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7/30/2019 Science Chapter 3 Form 4
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Chapter 3HEREDITY AND VARIATION.
3.1 Cell Division.
Chromosomes.
are the nucleus of a cell contains many small thread-like structures.
made of deoxyribonucleic acid (DNA) and protein.
contain hereditary material called genes.
the nucleus of each organisms cell contain chromosomes which always
exist in pairs (except in gametes).
example : a human has 23 pairs of chromosomes.
in each pair of chromosomes, one chromosome inherited from the father
and one from the mother.
all somatic cells of the same type of organisms have the same number of
chromosomes.
by determining the number of chromosomes in the nucleus of a cell, the
type of organism can be known.
Genes.
are hereditary material or heritable characteristics located on
chromosomes.
is a DNA unit which functions to code the heritable characteristics.
example : passes on hereditary information from one generation to
another.
exist in pairs. one gene is inherited from the father and one from the mother.
genes determine characteristics like body height, hair colour, fingerprint,
and appearance.
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Relationship between gene, chromosome, and DNA.
Type of Cell Division.
the processes of reproduction and growth of any organism involve cell
division
there are two types, which is :
mitosis
meiosis
Mitosis
is a cell division process which takes place only in somatic cells.
for plants, it occurs at the tip of the root and shoot.
through mitosis, each daughter cell contains the same number of
chromosomes, and thus the same genes, as the parent cell.
the importance of mitosis :
a) forms new cells for growthb) forms new cells to replace cells or tissues which are damaged
or have died.c) enables hereditary material in parent cell to be passed on to
daughter cells.d) enables asexual reproduction in some organisms.
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Human Cells Chromosomes
Deoxyribonucleic acid (DNA)GenesCharacteristics in human
builtof
containing containing of
formingdetermining
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Meiosis
the process of cell division to produce gametes with half of the number of
chromosomes of the parent cell.
occurs only in the reproductive organs.
occurs in the testis of a male and the ovary of female who have attained
sexual maturity. meiosis also occurs in the anther and ovary of plants.
meiosis causes variation among species of the same organism.
this is because the chromatids overlaps with one another and the
exchange of genetic material (chromosomes and genes) takes place.
this occurrence is called crossing over.
exchange of genetic material among chromatids during crossing over
produces a combination of new genes in gametes.
this process causes genetic variation.
this also explains why children that are born to the same parents show
different characteristics. the importance of meiosis :
a) ensures that the number of chromosomes is diploid in thezygote when the nucleus of sperm fuses with the nucleus of anovum during fertilisation.
b) combination of different chromosomes and crossing over whichoccur causing variation to take place in the next generation.
Comparison between meiosis and mitosis
Similarities
chromosome replication takes place
call division occurs new cells are produced
Differences
When this process happens
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Meiosis Mitosis
During theformation ofsomatic cells
During theformation of
gametes
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Where this process happensin animals
Where this process happens
in plants
Number of cell division
Replication Process
Number of daughter cellsproduced
Number of chromosomes indaughter cells comparedto those in parent cell
Genetic make-up in daughtercells compared to those of
parent cell
Crossing Over
3.2 Principles and Mechanism of Inheritance.
Dominant genes and recessive genes.
gametes involved in fertilisation contain chromosomes, chromosomes are
made up of DNA molecules, certain segments in the DNA are made up ofgenes, these genes are responsible for traits.
examples of traits passed on from parents are skin color, hair color, blood
group and height.
genes that exist in pairs in chromosomes control the characteristics
inherited from the parents. there are two types of genes :
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Somaticcells
Testes andovaries
Tip of the rootand shoot
Anther andovary
OnceTwice
OccursOccurs duringthe first celldivision only
TwoFour
SameHalf
SameDifferent
Doesnthappen
Happens
Dominant genes Recessive genes
are genes whichshow the
characteristicsthat they controlwhen paired witha dominant gene
or a recessivegene.
are genes whichonly show thecharacteristics
that they controlwhen these
genes are paired
with anotherrecessive gene.
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dominant genes are represented by the letter T.
recessive genes are represented by the letter t.
Mechanism of trait inheritance.
the studies of Gregor Mendel on the pea plant can explain trait inheritance
in organisms.
Mendel cross-bred tall pea plats which were pure breed with short pea
plants which were also pure breed.
