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CHAPTER 3
Heredity and Variation
Human Cells Chromosomes
DNAGenes Characteristics/ traits
Built up of containing
determiningcontaining
containing
3.1 CELL DIVISION
• Chromosomes, genes and DNA• Life cycle and cell division– Mitosis: growth– Meiosis: sexual reproduction
Chromosomes, genes and DNA Cik Emy ®
NUCLEUS
CHROMOSOMES-threadlike structure
-human beings have 46 (23 pairs-contain protein and DNA
DNA- Double helix structure
- -Made up of series of genes
GENE- Segment of DNA-Genetic codes which
determine an organism characteristics
Mitosis
• Definition: A cell division process in which a parent cell
(original cell) divides into two identical daughter cells (new cells)
• Occurs in:– Human and animal : somatic cells– Plants: tips of roots and shoots
Stages of mitosis
•Chromosomes in the nucleus shorten, thicken and more visible.
• Replication of chromosomes occur.• Each strand of chromosome is called
chromatid.
chromatid
centromere
nucleus membrane
• The nucleus membrane forms again.• Two daughter cells are formed.
• Each daughter cell has the same number of the chromosomes as the parent cell.
• Spindle fibres emerge.•Chromosomes align themselves in the centre
of the cell.• The nucleus membrane disappears.
• Chromatids divide and separate to the opposite ends of the cell.
• At the same time the cell starts to divide.
spindle fibres
The Importance of MitosisProduce new cells for
growth – unicellular zygote to multicellular organism
Asexual reproduction for simple organism like amoeba
Regenerate body parts such as lizard grows a
new tail
Vegetative reproduction such as rhizomes, tubers,
leaves
Produce new cells for replacing dead
cells/repairing tissues
Meiosis • Definition: A process in which a nucleus of a parent cell divides
into four daughter cells through two divisions.
• The daughter cells will have half the number of chromosomes in parent cell.
• Occurs in reproductive organs to produce gametes (ovum and sperms):– Human and animals: testes (male) and ovary (female)– Plants : anthers and ovaries
Stages Of Meiosis : Meiosis 1 And Meiosis 2
• Chromosomes in the nucleus shorten, thicken and more visible.
• Crossing over and replication process occur on chromatids.
Meosis 1
•Chromosomes align themselves in the centre of the cell.
• The nucleus membrane disappears.
•Chromatids separate and move to the opposite ends of the cell.
• The cell begins to divide into two.
• Two daughter cells are produced.• The nucleus membrane reforms.
•The nucleus membrane is formed again.• Four daughter cells are formed.• Each daughter cell has half of the
number of chromosomes in the parent cell.
• Spindle fibres emerge.• Chromosomes align themselves in the
centre of the cell.• The nucleus membrane disappears.
• Chromatids divide and separate to the opposite ends of the cell.• The cell starts to divide.
Meosis 2
The Importance of Meiosis
Crossing over process
Independent assortment of chromosomes
(characteristics are separated during formation of gamete and randomly
combined during fertilization)
1. To produce gametes
2. To allow variation through :
Conclusion Of Mitosis And Meiosis
Meiosis
Fertilisation
Mitosis
Male (46 chromosomes)
Female (46 chromosomes)
Sperm (23 chromosomes)
Ovum (23 chromosomes)
Zygote(46 chromosomes)
Baby(46 chromosomes)
Similarities And Differences Between Mitosis And Meiosis
SIMILARITIES
Replication of DNA
takes place
Involves division of
nucleus and cytoplasm
Cell division occurs
The Difference Between Mitosis And Meiosis
DIFFERENCESMITOSIS MEIOSIS
Replication process
Number of division
Number of daughter cells produced
Number of chromosomes in
daughter cell
Combination of gene
Occur
One only
Four
Same as in parent cell
Same as parent cell
Occur in the first division
Two times
Two
Half of parent cell
Different from parent cell
Crossing over processDoes not occur Occur
3.2 THE PRINCIPLES AND MECHANISM OF INHERITANCE
GeneExist in pair
Dominant gene-A gene which expresses the trait when paired with a dominant or
recessive gene-Denote as capital letter, eg
T (tall)
Dominant traitCharacteristics controlled by
dominant genes
Recessive gene-A gene which expresses the trait
when paired with a recessive gene only (in the absence of a dominant
gene)-Denote as small letter, eg t (short)
Recessive traitCharacteristics controlled by
recessive genes
Chromosomes are in pair. One from the father while the other one from the mother.
