science chapter 3 form 4

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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science chapter 3 form 4

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