Form 4 Science Chapter 3: Heredity and Variation 3.1 Cell division
All living organisms grow and reproduce & are made up of cells
Cells reproduce by dividing and passing on their genes to daughter cells
Each cell has its own nucleus which controls the cell’s activities through the genetic material, DNA which acts as a set of instruction or code for life
What is genes?
Genes are sections of DNA that code for the production of protein and are arranged along the chromosomes
Carry the inherited characteristics of a person
Genes occurs in pairs, therefore each inherited characteristics is carried by two genes
ne gene from each pair comes from the father and the mother
E!ample of inherited characteristics carried by genes are"
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Colour of the eyes
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#attern of $nger prints
%ype of hair - %ype
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lood group
-
'ntelligence
Deoyri!on"cleic acids #D$%&
's a double heli!, made up of a series of genes to encode genetic information %his determine determine the individual individual characteristics characteristics of an an organism organism
All the information in DNA is transmitted in the form of a genetic code
(hould any information not be followed, the baby will be born with physical or mental defects
DNA molecule is very complicated and resembles a twisted ladder or staircase
Chromosomes
's a thread)li*e structure found in the nucleus of a cell
Chromosome and the genes on them are composed of DNA
't e!ist in pair and are identical
homologous
A pair of chromosomes has genes arranged in the same way+ As a result the genes also e!ist in pairs
uman has two sets of chromosomes in the body-one set from the father and another set from the mother.
'itosis
ccur in somatic cells
's the type of cell division that produces genetically identical cells
During mitosis, DNA replicates in the parent cell, which divides to produce two new cells, each containing an e!act copy of the DNA as in the parent cell
all cells body e!cept reproductive cells
%his allows growth of multicellular organism from unicellular /ygotes
0eplacements of cells and repair of tissues is possible through mitosis followed by cell division
Cells are constantly dying and being replaced by identical cells
's the basis of ase!ual reproduction species by one parent organism
the production of new individuals of a
1itosis is the type of cell division that produces genetically identical cells
During mitosis, DNA replicates in the parent cell, which divides to produce two new cells, each containing an e!act copy of the DNA as in the parent cell
Cells produced are diploid -2n.
0eplacement of cells and repair of tissues is possible through mitosis -E!" cell replacement is rapid in the s*in & in the lining of the gut
's actively carried out at the ends of plant shoots and roots
Step 1
Chromosomes thic*en and shorten, and become clearly visible
Each chromosome duplicates itself and forms two chromatids attached at the centromere
%he nuclear membrane disappears
Step (
A $brous spindle is formed between the chromosomes
Each chromosome is attached to a spindle by its centromere
%he chromosome arrange themselves at the e3uator of the spindle
Step 3
%he centromere of each chromosome splits into two
%he chromatids separate and move to opposite poles+
Step 4
A nuclear membrane is formed around the chromosomes at each pole %he cell begins to divide and form two daughter cells
Each daughter cell has the same number of chromosomes as the parent cell
'n a plant cell, the cytoplasm does not constrict+ 'nstead a new cell wall is developed across the cytoplasm
)mportance o* mitosis
'mportant for the following reasons" -a. Growth -b. 0eplacing old or damaged cells
Enables organisms to reproduce themselves -E!" by ase!ual reproduction4 binary $ssion in amoeba.
Ensures that the new cells produced have the same number of chromosomes as the parent cells
'eiosis
1eiosis is the process by which a nucleus divides by two divisions into four nuclei, each containing half the number of chromosome of the mother cell
0esulting nuclei are haploid -n.
%he cells produced are genetically di5erent to the parent cell and to each other
ccurs in 2 stages that is meiosis 6 and meiosis 2
#roduces genetic variation -caused by crossing over between homologous chromosome.
