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 organism’s 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.

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.

Human Cells Chromosomes

Deoxyribonucleic acid (DNA)GenesCharacteristics in human

builtof

containing containing of

formingdetermining

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 the zygote when the nucleus of sperm fuses with the nucleus of an ovum during fertilisation.

b) combination of different chromosomes and crossing over which occur causing variation to take place in the next generation.

Comparison between meiosis and mitosisSimilarities

chromosome replication takes place call division occurs

new cells are produced

Differences

When this process happens

Where this process happensin animals

Meiosis Mitosis

During the formation of somatic cells

During the formation of

gametes

Somatic cells

Testes and ovaries

Where this process happensin plants

Number of cell division

Replication Process

Number of daughter cellsproduced

Number of chromosomes indaughter cells compared

to 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 of genes, 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 :

Tip of the root and shoot

Anther and ovary

OnceTwice

OccursOccurs during the first cell division only

TwoFour

SameHalf

SameDifferent

Doesn’t happen

Happens

Dominant genes Recessive genesare genes which

show the characteristics

that they control when paired with a dominant gene

or a recessive gene.

are genes which only show the characteristics

that they control when these

genes are paired with another

recessive gene.

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. 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.

the characteristics determined by this genes are

called dominant traits.

the characteristics determined by this genes are

called recessive traits.

dominant traits are shown if one or both dominant

genes which control the particular

characteristics are present.

recessive traits are only shown if

the recessive genes are not

paired with dominant genes.

human traits controlled by this

genestall, curly hair, free ear lobe,

black hair, able to roll the tongue,

dimples, long eye lashes, right-

handed, pigments in skin, black or brown

iris, normal eyesight

human traits controlled by this

genesshort, straight hair, attached

ear lobe, blond hair, unable to roll the tongue,

no dimples, short eye lashes, left-handed, albino, blue iris, colour-

blindness

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, that inheritance 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 in

the Punnett Square.

Gen T tTtttT TT Tt

t

Punnet square

TT tt

t tTT

tall plant(pure breed)

dwarf plant(pure breed)

xParents

Meiosis

Gametes

Fertilisation

First filialgeneration

all plants in the first filial generation, F1,

are tall (100%) this situation

shows that tall trait is controlled by the dominant gene while short trait is controlled by the recessive gene.

Tt Tt Tt Tttall plant(hybrid)

tall plant(hybrid)

Tt Tt

tall tall tall tall

xFirst filialgeneration

Meiosis

ratio of plants that are tall to dwarf plants are 3 : 1.

probability of obtaining tall plants in the second generation is 75% while the short plants is 25%.

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+Xo 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 sperm

that 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

T ttT

Tt Tt Tt tt

tall tall tall dwarf

Gametes

Fertilisation

Second filialgeneration

22 +X

22 +X

22 +Y

22+X

44 +XX

44 +XX

44 +XY

44 +XY

44 +XY

44 +XX

Parents

Gametes

Fertilisation

Offspring

Meiosis

x

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 ovum fertilised by a sperm divides into two to form two similar embryos.

Formed when two ova are released by the ovary at the same time and fertilised separately by two sperms to form two different embryos.

PlacentaThe two embryos formed will grow in one shared placenta in the mother’s uterus.

The two embryos formed when grow in separate placentas in the mother’s uterus

Genetic make-up

Same genetic make-up. Therefore, identical twins have the same appearance and the same sex.

Different genetic make-up since they originate from two different ova and sperms. Therefore, non-identical twins have different appearances.

SexThe sex of identical twins are the same.

The sex of non-identical twins may be the same or different.

sometimes the division of the embryo is not complete, therefore Siamese twins are formed.

female female male male 50 : 50

Siamese twins may share certain organs like brain, stomach, or heart and maybe joined to one another at the head, abdomen, breast, or buttocks.

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 disease caused 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 produce skin 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 the colours 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.

a patient who has wound will experience continuous bleeding or will take a long time for the blood to clot thus 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 Down’s

syndrome, Klinefelter’s syndrome, and Turner’s syndrome.

Down’s 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 down’s syndrome patient :

i. slanting eyesii. metal retardationiii. short fingersiv. wide distance between eyesv. small mouth but big tonguevi. protruding tongue causing the mouth to remain partially open

Klinefelter’s syndromeo a person who suffers from this hereditary diseases has one extra X

chromosome : XXY

Turner’s syndromeo a person who suffers from this hereditary lacks one X chromosome.o people with Turner;s syndrome have only one sex chromosome

which 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.

iii. chemical substances like pesticides, nicotine in cigarettes, drugs, nitrous acid and some preservatives, colouring and artificial sweetener can cause mutation.

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 mutate

and 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 to adapt to the environment.

species that are more resistant to disease, weather, and polluted 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 anaemia

can cause death. Klinefelter’s 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 hereditary diseases 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. 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

hereditary diseases like Down’s syndrome can be identified on the foetus in the mother’s womb.

A aaA

AA Aa Aa aa

Aa AaParents

Gametes

Fertilisation

Genotype of Offspring

Meiosis

Characteristics of Offspring

normal normal but normal but albino is a carrier is a carrier

X XbYX

XX XXb XY XbY

XY XXb

normal normal girl normal boy colour girl but is a carrier but is a carrier -blind boy

Parents

Fertilisation

Genotype of Offspring

Meiosis

Characteristics of Offspring

Gametes

x

x

foetal cells in the mother’s amniotic fluid are examined under a microscope.

if 47 chromosomes are observed in the foetal nucleus, the foetus is proved having Down’s 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 hybridPaddy PETA from Indonesia

and DGWG from China

IR-8 Bear more fruits, ripen faster and are more resistant to dry season

Oil palm Pisifera with dura Tenera Bear more fruits and have thinner shell with more content

Maize Sweetcorn from Taiwan and Mexico

Masmadu Fruits are sweeter, bigger, and more resistant to disease and dry season

Papaya Subang 6 and Sunrise Solo from Hawaii

Exotica Fruits are sweeter and have more content

examples of selective breeding in crops

Type of livestock Parents Characteristics of new breedCow Fresian and local New breed produces more

milk and are more adaptable to the change in the environment

Cow Female cow and male wild ox

New breed (selembu) that grows faster and produces better quality

Goat Jumnapari goat from India and local goat

New breed that can produce more meat and milk

Sheep Sheep from Australia and local Malin

New breed (Malin-X) is bigger in size, matures faster, supplies better quality meat and wool

Chicken Imported and local kampung chicken

New breed that grows and 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 placed into 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 :

i. heightii. body weightiii. intelligenceiv. skin colour

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 resistance to diseases or pests.

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 of

human cells.

Long-term effect new species of crops and livestock which

adapt better to the change in the environment might cause authentic species to become extinct.

v. width of shouldervi. 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

Factors that cause variation. there are 2 factors which cause variation :

number of

students

body weight

number of

students

blood groupsA B AB O

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 distinguish individuals in the same species.

2. Variations enable newly produced species to adapt themselves better to any changes in the environment.

3. In plants, features of variation like resistance to pathogens, enable the plants to grow rapidly and reproduce quickly.