But this doesn’t even account for the fact that
the gametes come from two different parents and also doesn’t consider crossing over.
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MUTATIONS

Mutations are changes in DNA that sometimes affect how genes do their jobs. Genes are
instructions, and if these instructions are damaged, the end result is not as it should be. It can
be compared to a recipe, where mistakes can lead to the recipe failing either in part or in full.
Mutations can affect small or large amounts of genetic material. Most mutations are harmless,
and some, called silent mutations, don’t have any effect. Rarely, mutations can cause serious
problems with health and development, whereas others actually benefit and will prevail in
populations through natural selection.

Types of Mutations

Some types of mutations include:

 Hereditary – also called germ line mutations, these mutations occur in the sex cells
(sometimes referred to as “germ cells”) and are therefore passed on from parent to
offspring.

 Somatic – also called acquired mutations, these happen to individuals in their lifetime.
A mistake may be made when the DNA is being copied to make new cells, or via
environmental damage, such as UV radiation. Since only somatic (body) cells are
affected, such mutations cannot be inherited.

If a mutation occurs in a somatic cell, it will be restricted to the body cells of a single
organism, but if a mutation occurs in a gamete, it may be transmitted to the organism’s
descendants.

There are two types of mutations.

 Gene mutations (includes point mutations)

 Chromosome mutations

Gene mutations involve a change in the structure of the DNA usually involving one gene.
A point mutation is a gene mutation that affects just one or two bases in the DNA molecule.
(point mutations were covered in detail in the previous section on DNA replication).

Chromosome mutations in involve a change in the number or structure of whole

chromosomes
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Gene mutations

Gene mutations occur when there is a chemical change within a gene. An alteration in the
order of nucleotides in that part of the DNA molecule could result in a change in the order
of amino acids making up a protein. These mutations occur naturally in DNA but can also
be caused by X-rays, atomic rays and some chemicals.

The following indicates types of mutation where whole genes are moved:

Deletion

As the name implies, genes of a chromosome are permanently lost as they become
unattached to the centromere and are lost forever

1. Normal chromosome before mutation

2. Genes not attached to centromere become loose and lost forever
3. New chromosome lacks certain genes which may prove fatal depending on how
important these genes are

Duplication

In this mutation, the mutant genes are displayed twice on the same chromosome due to
duplication of these genes. This can prove to be an advantageous mutation as no genetic
information is lost or altered and new genes are gained.

1. Normal chromosome before

mutation

2. Genes from the homologous

chromosome are copied and inserted into
the genetic sequence

3. New chromosome possesses

all its initial genes plus a duplicated
one, which is usually harmless.
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Inversion

This is where the order of a particular order of genes are reversed as seen below

1. Normal chromosome un-altered

2. The connection between genes break and the sequence of these genes are reversed
3. The new sequence may not be viable to produce an organism, depending on which genes
are reversed. Advantageous characteristics from this mutation are also possible

Translocation

This is where information from one of two homologous chromosomes breaks and binds to the
other. Usually this sort of mutation is lethal

1. An un-altered pair of homologous chromosomes

2. Translocation of genes has resulted in some genes from one of the chromosomes
attaching to the opposing chromosome
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An example of a gene mutation is the hereditary disease known as sickle cell anaemia. A
gene mutation causes the normal haemoglobin in red blood corpuscles to be replaced by
an abnormal haemoglobin, haemoglobin-S. This mutant protein causes the red blood
corpuscles to form a sickle shape and they are therefore unable to carry oxygen effectively.
The victim suffers from oxygen shortage, weakness, emaciation, kidney and heart failure.

All of the genetic mutations discussed above have a negative impact and are undesired,
however, in some cases they can prove advantageous.

Genetic mutations increase genetic diversity and therefore have an important part to play.
They are also the reason many people inherit diseases.
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Chromosome mutations

Chromosome mutations occur when there is a change in:

 The structure of chromosomes

 The number of chromosomes

Abnormalities of both types can occur during meiosis.

Failure of chromosomes or chromatids to segregate in meiosis is called non-disjunction.

Aneuploidy is caused by non-disjuncture. Aneuploidy occurs when there is not an even

number of chromosomes. A chromosome has either been lost or gained.