'n animals, meiosis occurs in testes and ovaries, whereas in plants, it occurs in the anthers and ovaries
)mportance o* meiosis 6+ #roduction of gamete 7 the only process that produces gametes 2+ 0eduction in chromosome number 7 ensures that each gamete has only half the number of chromosomes of the parent cell 8+ 1aintenance of chromosome number in somatic cells 7 the somatic cells have their number of chromosomes maintained 9+ #roduction of variation 7 causes genetic variation in the descendants-can produce individuals who do not resemble their parents closely.
+ :ariation occurs in %; ways"
6+ Di5erent combination of chromosomes 2+ Crossing over
Comparing and contrasting mitosis ,ith meiosis
(imilarities of 1'%('( and 1E'('("
) oth are processes of cell division ) 'n each process, duplication of chromosomes occurs once
D)FF--$C-S /F ')0/S)S %$D '-)/S)S ')0/S)S %o produce new cells for growth and repair of damaged tissues
D)FF--$C-S %im o* process
'-)/S)S %o produce gametes -reproductive cells.
0ype o* cell involved
0eproductive cells which produce gametes
rod"ct
Nucleus of parent cells divides once
Division o* n"cle"s
Nucleus of parent cell divides twice
Duplication of chromosomes ta*es place
D"plication o* chromosomes
Duplication of chromosomes ta*es place in the $rst cell division
(ame as in parent cell,9=
$"m!er o* chromosomes in da"ghter cells
alf the number of those in the parent cell,28
Does not occur
Crossing over
ccurs when the chromosomes are at the e3uatorial plane
(ame as in parent cell
Com!ination o* chromosomes
Di5erent from that of the parent cell
Does not cause genetic variation
2enetic variation
Causes genetic variation
(omatic cells -non reproductive cells. %wo daughter cells
3.( 0he rinciples and 'echanism o* )nheritance Dominant 2enes and ecessive 2enes
Dominant Genes 7 A gene which is powerful and can mas* the e5ect of a less powerful or recessive gene in a pair 0ecessive Genes 7 A wea* gene and cannot show its e5ect if it is paired with a dominant gene
Each pair of genes is represented by two letters, a capital letter used for a dominant gene and a small letter for a recessive gene
#henotype is to describe the physical traits and the physiological traits of a person
Genotype is to describe the ma*e)up or constitution of a person+ ;hich includes the genes from both parents either dominant or recessive+
Dominant %raits 0ight anded %all lac*?Curly air lac*?rown eyes ;ith Ear lobes Can 0oll %ongue as Normal :ision as Dimples #igmented (*in >ong Eye >ashes
0ecessive %raits >eft anded (hort rown?(traight hair lue Eyes ;ithout Ear lobes Cannot 0oll %ongue Color lind as No Dimples Non #igmented (*in -Albino. (hort Eye >ashes
'onohy!rid inheritance
1endel chose true)breeding varieties of the garden pea as the parents
1endel crossed a tall plant with a dwarf plant+
#rogeny from this cross, called the <6 generation -$rst $lial generation. were all tall
All the <6 plants were then self)pollinated to obtain the <2 generation -second $lial generation.
%he <2 generation included both tall and dwarf plants in the ratio of 8 tall to 6 dwarf-phenotype ratio.
't is shown that only the dominant-strong. trait appeared in the <6 generation+ %he recessive -wea*. trait is bloc*ed by the strong trait from e!pressing itself and is therefore hidden in the <6 generation
0he mechanism o* trait inheritance
%he character of height is determined by two contrasting traits, tall and dwarf+
%he gene for tall is dominant while the gene for dwarf is recessive
'f the gene for tall is represented by @% and the gene for dwarf is represented by @t, there will be three genetic combinations-genotype. for plant height
2enotype is the term used to describe the genetic ma*e)up or constitution of a person+ 't includes the genes inherited from both parents,whether they are dominant or recessive
henotype is the physical traits such as colour of the eyes, height and ect++
8 possible combination are"
henotype
2enotype
%all -pure strain.
%% -homo/ygous dominant.
%all -monohybrid.
%t -hetero/ygous.
Dwarf -pure strain.
tt -homo/ygous recessive.