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thecharacteristicsdetermined bythis genes are
called dominanttraits.
thecharacteristicsdetermined bythis genes are
called recessivetraits.
dominant traitsare shown if oneor both dominant
genes whichcontrol theparticular
characteristicsare resent.
recessive traitsare only shown if
the recessivegenes are not
paired withdominant genes.
human traitscontrolled by this
genes
tall, curly hair,
free ear lobe,black hair, ableto roll the tongue,
dimples, longeye lashes, right-
handed,pigments in skin,black or brown
iris, normaleyesight
human traitscontrolled by this
genes
short, straight
hair, attachedear lobe, blondhair, unable toroll the tongue,
no dimples, shorteye lashes, left-handed, albino,blue iris, colour-
blindness
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he found that all pea plants in the first generation were tall.
the tall pea plants in the first generation were then cross-bred among
themselves.
Mendel found that three quarters of the pea plants in the second
generation were tall and one quarter of them short.
in this experiment, only one trait inheritance is studied, hence, thatinheritance is called monohybrid inheritance.
genotype refers to the genetic make-up of an organism.
phenotype refers to the physical appearance which can be seen in an
organism.
example, if a tall person has a TT gene, he is said to have TT genotype
and shows tall phenotype.
organisms in the first filial generation, F1, are called hybrids.
a hybrid contains a dominant gene and recessive gene. ex : impure breed
(Tt)
combination of gametes at the first filial generation, F1, can be shown inthe Punnett Square.
Gen T t
TtttT TT Tt
t
Punnet square
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TT tt
t tTT
tall plant(pure breed)
dwarf plant(pure breed)
xParents
Meiosis
Gametes
Fertilisation
First filialgeneration
all plants in the
first filialgeneration, F1,are tall (100%)
this situation
shows that talltrait is controlledby the dominantgene while shorttrait is controlledby the recessivegene.
Tt Tt Tt Tt
T ttT
Tt Tt Tt tt
tall plant(hybrid)
tall plant(hybrid)
Tt Tttall tall tall tall
xFirst filialgeneration
Gametes
Fertilisation
Meiosis
ratio of plantsthat are tall todwarf plants are3 : 1.
probability of
obtaining tallplants in thesecondgeneration is75% while theshort plants is25%.
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3.3 Sex Determination and The Occurrence of Twins in Human Beings.
Sex Chromosomes
a human somatic cell has 23 pairs of chromosomes (46 chromosomes).
from the 23 pairs of chromosomes, 22 pairs are autosomes and one pair
is sex chromosomes.
sex chromosome determine the sex of a person.
male sex chromosomes are XY.
male somatic cell contains 22 pairs of autosomes with one X chromosome
and one Y chromosome. 44 + XY
in meiosis, a male produces two types of gamete which :
o carries 22 autosomes and one X chromosome. 22+X
o carries 22 autosomes and one Y chromosome. 22+Y
female sex chromosome is XX
female somatic cell contains 22 pairs of autosomes and two X
chromosomes. 44 + XX
in meiosis, a female gamete only carries 22 autosomes and one X
chromosome. 22+X
Sex Determination.
the sex of child is determined by the father according to the type of spermthat fertilises the ovum.
if a sperm that carries
o the x chromosomes fertilises an ovum, a baby girl is born.
o the y chromosomes fertilises an ovum, a baby boy is born.
the probability of having a male child or female child is the same> 50 : 50
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tall tall tall dwarf
Second filialgeneration
22 +X
22 +X
22 +Y
22+X
44 +XX
44 +XX
44 +XY
44 +XY
44 +XY
44 +XX
female female male male50 : 50
Parents
Gametes
Fertilisation
Offspring
Meiosis
x
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Occurrence of twins. in humans, normally one ovum fertilised by one sperm to form one zygote
which will develop to form an embryo.
sometimes more than one embryo is produced in a pregnancy.
twins - two babies born to a mother in a pregnancy and generally at the
same time.
- there are two types of twins identical twins
non-identical twins
Comparison between identical and non-identical twins.
Similarities two babies born in a pregnancy
Differences
Differences Identical Twins Non-identical twins
Formation
Formed when an ovumfertilised by a sperm dividesinto two to form two similarembryos.
Formed when two ova arereleased by the ovary at thesame time and fertilisedseparately by two sperms toform two different embryos.
PlacentaThe two embryos formed willgrow in one shared placentain the mothers uterus.
The two embryos formedwhen grow in separateplacentas in the mothersuterus
Genetic make-up
Same genetic make-up.Therefore, identical twinshave the same appearanceand the same sex.
Different genetic make-upsince they originate from twodifferent ova and sperms.Therefore, non-identicaltwins have differentappearances.
SexThe sex of identical twins arethe same.
The sex of non-identicaltwins may be the same ordifferent.
sometimes the division of the embryo is not complete, therefore Siamese
twins are formed.