Same genes:Dominant gene and recessive gene are placedopposite to each other
Homologous chromosomes
t T
Examples of dominant and recessive traitsTrait Dominant Recessive
Earlobes Free earlobes Attached earlobes
Tounge Ability to roll Inability to roll
Dimples Presence Absence
Hair Curly Straight
Iris colour Brown Blue
Height Tall Dwarf
The Mechanism Of Trait Inheritance
• Gregory Mendel was the first person who discovered the principles of heredity.
Genotype
Phenotype
The genetic information of an organism
The physical characteristics of an organism
Monohybrid Inheritance By Gregory Mendel
TT ttTall (pure breed) Dwarf (pure breed)
T T tt
T t tT
Tall Tall Tall Tall
Tall Tall Tall Dwarf
Phenotype :Genotype :
Phenotype :
Genotype :
Phenotype :Genotype :
F2 Generation
F1 Generation
Gametes
Gametes
Tt Tt Tt Tt
TtTtTT t t
Key: Homozygous: both genes are
same Eg: TT, tt
Heterozygous: genes are different
Eg: Tt
Exercise 1
• In the pea plant, the gene for round seed(R) is dominant to the gene for wrinkled seed(r). a) If a homozygous round-seeded pea plant is
crossed with a homozygous wrinkled-seeded plant, what will be the phenotype of F1 plants?
b) If the F1 plants are self-pollinated, what will be the genotype and phenotype ratios of the F2
plants?
RR rrRound seed Wrinkled seed
R R rr
Round Round Round Round
Parental Phenotype :Parental Genotype :
F1 Generation
Gametes
Rr Rr Rr Rr
Hence all the F1 plants have round seeds.
a) Key: R: round seededr : wrinkled seeded
b)
R r rR
Round Round Round
Wrinkled
F2 Phenotype :F2 Genotype :
F1 Generation
Gametes
Rr Rr
RrRrRR r r
Genotype ratio is 1 RR:2 Rr: 1rr
Phenotype ratio is 3 plants with round seeds : 1 plant with wrinkled seed
Exercise 2
• B represents the dominant gene for black cat, and b represents the recessive gene for a white cat.
What is the phenotype ratio of the number of black cats to the number of white cats if black fur cat (Bb) is crossed with another black fur cat (Bb)?
B b bB
Black Black Black White
F1 Phenotype :F1 Genotype :
Parents
Gametes
Bb Rb
BbBbBB bb
Phenotype ratio is 3 black fur cats: 1 white fur cat
Key: R: black catr: white cat
Answer:
Exersice 3
• In the pea plant, the gene for purple flowers (P) is dominant to the gene for white flowers (p). A cross between a heterozygous pea plant with purple flowers and a homozygous pea plant with white flowers is done. What are the genotypes of the offsprings?
P p pp
Purple White Purple
White
F1 Phenotype :F1 Genotype :
Parents
Gametes
Pp pp
ppPpPp pp
Genotype ratio is 2 Pp : 2 pp
Key: Pp: heterozygous , purple flowerpp: homozygous , white flower
Answer:
3.3 SEX DETERMINATION AND THE OCCURRENCE OF TWINS IN HUMAN BEINGS
• Sex chromosomes: the last pair in human chromosomes
Male: 22 pairs of autosomes and a pair of sex chromosomes XY.
Female: 22 pairs of autosomes and a pair of sex chromosomes XX.
• Sex determination : the sex of baby is determined by the father’s sperm (XY)
44 + XY 44 + XX
Y XGamete cells: orX
Fertilisation
Baby girl 44+XXBaby boy44+ XY
44 + XX 44 + XY
22 + X 22 + X
44 + XX 44 + XY
22 + Y22 + X
44 + XX 44 + XY
Mother Father
Gametes :
Parents :
Offsprings:
Ratio of daughter : son is 1:1
Therefore , the chance to get daughter or son is 50:50.
Daughter Daughter Son Son
The Formation Of Identical And Non-identical Twins
An ovum is fertilised by a sperm to form a zygote.
