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BIOLOGY FOR CLASS IX
Chapter #16
Class IX
Genes And Inheritance
Content
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Inheritance By Genes
Crossing Over And Its Significance
Significance Of Crossing Over
Brief Structure Of Dna
Significance Of Base Sequence
Replication Of Dna
Common Terms Used In Genetics
Mendel And His Laws Of Inheritance
Mendel’s Law Of Segregation
Law Of Independent Assortment
Determination Of Sex In Man By X And Y Chromosomes
Sex Linked Inheritance
Other Hereditary Diseases In Human Beings
GENE
Gene is a unit of heredity which is transferred from a parent to offspring
and is held to determine some characteristic of the offspring.
Some of these traits may be physical for example hair and eye color and
skin color etc. On the other hand some genes may also carry the risk of
certain diseases and disorders that may pass on from parents to their
offspring
What is genetic variation?
Individuals in a population are not exactly the same.
 Each individual has its unique set of traits, such as size, color,
height, body weight, skin colour and even the ability to find
food.
 Sometimes, offspring’s of the same parents still differ a lot
among themselves. You can find that among 3 sisters, one may
be very tall, the other may have dark hair and the third may
have a rounded nose tip. Such differences in individuals from the
same parents are called variation.
 Characteristics or traits that are inherited are determined by
genetic information. Some other traits like dialect or accent,
scars, skin texture or even body weight may be determined by
some external
or environmental factors.
How genetic conditions are inherited?
 Each cell in the body contains 23 pairs of chromosomes. One
chromosome from each pair is inherited from your mother and
one is inherited from your father.
 The chromosomes contain the genes you inherit from your
parents. There may be different forms of the same gene – called
alleles.
 For example, for the gene that determines eye colour, you may
inherit a brown allele from your mother and a blue allele from
your father. In this instance, you will end up with brown eyes
because brown is the dominant allele. The different forms of
genes are caused by mutations (changes) in the DNA code.
Crossing Over
 Crossing over, or recombination, is the exchange of chromosome
segments between nonsister chromatids in meiosis. Crossing
over creates new combinations of genes in the gametes that are
not found in either parent, contributing to genetic diversity.
Significance of Crossing Over
a.It produces new individuals having new combinations of traits.
b. Crossing over has helped in establishing the concept of linear
arrangement of genes.
c. The frequency of crossing over helps in the mapping of
chromosomes. i.e., determining the location of the genes in the
chromosomes.
d. Selection of useful recombination by geneticists has brought
about green revolution in our country.
The Structure of DNA
 DNA is made up of six smaller molecules
-- a five carbon sugar called deoxyribose,
a phosphate molecule and four different
nitrogenous bases (adenine, thymine,
cytosine and guanine). Using research
from many sources, including chemically
accurate models, Watson and Crick
discovered how these six subunits were
arranged to make the the structure of
DNA..
 The model is called a double helix because two long strands
twist around each other like a twisted ladder. The rails of the
ladder are made of alternating sugar and phosphate molecules.
The steps of the ladder are made of two bases joined together
with either two or three weak hydrogen bonds
Nucleotides
The basic building block of DNA is called a NUCLEOTIDE. A
nucleotide is made up of one sugar molecule, one phosphate
molecule and one of the four bases. Here is the structural
formula for the four nucleotides of DNA. Note that the purine
bases (adenine and guanine) have a double ring structure
while the pyrimidine bases (thymine and cytosine) have only a
single ring. This was important to Watson and Crick because
it helped them figure out how the double helix was formed.
Watson Crick Model of DNA Structure
In 1953 Watson and Crick surprised the scientific world with a
concise one page paper in the British Journal Nature. The paper
reported their molecular model of DNA, the double helix,
which has since become the symbol of molecular biology. The
beauty of the model was that its structure suggested the basic
mechanism of DNA replication. Watson and Crick suggested
ladder type organization of DNA. Each molecule of DNA is
made up of two poly nucleotide chains which are twisted
around each other and form a double helix. The uprights of the
ladder are made up of sugar and phosphate part of nucleotide
and the rungs are made up of paired nitrogenous bases. The
pairs are always as follows.
