Form 4 Science Chapter 3: Heredity and Variation3.1 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 cells activities through the genetic material, DNA which acts as a set of instruction or code for lifeWhat 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 One gene from each pair comes from the father and the mother Example of inherited characteristics carried by genes are: Colour of the eyes Pattern of finger prints Type of hair Blood group Intelligence

Deoxyribonucleic acids (DNA) Is a double helix, made up of a series of genes to encode genetic information This determine the individual characteristics of an organism All the information in DNA is transmitted in the form of a genetic code Should 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 staircaseChromosomes Is a thread-like structure found in the nucleus of a cell Chromosome and the genes on them are composed of DNA It exist in pair and are identicalhomologous A pair of chromosomes has genes arranged in the same way. As a result the genes also exist in pairs Human has two sets of chromosomes in the body(one set from the father and another set from the mother)Mitosis Occur in somatic cells all cells body except reproductive cells 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 exact copy of the DNA as in the parent cell This allows growth of multicellular organism from unicellular zygotes Replacements of cells and repair of tissues is possible through mitosis followed by cell division Cells are constantly dying and being replaced by identical cells Is the basis of asexual reproductionthe production of new individuals of a species by one parent organism

Mitosis 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 exact copy of the DNA as in the parent cell Cells produced are diploid (2n) Replacement of cells and repair of tissues is possible through mitosis (Ex: cell replacement is rapid in the skin & in the lining of the gut Is actively carried out at the ends of plant shoots and roots

Step 1 Chromosomes thicken and shorten, and become clearly visible Each chromosome duplicates itself and forms two chromatids attached at the centromere The nuclear membrane disappearsStep 2 A fibrous spindle is formed between the chromosomes Each chromosome is attached to a spindle by its centromere The chromosome arrange themselves at the equator of the spindleStep 3 The centromere of each chromosome splits into two The chromatids separate and move to opposite poles.Step 4 A nuclear membrane is formed around the chromosomes at each pole The cell begins to divide and form two daughter cells Each daughter cell has the same number of chromosomes as the parent cell In a plant cell, the cytoplasm does not constrict. Instead a new cell wall is developed across the cytoplasm

Importance of mitosis Important for the following reasons:(a) Growth(b) Replacing old or damaged cells Enables organisms to reproduce themselves (Ex: by asexual reproduction; binary fission in amoeba) Ensures that the new cells produced have the same number of chromosomes as the parent cellsMeiosis Meiosis 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 Resulting nuclei are haploid (n) The cells produced are genetically different to the parent cell and to each other Occurs in 2 stages that is meiosis 1 and meiosis 2 Produces genetic variation (caused by crossing over between homologous chromosome) In animals, meiosis occurs in testes and ovaries, whereas in plants, it occurs in the anthers and ovariesImportance of meiosis1. Production of gamete the only process that produces gametes2. Reduction in chromosome number ensures that each gamete has only half the number of chromosomes of the parent cell3. Maintenance of chromosome number in somatic cells the somatic cells have their number of chromosomes maintained4. Production of variation causes genetic variation in the descendants(can produce individuals who do not resemble their parents closely)* Variation occurs in TWO ways:1. Different combination of chromosomes2. Crossing over

Comparing and contrasting mitosis with meiosis Similarities of MITOSIS and MEIOSIS:- Both are processes of cell division- In each process, duplication of chromosomes occurs onceDIFFERENCES OF MITOSIS AND MEIOSISMITOSISDIFFERENCESMEIOSIS

To produce new cells for growth and repair of damaged tissuesAim of processTo produce gametes (reproductive cells)

Somatic cells (non reproductive cells)Type of cell involvedReproductive cells which produce gametes

Two daughter cellsProductFour gametes

Nucleus of parent cells divides onceDivision of nucleusNucleus of parent cell divides twice

Duplication of chromosomes takes placeDuplication of chromosomesDuplication of chromosomes takes place in the first cell division

Same as in parent cell,46Number of chromosomes in daughter cellsHalf the number of those in the parent cell,23

Does not occurCrossing overOccurs when the chromosomes are at the equatorial plane

Same as in parent cellCombination of chromosomesDifferent from that of the parent cell