Zygote undergoes mitosis and separates into two to develop into two embryos.
Genetically identical.
Share the same placenta.
Same sex.
IDENTICAL TWINS
Two different ova fertilised by two different sperms at the same time.
Two different zygotes are formed and develop into two embryos.
Genetically different.
Each has own placenta.
Same sex or different sex.
Known as fraternal twins.
NON-IDENTICAL TWINS
IDENTICAL TWINS NON-IDENTICAL TWINS
Zygote undergoes mitosis
Two embryos
Foetus share the same placenta
Two ova are fertilised
Two different zygotes are formed
Two separate embryos
Each foetus has his own placenta
• Siamese twins– Identical twins that are joined at some parts of the
bodies.– Formed when zygote fails to divide completely in
the course of formation.
Similarities And Differences Between Identical And Non-identical Twins
IDENTICAL TWINS NON-IDENTICAL TWINS
Similarities:The twins are formed in the uterus.
Two babies are formed.
One
One
Same sex
Yes
Same / identical
Yes / sharing placenta
Two
Two
Same or different sex
No
Different
No / separate placenta
Number of sperms
Fertilised ovum splits into two
Genetic make up
Sharing of placenta
Sex
Number of ova
Differences
3.4 MUTATION
• Mutation: the spontaneous change to the structure of genes or chromosomes
Occurs in
Somatic/body cells(can be passed down
through mitosis)
Gametes/reproductive cells
(inherited)
TYPES OF MUTATION
Gene mutation
Change in the structure of DNA
Chromosome mutation
A change in the number of chromosomes or in
the structure
Examples Of Chromosome Mutation
• Down’s syndrome: an extra chromosome in autosome 21 (47 chromosomes)
• Klinefelter’s syndrome: a male with extra X chromosome (XXY)
• Turner’s syndrome: a female with one less X chromosome (XO)
• Polyploidy: having one or more chromosomes in a set of chromosome
Examples Of Gene Mutation
• Colour blindness: a mutated gene in the X chromosome
• Sickle cell anemia: mutation in gene that produces haemoglobin; inefficient in transporting oxygen in the blood
• Albinism: the gene for skin colour is mutated; unable to produce black pigment (melanin)
• Haemophilia: mutation in gene that is important for blood-clotting
Examples Of Chromosome Mutation
Down’s syndromeAdditional chromosome at autosome 21.
•Physical retardation•Thick neck•Slanted eyes•Protuding tounge•Reduced resistance to diseases
Klinefelter’s syndromeA male with extra X chromosomes (having XXY)
•Infertile•Underdeveloped testes•Narrow shoulder •Enlarged breasts
Turner’s syndromeA female having one less X chromosome (XO)
•Infertile•Short•Mentally retarded•Has no ovaries•Small breasts•Folded skin on neck•No menstrual cycle
Polyploidy– The number of chromosomes is increased by one
or more sets.– More common in plants.
Examples Of Gene MutationColour blindness
Mutant gene (recessive gene) on the X chromosome.