 Adenine always pars with thymine and cytosine with guanine.
There is no other alternate possible two polynucleotide chains
which are complimentary to each other, are held together by
hydrogen bonds. There are two hydrogen bonds between A = T,
and three between C = G. Both polynucleotides strands remain
separated by 2OA´´ distance. The coiling of double helix is right
handed and complete turn occurs after 34A´´.
SIGNIFICANCE OF BASE SEQUENCE
 Watson Crick model explained chargaff’s rules. Wherever one
strand of DNA molecule has an A, the partner strand has a T and
G is one strand is always paired with a C in the complimentary
strand. Therefore in DNA of any organism, the amount of
academic equals the amount of thymine and the amount of
guanine equals the amount of cytosine. Although the basepairing rules dictate the combinations of nitrogenous bases that
form the rungs of the double helix; they do not restrict the
sequence of nucleotide along each DNA strand. Thus the linear
sequence of four bases can vary in countless ways and each
gene has unique order, or base sequence.
Watson Crick model suggested that the basis or occupying
the genetic information is complimentary one chain of
DNA molecule may have any conceivable base sequence but
this sequence completely determines that of its partner in
the duplex. If the sequence of one chain is ATTGCAT, the
sequence of its partner in duplex must be TAAGGTA. Each
chain in duplex is a complimentary mirror image of the
other. To copy the DNA molecule one need only unzip it
construct new complimentary chain along each naked
strand.
DNA REPLICATION
DNA replication is the process by which DNA makes a
copy of itself during cell division.
 The first step in DNA replication is to ‘unzip’ the double
helix structure of the DNA? molecule.
 This is carried out by an enzyme? called helicase which
breaks
the
hydrogen
bonds?
holding
the complementary? bases? of DNA together (A with T, C
with G).
 The separation of the two single strands of DNA creates a ‘Y’ shape
called a replication ‘fork’. The two separated strands will act as
templates for making the new strands of DNA.
 One of the strands is oriented in the 3’ to 5’ direction (towards the
replication fork), this is the leading strand?. The other strand is
oriented in the 5’ to 3’ direction (away from the replication fork), this
is the lagging strand?. As a result of their different orientations, the
two strands are replicated differently
Basic term of genetics
ALLELES An alternative form of a gene that occurs at the same
locus on homologous chromosomes, e.g., A, B, and O genes are
alleles.
ChromosomeRod-shaped structures within the cell nucleus that
carry genes encoded by DNA.
Co-dominantGenes are co-dominant if both alleles are expressed
in the heterozygous state, e.g., K and k genes
GameteA reproductive sex cell (ovum or sperm) with the haploid
number (23) of chromosomes that results from meiosis.
GeneA segment of a DNA molecule that codes for the synthesis of
a single polypeptide.
 Genotype All of the alleles present at the locus (or closely linked
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loci) of a blood group system, indicating chromosomal alignment
if appropriate, e.g., AO in the ABO BGS, CDe/cde in the Rh BGS,
or MS/Ns in the MNSs BGS. Genotypes are indicated by
superscripts, underlining, or italics.
Haploid number of chromosomes the number of chromosomes
found in sex cells, which in humans is.
HeterozygousThe situation in which allelic genes are different,
e.g. the Kk genotype in the Kell BGS or the Fya Fyb genotype in
the Duffy BGS.
Homologous chromosomes A matched pair of chromosomes,
one from each parent, e.g., two #6 chromosomes.
HomozygousThe situation in which allelic genes are identical,
e.g., the KK genotype or the Fya Fya genotype.
 LocusThe location of allelic genes on the chromosome, e.g., A, B,
and O genes occur at the ABO locus. (Plural.
 RecessiveGenes are recessive if the phenotype that they code
for is only expressed when the genes are homozygous, e.g., le
le genes, in the Lewis system or h h genes in the ABO BGS. = loci)
The Mendelian Concept of a Gene
 In the 1860’s, an Austrian monk
named Gregor Mendel introduced a
new theory of inheritance based on
his experimental work with pea
plants.