3.3 Se Determination and the occ"rrence o* t,ins in h"man !eings Se Chromosome •
%"tosomes are chromosomes which are not se! chromosomes
•
%he characteristic features of a person are determined by the autosomes
•
%he se! of a person is determined by the se! chromosome
•
•
•
'n a woman, the se! chromosomes are a pair of identical chromosomes called B chromosomes 'n a man, the se! chromosomes consists of one B chromosome and a smaller chromosome %herefore, chromosomes in female can be written as 99BB while male as 99B
'echanism o* Se Determination
'f the baby inherits chromosome from its father
'f the baby inherits B chromosome from its father
boy
girl
%(, the se! of the baby depends on which sperm fertili/es the egg cell-ovum.+
)dentical and nonidentical t,ins #1& )dentical t,ins
;hen an ovum is fertili/ed by a sperm, a /ygote is formed %he /ygote then divides into 2 separate cells in the womb producing 2 embryos identical twins
)dentical t,ins are two normal, genetically identical individuals of the same se! and share a common placenta during developmental stage
#(& $on)dentical t,ins
(ome women may produce two ovum at the same time+
;hen the two di5erent ovum are fertili/ed at the same time by two di5erent sperms, two /ygotes are formed non)identical twins-fraternal twins.
Non)identical twins are genetically di5erent individuals, can may be of the same se! or di5erent se! and each have their own placenta during developmental stage
Comparing and contrasting identical t,ins ,ith nonidentical t,ins #1& S)')%)0)-S:
%wo foetuses develop in the uterus at the same time
%wo babies are born at the same time
#(& Di5erences )dentical t,ins
Di5erences
$on)dentical t,ins
ne ovum and one sperm
$o o* gametes involved
%wo ovum and two sperms
ne ovum is fertili/ed and one /ygote is formed
Fertili6ation
%wo ovum are fertili/ed and two /ygotes are formed
%he single /ygote divides into two /ygotes
Division o* 6ygote *ormed
%he two /ygotes formed do not divide
%he two foetuses share one placenta
$"m!er o* placentas
%he two foetuses have two separate placentas
ave identical chromosomes and genes
Chromosome s and genes
ave di5erent chromosomes and genes
ave the same se!)both boys or both girls
Se o* t,ins
%he twins may have the same se! or opposite se!es
Characteristi c *eat"res
%he twins have di5erent characteristic features
%he twins have similar characteristic feature
Siamese t,ins
(ometimes a /ygote can not divides into two completely, so the two /ygotes are still lin*ed to each other
%he lin*ed /ygote develop into foetuses and are born as twins Foined together in some parts+
(iamese twins are identical twins that are formed when the two embryos do not separate completely during the developmental stage and are attached to one another
3.4 '"tation
's a change in the amount or the chemical structure of DNA
Can occur spontaneously or induced by substances called m"tagens
%he new genetic material produced are called m"tants
1utation occur in gamete cell
1utation occur in somatic cell produced by mitosis
0ypes o* m"tation 6+ Chromosomal mutation
can be inherited
can only be inherited by daughter cells
2+ Gene mutation
1. Chromosomal m"tation
's the result of changes in the number or structure of chromosomes
Can cause changes in the characteristics of the organism
Changes in the no of chromosomes are usually the result of errors occurring during meiosis or mitosis
%hese changes may involve the loss of a pair or a single chromosome, or the increase in an entire haploid set of chromosomes polyploidy
Conse7"ences o* chromosomal m"tation 1. Do,n8s syndrome
'ndividuals su5ering from Down’s syndrome have 9 chromosomes
Additional chromosome is autosome no 26
(ymptoms" mental retardation, reduced resistance to disease, congenital heart abnormalities, a short stoc*y body, a thic* nec* and the characteristics folds of s*in over the inner corner of the eye
%ermed as mongolism
(. 9line*elter8s syndrome
ave an e!tra B chromosome, so that the usual *aryotype of B is replaced by one of BB
1ale with underdeveloped testis, infertility and some female characteristic such as breast enlargement
3. 0"rner8s syndrome
>ac* one B chromosome
(uch women are B, rather than the normal BB+
ave ovaries and breast that do not develop, do not e!perience the menstrual cycle and do not ovulate
++ Down’s syndrome and other related chromosomal abnormalities occur more fre3uently in children born to older women related to the age of the mother’s egg cells
(. 2ene '"tationpoint m"tation
's a change in the structure of the DNA at a single locus
DiHcult to detect because physical changes on speci$c genes are not visible and cannot be observed under a microscope
ften have undesirable e5ects but are sometimes bene$cial to the mutant organism
'f gene mutation produces a bene$cial phenotype, the fre3uency of the mutation in the population will increase from generation to generation as a result of natural selection
Conse7"ences o* gene m"tation 1. Colo"r ;lindness
's a se!)lin*ed recessive trait
0ecessive gene for colour blindness is found on the human B chromosome
1ost common form is red)green colour blindness-cannot distinguishes red & green.