Does not cause genetic variationGenetic variationCauses genetic variation

3.2 The Principles and Mechanism of InheritanceDominant Genes and Recessive Genes Dominant Genes A gene which is powerful and can mask the effect of a less powerful or recessive gene in a pair Recessive Genes A weak gene and cannot show its effect 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 Phenotype is to describe the physical traits and the physiological traits of a person Genotype is to describe the make-up or constitution of a person. Which includes the genes from both parents either dominant or recessive.Dominant TraitsRecessive Traits

Right HandedLeft Handed

TallShort

Black/Curly HairBrown/Straight hair

Black/Brown eyesBlue Eyes

With Ear lobesWithout Ear lobes

Can Roll TongueCannot Roll Tongue

Has Normal VisionColor Blind

Has DimplesHas No Dimples

Pigmented SkinNon Pigmented Skin (Albino)

Long Eye LashesShort Eye Lashes

Monohybrid inheritance Mendel chose true-breeding varieties of the garden pea as the parents Mendel crossed a tall plant with a dwarf plant. Progeny from this cross, called the F1 generation (first filial generation) were all tall All the F1 plants were then self-pollinated to obtain the F2 generation (second filial generation) The F2 generation included both tall and dwarf plants in the ratio of 3 tall to 1 dwarf(phenotype ratio) It is shown that only the dominant(strong) trait appeared in the F1 generation. The recessive (weak) trait is blocked by the strong trait from expressing itself and is therefore hidden in the F1 generationThe mechanism of trait inheritance The character of height is determined by two contrasting traits, tall and dwarf. The gene for tall is dominant while the gene for dwarf is recessive If the gene for tall is represented by T and the gene for dwarf is represented by t, there will be three genetic combinations(genotype) for plant height Genotype is the term used to describe the genetic make-up or constitution of a person. It includes the genes inherited from both parents,whether they are dominant or recessive Phenotype is the physical traits such as colour of the eyes, height and ect.. 3 possible combination are:Phenotype Genotype

Tall (pure strain)TT (homozygous dominant)

Tall (monohybrid)Tt (heterozygous)

Dwarf (pure strain)tt (homozygous recessive)

3.3 Sex Determination and the occurrence of twins in human beingsSex Chromosome Autosomes are chromosomes which are not sex chromosomes The characteristic features of a person are determined by the autosomes The sex of a person is determined by the sex chromosome In a woman, the sex chromosomes are a pair of identical chromosomes called X chromosomes In a man, the sex chromosomes consists of one X chromosome and a smaller Y chromosome Therefore, chromosomes in female can be written as 44+XX while male as 44+XYMechanism of Sex Determination Fathers sperm determine the sex of the baby If the baby inherits Y chromosome from its father boy If the baby inherits X chromosome from its fathergirl THUS, the sex of the baby depends on which sperm fertilizes the egg cell(ovum).

Identical and non-identical twins(1) Identical twins When an ovum is fertilized by a sperm, a zygote is formed The zygote then divides into 2 separate cells in the womb producing 2 embryos identical twins Identical twins are two normal, genetically identical individuals of the same sex and share a common placenta during developmental stage(2) Non-Identical twins Some women may produce two ovum at the same time. When the two different ovum are fertilized at the same time by two different sperms, two zygotes are formed non-identical twins(fraternal twins) Non-identical twins are genetically different individuals, can may be of the same sex or different sex and each have their own placenta during developmental stageComparing and contrasting identical twins with non-identical twins(1) SIMILARITIES: Two foetuses develop in the uterus at the same time Two babies are born at the same time

(2) DifferencesIdentical twinsDifferencesNon-Identical twins

One ovum and one spermNo of gametes involvedTwo ovum and two sperms

One ovum is fertilized and one zygote is formedFertilizationTwo ovum are fertilized and two zygotes are formed

The single zygote divides into two zygotesDivision of zygote formedThe two zygotes formed do not divide

The two foetuses share one placentaNumber of placentasThe two foetuses have two separate placentas

Have identical chromosomes and genesChromosomes and genesHave different chromosomes and genes

Have the same sex-both boys or both girlsSex of twinsThe twins may have the same sex or opposite sexes

The twins have similar characteristic featureCharacteristic featuresThe twins have different characteristic features

Siamese twins Sometimes a zygote can not divides into two completely, so the two zygotes are still linked to each other The linked zygote develop into foetuses and are born as twins joined together in some parts. Siamese 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 Mutation Is a change in the amount or the chemical structure of DNA Can occur spontaneously or induced by substances called mutagens The new genetic material produced are called mutants Mutation occur in gamete cell can be inherited Mutation occur in somatic cell can only be inherited by daughter cells produced by mitosis