 Mendel instead believed that
heredity is the result of discrete units
of inheritance, and every single unit
(or gene) was independent in its
actions in an individual’s genome.
 For any given trait, an individual
inherits one gene from each parent
so that the individual has a pairing of
two genes.
Mendel observed seven pea plant
traits that are easily recognized in one
of two forms:
1.
2.
3.
4.
5.
6.
7.
Flower color: purple or white
Flower position: axial or terminal
Stem length: long or short
Seed shape: round or wrinkled
Seed color: yellow or green
Pod shape: inflated or constricted
Pod color: green or yellow
Mendel's Laws are as follows
1. The Law of Dominance
2.
The Law of Segregation
3. The Law of Independent Assortment
The Law of Dominance
In a cross of parents that are pure for contrasting traits, only
one form of the trait will appear in the next generation. Offspring
that are hybrid for a trait will have only the dominant trait in the
phenotype.
Law of Segregation
 The pair of alleles of each parent separate and only
 one allele passes from each parent on to an
offspring
 which allele in a parent's pair of alleles is inherited
is a matter of chance
 Segregation of alleles occurs during the process of
gamete formation (meiosis)
 randomly unite at fertilization
Law of independent assortment
 law of independent assortment. It states that the alleles of one
gene sort into the gametes independently of the alleles of
another gene.
 Since we're talking about a cross with double heterozygotes,
what we're monitoring here in the F1 generation is called
a dihybrid cross. That is a cross between individuals that
areheterozygous at two different loci. Adrian expects that his F1
cross will produce a 3:1 ratio between the dominant
and recessive traits. However, he's perplexed to observe that
instead he sees a 9:3:3:1 ratio among four different phenotypes.
Chromosomes X and Y and Sex Determination
 In a human, the normal chromosomes complement is 46, 44
of which are autosomes while 2 distinct chromosomes are
deemed sex chromosomes, which determine the sex of an
organism and various sex linked characteristics.
 In most animals, those who possess XX chromosomes are
female while male animals possess an X and a Y
chromosome. However, this is not true of all organisms, as it
can be reversed in some species.
Sex Chromosomes
 A humans' sex is predetermined in the sperm gamete.
 The egg gamete mother cell is said to be homogametic,
because all its cell possess the XX sex
chromosomes. sperm gametes are
deemed heterogametic because around half of them contain the
X chromosome and others possess the Y chromosome to
compliment the first X chromosome.
 In light of this, there are two possibilities that can occur during
fertilisation between male and female gametes, XX and XY.
Since sperm are the variable factor (i.e. which sperm fertilises
the egg) they are responsible for determining sex.
Chromosomes X and Y
 Chromosomes X and Y do not truly make up a homologous
pair. They act similarly in their roles, but they are not
homologous (the same). The X chromosome in humans is
much longer than the Y chromosome and also contains
many more genes.
 These genes are said to be sex linked, due to the fact they
are present in one of the sex chromosomes. During
fertilisation, when the opposing homologous chromosomes
come together, the smaller Y chromosome offers no
dominance against the 'extra' X chromosomes as indicated
below.
 Chromosomes X and Y do not truly make up a homologous
pair. They act similarly in their roles, but they are not
homologous (the same). The X chromosome in humans is
much longer than the Y chromosome and also contains
many more genes.
 These genes are said to be sex linked, due to the fact they
are present in one of the sex chromosomes. During
fertilisation, when the opposing homologous chromosomes
come together, the smaller Y chromosome offers no
dominance against the 'extra' X chromosomes .
 The arrows indicate sex linked genes in the X chromosome.
In this homologous pairing, all those genes are dominant,
because there are no opposing genes in the Y chromosome
to offer dominance.
 So when the organism has an XY chromosome compliment
(i.e. a male), these sex linked genes are freely expressed in
the organisms phenotype, an example being hairy ears
developing in old age.
Sex Linked Characteristics
 These sex linked genes on the X chromosome display a
number of characteristics. The following are just some
examples of phenotypes as a result of these genes in
expression;
 Red-Green colour blindness
 Haemophilia - A condition which prevents the clotting of
the blood
 Hairy ears in men through advancing age
 More information on sex linked characteristics and
how they are passed on from generation to
generation will be available in new areas of the site
soon.