Siamese twins may share certain organs like brain, stomach, or heart and
maybe joined to one another at the head, abdomen, breast, or buttocks.
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they can be separated by surgery if they do not share important organs
like brain, heart, kidneys, or lungs.
3.4 Mutation
is the spontaneous change to the structure of genes or chromosomes of
an organism.
causes change of characteristics in a child.
occurs in somatic cells or in gametes.
the effect of mutation will be inherited by one generation to another.
there are two types of mutation gene mutations
chromosome mutations
Gene Mutations
involve changes in the structure of gene.
gene mutations change or produce the new genes to replace normal
genes.
genes that have undergone mutation are called mutants.
Heredity diseasecaused by gene
mutation
Brief explanation
Albinism albinisms is caused by the change in the gene that
controls skin colour.
the newly produced genes are unable to produceskin pigment.
the skin and hair of albinos are white and their eyes
are pink.
Sickle cell anaemia sickle cell anaemia is a type of disease caused by the
change in the genes that produce haemoglobin.
haemoglobin that has undergone mutation is not
efficient in transporting oxygen.
Colour-blindness is a sex-linked disease determined by a recessive gene
on the X chromosome.
a colour-blind person cannot see or differentiate thecolours red and green.
Haemophilia caused by the deficiency if a type of protein in the blood
required for blood clotting.
haemophilia is caused by the deficiency of a type of
protein in the blood required for blood clotting.
haemophilia is a sex-linked disease determined by a
recessive gene on the X chromosome.
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a patient who has wound will experience continuous
bleeding or will take a long time for the blood to clotthus this condition way lead to death.
Chromosome Mutations
involve changes in the number of chromosomes or arrangement of genes
in chromosomes.
sometimes, a portion of a chromosome may break away and go missing,
or become attached itself to other chromosomes.
this causes the arrangement of a chromosome to change and differ from
normal.
the number of chromosomes can be increased or decreased
chromosome mutations can cause hereditary diseases like Downs
syndrome, Klinefelters syndrome, and Turners syndrome.
Downs syndromeo this heredity disease is caused by the presence of one extra
chromosome at chromosomes 21 in the somatic cells.o incomplete separation of chromosomes during meiosis causes the
zygote to have an extra chromosome : 47 in total.o characteristics of a downs syndrome patient :
i. slanting eyesii. metal retardation
iii. short fingersiv. wide distance between eyesv. small mouth but big tonguevi. protruding tongue causing the mouth to remain partially open
Klinefelters syndromeo a person who suffers from this hereditary diseases has one extra X
chromosome : XXY
Turners syndromeo a person who suffers from this hereditary lacks one X chromosome.
o people with Turner;s syndrome have only one sex chromosomewhich is XO.
Factors that cause mutation.i. mutation can occur naturally, caused by substances found in food or
environment.ii. agents causes mutations are called mutagens.
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iii. chemical substances like pesticides, nicotine in cigarettes, drugs, nitrousacid and some preservatives, colouring and artificial sweetener can causemutation.
iv. radiation radioactive radiation can cause mutation gamma ray can affect growth and cell division
ultraviolet rays and X-ray rays can cause skin cells to mutateand this may cause cancerv. temperatures which are too high or too low can also cause mutation.
Advantages and disadvantages of mutation.
not all occurrences of mutation bring adverse effects to organisms.
advantages of mutation
mutation causes variations in organisms which allow them toadapt to the environment.
species that are more resistant to disease, weather, andpolluted environment can be produced.
disadvantages of mutation some mutations cause diseases like colour-blindness and
haemophilia cannot be cured.
sometimes, disease like haemophilia and sickle cell anaemiacan cause death.
Klinefelters syndrome causes infertility.
mutation causes physical, mental, or foestus retardation.
3.5 Effects of Genetic Research on Human Life.1. Research in genetics has contributed greatly to medicine and agriculture.
2. In medicine, genetic research has helped to identify various hereditarydiseases and ways to prevent those diseases.
3. In agriculture, genetic research aims at :
improving the quality of breeds through selective breeding
bringing in new species with higher resistance to diseases or pests
obtaining the production of large quantities of crops and livestock in the
shortest time.
Medicine.
most of the hereditary diseases like colour-blindness, haemophilia and
albinism are caused by recessive genes.
genetic research can tell us how albinism caused by recessive genes an
autosomes is inherited.
