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Gene and Chromosome
A gene is the basic physical and functional unit of heredity .Genes ,
which are made up of DNA , that provide the coded instructions for
synthesis of RNA, which, when translated into protein, leads to the
expression of hereditary character. In humans, genes vary in size from a
few hundred DNA bases to more than 2 million bases. The Human
Genome Project has estimated that humans have between 20,000 and
25,000 genes.
Different forms of genes, called alleles, determine how these traits are
expressed in a given individual. Humans are thought to have about 3
5,000 genes, while bacteria have between 500 and 6,000. Humans have
23 pairs of chromosomes - or a total of 46
anything that lives depends on genes. They possess the data to build
and maintain cells and pass genetic information to offspring.
 Function of gene
Your genes decide virtually everything about you. Here is a list of
some of the basic features and characteristics than your genes are
programmed to do.

whether you are tall or not

the color of your hair

the color of your skin

whether you are more likely to develop certain diseases

whether you are good at sports

how you respond to environmental triggers

what you look like inside and out
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Mr. Ali Hasanain Alhamdany
Msc. physiology
• Gene component
A gene consists of a long combination of four different nucleotide bases
(chemicals). There are many possible combinations. The four
nucleotides are:
A(adenine)
C(cytosine)
G(guanine)
T(thymine)
Different combinations of the letters ACGT give people different
characteristics. For example, a person with the following combination
- AAACCGGTTTTT - may have green eyes, while somebody whose
combination is - AAACCGGTTTAA - may have blue eyes.
• DNA
DNA, or deoxyribonucleic acid, is the hereditary material in humans
and almost all other organisms. Nearly every cell in a person‟s body has
the same DNA. Most D NA is located in the cell nucleus (where it is
called nuclear DNA),
.
Nucleus
Chromosome
s
Cell
but a small amount of DNA can also be found in the mitochondria
(where it is called mitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical
bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human
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Mr. Ali Hasanain Alhamdany
Msc. physiology
DNA consists of about 3 billion bases, and more than 99 percent of
those bases are the same in all people.
DNA bases pair up with each other, A with T and C with G, to form
units called base pairs. Each base is also attached to a sugar molecule
and a phosphate molecule. Together, a base, sugar, and phosphate are
called a nucleotide. Nucleotides are arranged in two long strands that
form a spiral called a double helix
• Chromosomes
Chromosomes comes from the Greek Chroma, which means color,
and Soma, which means body (chromosomes are stained very strongly
by some dyes) Humans have 23 pairs of chromosomes.
A chromosome consists of DNA and has proteins attached to it called
histones that support its structure. These chromosomes are located in
your body's cells, which then contain this important genetic information
held in the long strands of DNA.
Each chromosome has a constriction point called the centromere, which
divides the chromosome into two sections, or “arms.” The short arm of
the chromosome is labeled the “p arm.” The long arm of the
chromosome is labeled the “q arm.” The location of the centromere on
each chromosome gives the chromosome its characteristic shape, and
can be used to help describe the location of specific genes
DNA and
histone
proteins are
packaged into
structures
called
chromosomes
.
Each of your cells contains two sets of chromosomes - one set came
from your mother while the other came from your father - male sperm
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Mr. Ali Hasanain Alhamdany
Msc. physiology
and the female egg carry a single set of 23 chromosomes each - 22
autosomes and an X or Y sex chromosome.
If you are female you inherited an X chromosome from each parent - if
you are male you inherited an X chromosome from your mother and a
Y from your father
• Alteration of gene
the alteration (mutation) means that the information contained in the
particular gene is either changed or absent. The four main ways of
inheriting an altered gene are:




Autosomal dominant – the alteration is present in every generation
and may cause the condition in every person who has the alteration.
This is because the altered copy of the gene is dominant over the
healthy copy. Examples include Huntington‟s disease and familial
hypercholesterolaemia (genetically linked high cholesterol levels).
Autosomal recessive – the affected person has two copies of the
altered gene (they have inherited an altered copy of the gene from both
parents). They develop the disorder because they do not have a
functioning copy of the gene. Examples of autosomal recessive genetic
disorders include cystic fibrosis, phenylketonuria (PKU) and sickle cell
anaemia .
X-linked dominant – this type of disorder generally occurs in females.
The „X‟ refers to one of the sex chromosomes that decide gender. The
mother always provides an X, while the father provides either X
(female child) or Y (male child). Women with an X-linked dominant
disorder have one altered copy and one normal copy of a gene that is on
the X chromosome. An example of an X-linked dominant genetic
disorder is a rare form of rickets known as hypophosphataemic or
vitamin D resistant rickets.
X-linked recessive – this type of disorder is more common in males. It
is caused by an alteration in a gene on the X chromosome. Since a male
has one X and one Y (XY), he does not have a second „healthy‟ copy of
the gene. Examples of X-linked recessive genetic disorders include
Duchenne muscular dystrophy and haemophilia.
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Mr. Ali Hasanain Alhamdany
Msc. physiology
• Genetic Disorder
A genetic disorder is caused by an altered or faulty gene or set of genes.
The four broad groups of genetic disorders are single gene disorders,
chromosome abnormalities, mitochondrial disorders and multifactorial
disorders.
• Single gene disorder
single gene disorder is the result of a single mutated gene. Over 4000
human diseases are caused by single gene defects. Single gene disorders
can be passed on to subsequent generations in several ways for example
sickle cell disease, Fragile X syndrome, muscular dystrophy, or
Huntington disease.

Chromosome abnormalities
A chromosome disorder means there is a change in either the structure
or the number of chromosomes. This can happen in three main ways:
• The altered chromosome is passed from the parent to the child
• The abnormality happens when either the sperm or egg (germ
cells) is created
• Soon after conception.
Chromosomal aberrations are disruptions in the normal chromosomal
content of a cell and are a major cause of genetic conditions in humans,
such as Down syndrome, although most aberrations have little to no
effect. Genetic counseling is offered for families that may carry a
chromosome rearrangement.
The gain or loss of DNA from chromosomes can lead to a variety
of genetic disorders. Human examples include:
• Down syndrome, the most common trisomy, usually caused by an
extra copy of chromosome 21 (trisomy 21). Characteristics
include decreased muscle tone, stockier build, asymmetrical skull,
slanting eyes and mild to moderate developmental disability.
• Edwards syndrome, or trisomy-18, the second most common
trisomy.[ Symptoms include motor retardation, developmental
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Mr. Ali Hasanain Alhamdany
Msc. physiology
disability and numerous congenital anomalies causing serious
health problems. Ninety percent of those affected die in infancy.
They have characteristic clenched hands and overlapping fingers.
• Diagnosis
Due to the wide range of genetic disorders that are presently known,
diagnosis of a genetic disorder is widely varied and dependent of the
disorder. Most genetic disorders are diagnosed at birth or during early
childhood, however some, such as Huntington's disease, can escape
detection until the patient is well into adulthood.
The basic aspects of a genetic disorder rests on the inheritance of
genetic material. With an in depth family history, it is possible to
anticipate possible disorders in children which direct medical
professionals to specific tests depending on the disorder and allow
parents the chance to prepare for potential lifestyle changes,
anticipate
the
possibility
of
still
birth,
or
contemplate termination. Prenatal diagnosis can detect the presence of
characteristic abnormalities in fetal development through ultrasound, or
detect the presence of characteristic substances via invasive
procedures which involve inserting probes or needles into the uterus
such as in amniocentesis.
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Mr. Ali Hasanain Alhamdany
Msc. physiology