 The next page looks at genetic mutations and the
consequences as a result of them.
X-linked recessive condition
Red-green color blindness
Red-green color blindness simply means that a person cannot distinguish
shades of red and green (usually blue-green). Their visual acuity (ability
to see) is normal. There are no serious complications; however, affected
individuals may not be considered for certain occupations involving
transportation or the Armed Forces where color recognition is required.
Males are affected 16 times more often than females, because the gene is
located on the X chromosome.
Hemophilia A
Hemophilia A is a disorder where the blood cannot clot properly due to
a deficiency of a clotting factor called Factor VIII. This results in
abnormally heavy bleeding that will not stop, even from a small cut.
People with hemophilia A bruise easily and can have internal bleeding
into their joints and muscles. Hemophilia A is seen in one in 10,000 live
male births. Treatment is available by infusion of Factor VIII (blood
transfusion). Female carriers of the gene may show some mild signs of
Factor VIII deficiency such as bruising easily or taking longer than usual
to stop bleeding when cut. However, not all female carriers present
these symptoms. One third of all cases are thought to be new mutations
in the family (not inherited from the mother).
Sickle-cell Anemia
 Sickle-cell anemia, also called sickle-cell disease, is a hereditary dis
order in which abnormal hemoglobin * within the red blood cells
(RBCs) causes the cells to take on abnormal sickle (crescent) shapes.
This decreases the ability of the hemoglobin to transport oxygen
throughout the body. The sickled cells tend to bunch up and clog the
blood vessels, and they tend to break apart more easily than normal
RBCs. This may cause inflammation, pain, tissue damage, and anemia
Diabetes
 Diabetes is a chronic condition associated with abnormally high levels
of sugar (glucose) in the blood.
 Insulin produced by the pancreas lowers blood glucose.
Genetic Engineering
Genetic engineering is the process of manually adding new DNA to an
organism. The goal is to add one or more new traits that are not already
found in that organism. Examples of genetically engineered (transgenic)
organisms currently on the market include plants with resistance to
some insects, plants that can tolerate herbicides, and crops with
modified oil content.
Genetic Engineering
Genetic engineering is the process by which scientists modify the
genome of an organism. Creation of genetically modified organisms
requires recombinant DNA. Recombinant DNA is a combination of DNA
from different organisms or different locations in a given genome that
would not normally be found in nature.
In most cases, use of recombinant DNA means that you have added an
extra gene to an organism to alter a trait or add a new trait. Some uses
of genetic engineering include improving the nutritional quality of food,
creating pest-resistant crops, and creating infection.
Genetic Engineering
Uses of Transgenic plants: In order to improve the quality and quantity
of plants, traditional method of plant breeding is replaced by the
creation of transgenic plants. The transgenic plants are plants carrying
foreign genes introduced deliberately into them to develop a new
character useful for the plant. The infection of plants by microorganism
mostly viruses, poor production and decline in quality of plants due to
attack by insects and the plants inability to withstand the pesticide or
the weedicide used in the agriculture process welcomed the genetic
engineering technology to develop transgenic plants with new
characters like resistance to infections, defensive against the attacking
insects and resistance to pesticides or weedicide(wild plant).
Genetic Engineering
Uses Transgenic animals: Transgenic animals are animals carrying
foreign genes deliberately introduced into them and exhibiting the
characteristics of the introduced gene. Animals are suitable for various
research activities trying to help mankind. In that way transgenic
animals are created to study human diseases to derive appropriate
treatment methods and to develop and identify the drug useful to treat
the disease. The presence of human proteins in milk of animals is made
possible by genetic engineering. Gene transfer is done in animals to
increase the milk production and to increase the growth.
Genetic Engineering
The recombinant proteins produced in the industry using the techniques
of genetic engineering acts as drugs for various human diseases. To
name a few, insulin produced for diabetes, alpha 1- antitrypsin in
treating emphysema, calcitonin(a hormone secreted by the thyroid that
has the effect of lowering blood calcium). to treat rickets(a disease of
children caused by vitamin D deficiency).