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several types of recessive genes that cause hereditary diseases are only
found in the X chromosome. (such genes are called sex-linked genes)
such genes are normally passed down by females who are the carrier to
their children.
colour-blindness and haemophilia are example of hereditary diseases
caused by sex-linked genes in humans. a female can only suffer from colour-blindness if she has both recessive
genes of this characteristics.
a male will suffer from this diseases if he has one recessive gene.
these are genes which are present in the X chromosomes but not in the Y
chromosomes.
inheritance of albinism in humans
inheritance of colour-blindness in humans
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A aaA
AA Aa Aa aa
Aa AaParents
Gametes
Fertilisation
Genotype ofOffspring
Meiosis
Characteristics ofOffspring
normal normal but normal but albinois a carrier is a carrier
X XbYX
XX XXb XY XbY
XY XXb
normal normal girl normal boy colourgirl but is a carrier but is a carrier -blind boy
Parents
Fertilisation
Genotype ofOffspring
Meiosis
Characteristics ofOffspring
Gametes
x
x
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hereditary diseases like Downs syndrome can be identified on the foetus
in the mothers womb.
foetal cells in the mothers amniotic fluid are examined under a
microscope.
if 47 chromosomes are observed in the foetal nucleus, the foetus is
proved having Downs syndrome. today, genetic engineering enables the transfer of DNA or genes from one
organism to another.
Agriculture.
selective breeding is done on crops and livestock.
plants or livestock are cross-bred from different varieties to produce new
varieties which have the desired characteristics.
Type of crop Parent plant Hybrid Characteristics of hybrid
Paddy PETA from Indonesiaand DGWG fromChina
IR-8 Bear more fruits, ripenfaster and are moreresistant to dry season
Oil palm Pisifera with dura Tenera Bear more fruits and havethinner shell with morecontent
Maize Sweetcorn fromTaiwan and Mexico
Masmadu Fruits are sweeter, bigger,and more resistant todisease and dry season
Papaya Subang 6 andSunrise Solo fromHawaii
Exotica Fruits are sweeter andhave more content
examples of selective breeding in crops
Type of livestock Parents Characteristics of new breed
Cow Fresian and local New breed produces moremilk and are more adaptable tothe change in the environment
Cow Female cow and male wildox
New breed (selembu) thatgrows faster and produces
better qualityGoat Jumnapari goat from Indiaand local goat
New breed that can producemore meat and milk
Sheep Sheep from Australia andlocal Malin
New breed (Malin-X) is biggerin size, matures faster,supplies better quality meatand wool
Chicken Imported and local New breed that grows and
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kampungchicken matures faster
examples of breeding in livestock
techniques of artificial insemination and embryo transfer are used in cross-
breeding
artificial insemination good quality semen from the male animal is placedinto the body of the female animal for fertilisation
embryo transfer an embryo of a good breed is placed into the uterus of a
female animal
Genetic engineering.
3.6 Variation Among Living Things.
variation is the differences in the characteristics which exists among
individuals of the same species
there are two types of variation : continuous variation
discontinuous variation
Continuous Variation.
refers to characteristics which do not show obvious variation in a trait
among individuals of the same species.
has continuous intermediate range between two extremes.
examples of continuous variation are :
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Genetic Modification
Advantages Disadvantages
Body health
food containing certain nutrients required
by the body can be produced.
Better quality crops and livestock
better quality of crops and livestock can
be produced.
quantity can be increased. crops and livestock that grow faster can
be produced.
Conservation of environment
the use of pesticide can be reduced with
the production of crops which have high
Nutritional value of food
generically modified food may not have
the same nutritional value.
Effect of new genes
new genes in food may be transferred to
the human body.
such genes may affect the functions ofhuman cells.
Long-term effect
new species of crops and livestock which
adapt better to the change in theenvironment might cause authentic
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i. heightii. body weightiii. intelligenceiv. skin colourv. width of shoulder
vi. length of sole continuous variation can be shown by a histogram
Discontinuous Variation.
discontinuous variation refers to characteristics which shows obvious and
definite variation in a trait among individuals if the same species examples of discontinuous variations are :
i. blood groupii. fingerprintiii. ability to roll the tonguesiv. ear lobev. left-handednessvi. presence of dimples
discontinuous variation can be shown by a bar chart
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number of
students
body weight
number ofstudents
blood groupsA B AB O
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Factors that cause variation.
there are 2 factors which cause variation :
genetic factor
environmental factor
Importance of variation.1. Variations which exist among the same species are important for the
formation of new species. Besides this, variations enable us to distinguishindividuals in the same species.
2. Variations enable newly produced species to adapt themselves better toany changes in the environment.
3. In plants, features of variation like resistance to pathogens, enable theplants to grow rapidly and reproduce quickly.
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