'f a male inherits one gene for C,he has this disorder
(. %l!inism
ccurs when the gene which is responsible for normal pigmentation undergoes mutation %he gene responsible for albinism is recessive
Albino human beings are unable to produce the blac* pigment -melanin.
(o, they have no distinct colour in their s*in, hair or eyes
Ca"ses o* m"tation 6+ 0adioactive substances
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0adioactive radiation such as alpha, beta and gamma radiation
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B)ray also can cause mutation
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Can leads to cancer
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Can change the structure of functions of gene and chromosome
2+ igh 'ntensity ltraviolet >ight
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ltraviolet from the sun can destroy the cells in the s*in and cause mutation
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Can leads to s*in cancer
8+ Chemical substances
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E!posure of certain chemical substances li*e formaldehyde, pesticides, ben/ene,nicotine
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Can cause cancer and foetal deformity
9+ Changes in environment
-
After a long period of time, changes in the environment can cause organisms to adapt themselves and mutate into new species
- %hese new species with their adapted or mutated features are most suitable for living in the changing environment
%dvantages and disadvantages o* m"tation %dvantages 6+ 1utation produces genetic variation+ %his can give rise to new species+
Disadvantages 6+ Can cause genetic disorder such as albinism and haemophilia which can be passed on future
generations 2+ #lants and animals mutate in response to adverse changes in the environment+ 1utation gives them a better chance to survive
2+ Can cause abnormalities in children
Variation
Each person has uni3ue features which enable you to di5erentiate him or her from other friends
No two people are e!actly the same
%hese di5erences among individuals in a population or a species are called variation
%he di5erences may be the result of genetic di5erences, the inIuences of the environment, or a combination of genetic and environmental inIuences+
'n human usually the character referred to are the physical characters such as height, weight, s*in colour, etc++
%he di5erences in physiological processes such as our blood glucose level and immunity level are also considered
%here are two types of variation" -6. Continuous variation -2. Discontinuous variation
Contin"o"s Variation
%he characteristics can be measured or graded on a scale such as human heights, weight,s*in colour, etc++
Cannot be grouped into distinct and discrete phenotypes
sually 3uantitative -they can be measured. and are determined by genes and inIuenced by environmental factors+
'n JC:, a normal distribution curve is obtained
Discontinuous variation
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%he characters are discrete and cannot be measured or graded on a scale such as A blood group,Download color blindness, or absence of With Freepresence Trial earlobe,etc++
Are 3ualitative-they cannot be measured or graded.
usually determined by a single gene with distinct e5ect+
Discrete distribution is obtained
0he di5erences o* contin"o"s variation and discontin"o"s variation
Factors that ca"se variation
)mportance o* variation
Causes natural di5erences in appearance among individuals of the same species enables us to identify di5erent individuals
'mproves the ability to survive in a changing environment
Gives rise to a large variety of plants and animals+