•Not able to differentiate colours especially green and red•More common in males because male has only one X chromosome
Albinism
The gene for skin colour is mutated (recessive gene)
•Unable to produce melanin (black pigment)•White hair•Pink eyes and pinkish skin•Skin is sensitive to sunlight
Sickle-cell anemiaMutation in gene that produce haemoglobin
•Sickle-cell shaped of red blood cells•Not efficient in transporting oxygen•Facing painful crises such as chest pain, fatigue, bone pain,•Yellowing of eyes •Inherited from both parents
HaemophiliaMutation in chromosome X (for blood clotting)
• Have problem in blood clotting• May bleed continuously or for a longer period• A woman carrier may inherit the disease to a son
Spontaneous changes during cell division
Exposure to physical mutagens: UV radiation,
X-rays, radioactive radiation
Environmental change: the change of temperature for a long period causes genetic
evolution in organism
Exposure to chemical mutagens: Benzene,
formaldehyde, nicotine, sodium nitrite, pesticides
Causes Of Mutation
Advantages And Disadvantages Of Mutation
Give rise to variation which leads to differences in traits
Better chance of survival when new species are more resistant to diseases
Advantages
Cause physical defects
Causes genetic diseases
Some chromosome mutations and gene mutation are lethal (fatal)
Causes mental defects
Disadvantages
3.5 THE EFFECTS OF GENETIC RESEARCH ON HUMAN LIFE
Gene therapy: to replace damaged/faulty genes during
the foetal stage
Genetic screening: to predict diseases likely to develop
Gene test: to screen unborn children for diseases
Genetic engineering: to transfer DNA from one
organism to anotherHuman genome project: maps all human genes to determine genes which
causes diseases
Field of Medicine
To produce plants which are resistant to diseases and
infections
To produce engineered plants which are able to live in unfavourable conditions
To produce plants which grow and mature fast
To produce higher yields
To produce high quality of crops and livestock
Field of Agriculture
Selective BreedingA process which involves cross
breeding and cloning of plants or animals
Advantages New varieties with
desired characteristics can be produced
To produce new and high quality crops
More resistant to diseases
Disadvantages May produce new
varieties with undesirable
characteristics
Example 1: Cow
is cross bred with
Friesian Cow Sahiwal Cow
Mafriwal Cow-Grow faster -Bigger in size-More milk -High resistant to disease
Example 2: Palm oil
is cross bred with
Dura Pisifera
Tenera-Thick mesocarp and large kennel :much oil-Thin shell: easier to extract
Thinmesocarp
Large kennel
Thickmesocarp
Smallkennel
Tissue CultureA process of cloning a plant by using a plant part or single cell in a test tube
under a controlled condition
AdvantagesTo produce many
copies of the same plants in a short time
To produce plants anytime we want
Shorter maturation time
No variation
Disadvantages
Complete wipe-out of crops if there is spread
of diseases
Steps of Tissue Culturea) Explant:
b) Multiplication:
c) Transplanting:
Cloning The process of producing an organism that is an exact genetic copy of another
Process:
Genetically Modified FoodFoods from plants and animals which
have had their genetic information changed by adding genes from
another organisms
Advantages:
For crops: enhanced taste and quality, increased yields,
matures faster, better resistance to diseases,
remain fresh longer
For livestock: increased resistance
to infections and diseases, higher yield
of milk, meat and eggs
Cause allergies and side effects to human beings
Disadvantages of Genetically Modified Food and Organisms:
Milk of genetically modified animals may be unsafe
Antibiotic-resistant genes in GM organisms may be pass on to bacteria;
difficult to kill them
Genetic research is expensive, poor farmers cannot afford
3.6 VARIATION AMONG LIVING THINGSVariation:
The differences among individuals of the same species
Continuous variation
Discontinuous variation
-Characteristics among individuals which do not show distinct differences
-Quantitative and can be measured
-Determined by genes and can be influenced by environmental factors
-Characteristics among individuals which show distinct differences
-Qualitative and cannot be measured
-Determined by a single gene and is not influenced by environmental factors
Continuous variation
Discontinuous variation
Height
Skin colour
Length of foot
Intelligence
Weight
Earlobes
Iris colourColour blindness
Tongue rolling
Blood group
Factors That Cause Variation
Genetic factors
Crossing over during meiosis: genetic material is exchanged
Random fusion of male and female gametes during fertilisations
Chromosome and gene mutation: creates new phenotypes
Independent assortment of chromosomes: different gametes
Environmental factors
Sunlight exposure: will have darker complexion
Water climate, temperature
pH: the Hydrangea flower is blue in acidic soil and pink in alkaline soil
Diet: a person who eats a lot will be fatter
Importance Of Variation
IMPORTANCE
Ensures the survival of a species under changing
environment
Enable any species to occupy a wider range of habitat
Enable natural selection: species with favourable characteristics will be
selected by nature
Enable us to distinguish
individuals of the same species
CONTINUOUS VARIATION
DISCONTINUOUS VARIATION
Similarity:Show differences in characteristics among
individuals of the same species
Yes
Not distinct
If it is influenced by environmental factors cannot be inherited
Two or more genes for the same character
Quantitative/ can be measured
No
Distinct
Can be inherited
Single gene
Qualitative/ cannot be measured
Influenced by environmental factors?
Inheritance
No of gene
Type of data
Characteristic
Differences
Similarities And Differences Between Continuous And Discontinuous Variation