Non-disjunction is when a homologous pair of chromosomes fail to separate during meiosis,

resulting in gametes with one extra chromosome and other gametes lacking a chromosome.

http://www.bio.miami.edu/~cmallery/150/mendel/c8.15x13.nondisjunction.jpg

If a gamete that underwent nondisjunction during its formation fuses with a normal gamete the
chromosome complement will differ from normal. It will be 2n +1 or 2n -1.
An extra chromosome results in trisomy, while one less results in monosomy.

If an individual has trisomy 21, that person will be afflicted with Down’s Syndrome. Trisomy is
the formation of an organism with an extra chromosome of a pair. If the trisomy is of a large
chromosome, it is often lethal.
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Non-Disjunction and Down's Syndrome

One well known example non-disjunction results in Down’s syndrome. This non-disjunction
occurs in chromosome 21 of a human egg cell. Mistakes in meiosis result in an egg or sperm
with 2 chromosome 21’s. During fertilization this gamete fuses with a gamete with the 1
chromosome 21, resulting in a zygote with 3 chromosomes 21 and a chromosome number of
47. As a result, a Down’s syndrome person will have cells which possess 47 chromosomes as
opposed to the normal chromosome compliment in humans of 46.

Karyotype of Down’ syndrome female

Down’s syndrome features

A Down’s syndroem sufferer may have one of more of the following characteristics:
 A short and stocky physique
 A skin fold at the inner corners of the eyes – an epicanthal fold.
 A large tongue
 Speech difficulties
 Small hands and fingers
 Defects of the heart, ears or eyes
 Mental retardation
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Here are some interesting statistics on Down's syndrome.

• The chances of non-disjunction of chromosome 21 occurring in women of 30 years of

age are about 1 in 900.

• The chances of non-disjunction of chromosome 21 in women of 35 years of age

increase to 1 in 350

• At the age of 40 years the chances of non-disjunction of chromosome 21 increase to 1

in 100.

• The non-disjunction of chromosome 21 occurs in 1 out of 800 babies, which results in

the child being a Down's syndrome sufferer.

Society and culture

In most societies people with Down’s syndrome have, historically, been separated from
mainstream society, often being housed in institutions. Since the early 1960s a policy of
inclusion has been advocated by many parents and professionals, especially in the USA. In
many cases inclusion into mainstream education has been successful, but there are
challenges, both for the individual with Down’s and the educators.

“Despite these changes, the additional support needs of people with Down syndrome can still
pose a challenge to parents and families. Although living with family is preferable
to institutionalisation, people with Down syndrome often encounter patronizing attitudes
and discrimination in the wider community.”[1]
Early intervention programmes, environmental enrichment programmes, assistance to families
and special education strategies have been shown by research to result in progress that is not
seen if such advantages are not available.

It is important that society develop attitudes that will permit people with Down syndrome to
participate in community life and to be accepted. They should be offered a status that
observes their rights and privileges as citizens, and in a real sense preserves their human
dignity.
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Non-disjunction in gonosomes

If the non--disjunction involves the gonosomes, the notation will indicate the chromosome
that is extra or missing: XXY, XXX or XO.

XXY: Klinefelter syndrome

 male
 sterile
 mentally retarded

Above: Karyotype of person with

Klinefelter syndrome

Right: Physical appearance of male with Klinefelter syndrome

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XO: Turner syndrome

 female
 infertile
 at risk for health problems such as diabetes and cataracts

Above: Karyotype of person with Turner’s Syndrome

Right: Physical appearance of person with Turner’s syndrome (short stature, low hairline,
short, thick neck, short metacarpals, elbow deformity, rudimentary ovaries)

Terminology that needs to be learned in connection with chromosomal abnormalities

Cause: something that brings about an effect or result

Effect: the result that is brought about by a cause

Diagnosis: the act or process of identifying or determining the nature and cause of a disease
or injury through evaluation of patient history, examination, and review of laboratory data.

Prognosis: a prediction of the course of a disease

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Cause of Mutations:
Anything that causes a mutation is called a mutagen.

Many chemical substances are mutagens, for example:

tars in tobacco smoke
Nitrous acids which can be made in the body from the nitrates that are used to preserve food.
Some dyes such as Sudan III which is used to colour food;
thiotepa which is used in the textile industry;
nitrosamines and benzene which are used in the petroleum industry.

Ultraviolet light in sunlight is one of the most important mutagens. It can cause skin cancer.

Natural sources of radiation which include radioactive elements in rocks, soil and the . air are
mutagens.

Artificial sources of radiation such as X-rays and nuclear power stations are mutagens