Types of mutation1. Chromosomal mutation2. Gene mutation

1. Chromosomal mutation Is 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 These changes may involve the loss of a pair or a single chromosome, or the increase in an entire haploid set of chromosomespolyploidy

Consequences of chromosomal mutation1. Downs syndrome Individuals suffering from Downs syndrome have 47 chromosomes Additional chromosome is autosome no 21 Symptoms: mental retardation, reduced resistance to disease, congenital heart abnormalities, a short stocky body, a thick neck and the characteristics folds of skin over the inner corner of the eye Termed as mongolism2. Klinefelters syndrome Have an extra X chromosome, so that the usual karyotype of XY is replaced by one of XXY Male with underdeveloped testis, infertility and some female characteristic such as breast enlargement3. Turners syndrome Lack one X chromosome Such women are XO, rather than the normal XX. Have ovaries and breast that do not develop, do not experience the menstrual cycle and do not ovulate** Downs syndrome and other related chromosomal abnormalities occur more frequently in children born to older women related to the age of the mothers egg cells

2. Gene Mutation/point mutation Is a change in the structure of the DNA at a single locus Difficult to detect because physical changes on specific genes are not visible and cannot be observed under a microscope Often have undesirable effects but are sometimes beneficial to the mutant organism If gene mutation produces a beneficial phenotype, the frequency of the mutation in the population will increase from generation to generation as a result of natural selectionConsequences of gene mutation1. Colour Blindness Is a sex-linked recessive trait Recessive gene for colour blindness is found on the human X chromosome Most common form is red-green colour blindness(cannot distinguishes red & green) Females have pair of genes that control color blindness but male have only one If a male inherits one gene for CB,he has this disorder Female can only has this disorder if she inherits two recessive genes,one from each parent2. Albinism Occurs when the gene which is responsible for normal pigmentation undergoes mutation The gene responsible for albinism is recessive Albino human beings are unable to produce the black pigment (melanin) So, they have no distinct colour in their skin, hair or eyes

Causes of mutation1. Radioactive substances Radioactive radiation such as alpha, beta and gamma radiation X-ray also can cause mutation Can leads to cancer Can change the structure of functions of gene and chromosome2. High Intensity Ultraviolet Light Ultraviolet from the sun can destroy the cells in the skin and cause mutation Can leads to skin cancer3. Chemical substances Exposure of certain chemical substances like formaldehyde, pesticides, benzene,nicotine Can cause cancer and foetal deformity4. Changes in environment After a long period of time, changes in the environment can cause organisms to adapt themselves and mutate into new species These new species with their adapted or mutated features are most suitable for living in the changing environment

Advantages and disadvantages of mutationAdvantagesDisadvantages

1. Mutation produces genetic variation. This can give rise to new species.1. Can cause genetic disorder such as albinism and haemophilia which can be passed on future generations

2. Plants and animals mutate in response to adverse changes in the environment. Mutation gives them a better chance to survive2. Can cause abnormalities in children

Variation Each person has unique features which enable you to differentiate him or her from other friends No two people are exactly the same These differences among individuals in a population or a species are called variation The differences may be the result of genetic differences, the influences of the environment, or a combination of genetic and environmental influences. In human usually the character referred to are the physical characters such as height, weight, skin colour, etc.. The differences in physiological processes such as our blood glucose level and immunity level are also considered There are two types of variation:(1) Continuous variation(2) Discontinuous variation Continuous Variation The characteristics can be measured or graded on a scale such as human heights, weight,skin colour, etc.. Cannot be grouped into distinct and discrete phenotypes Usually quantitative (they can be measured) and are determined by genes and influenced by environmental factors. In *CV, a normal distribution curve is obtained From the distribution curve, it can be seen that individuals show gradual difference from one extreme to another extreme There are very few individuals who are very short or very tall Most of them are of average height Discontinuous variation The characters are discrete and cannot be measured or graded on a scale such as ABO blood group, color blindness, presence or absence of earlobe,etc.. Are qualitative(they cannot be measured or graded) usually determined by a single gene with distinct effect. Discrete distribution is obtained The differences of continuous variation and discontinuous variation

Factors that cause variation

Importance of variation Causes natural differences in appearance among individuals of the same species enables us to identify different individuals Improves the ability to survive in a changing environment Gives rise to a large variety of plants and animals.