Form 4 Biology Notes

Form Four Genetics

Introduction

•  Genetics is the study of inheritance.
   The fact that the offspring of any species resemble the parents indicates that the characters in the parents are passed on to the offspring.
   Factors that determine characters (genes) are passed on from parent to offspring through gametes or sex cells.
   In fertilisation the nucleus of the male gamete fuses with the nucleus of the female gamete.
   The offspring show the characteristics of both the male and the female.
   Genetics is the study of how this heritable material operates in individuals and their offspring.

Variations within Plant and Animal Species

Variation

•  The term variation means to differ from a standard.
   Genetics also deals with the study of differences between organisms belonging to one species.
   Organisms belonging to higher taxonomic groups e.g. phyla or classes are clearly different.
   Although organisms belonging to the same species are similar, they show a number of differences or variations such that no two organisms are exactly the same in every respect.
   Even identical twins, though similar in many aspects, are seen to differ if they grow in different environments.
   Their differences are as a result of the environment which modifies the expression of their genetic make-up or genotype.
   The two causes of variations are the genes and the environment.
   Genes determine the character while the environment modifies the expression of that character.

 

Continuous and Discontinuous Variation

Continuous Variations

•  The differences between the individual are not clear-cut.
   There are intermediates or gradations between any two extremes.
   Continuous variations are due to action of many genes e.g. skin complexion in humans.
   In continuous variation, the environment has a modifying effect in that it may enhance or suppress the expressions of the genes.
   Continuous variation can be represented in form of a histogram.
   Example of continuous variation in humans is weight, height and skin complexion.
   Linear measurements:
   In humans, height shows gradation from tall, to tallest.
   So does the length of mature leaves of a plant.
   In most cases, continuous variation is as a result of the environment.

Discontinuous Variations

• These are distinct and clear cut differences within a species.

Examples include:

•  Ability to roll the tongue.
   An individual can either roll the tongue or not.
   Ability to taste phenylthiourea (PTC); some individuals can taste this chemical others cannot.
   Blood groups – and individual has one of the four blood groups A, B AB or O.
   There are no intermediates.
   Albinism – one is either an albino or not.
   Discontinuous variations is determined by the action of a single gene present in an individual.

Structure and Properties of Chromosomes

•  These are threadlike structures found in the nucleus.
   They are normally very thin and coiled and are not easily visible unless the cell is dividing.
   When a cell is about to divide, the chromosomes uncoil and thicken.
   Their structure, number and behaviour is clearly observed during the process of cell division.
   The number of chromosomes is the same in all the body cells of an organism.
   In the body cells, the chromosomes are found in pairs.
   Each pair is made up of two identical chromosomes that make up a homologous pair.
   However sex chromosomes in human male are an exception in that the Y-chromosome is smaller.

 

Number of Chromosomes

 

Diploid Number (2n)

•  This is the number of chromosomes found in somatic cells.
   For example, in human 2n = 46 or 22 pairs (44 chromosomes)are known as autosomes (body chromosomes”)
   while 1 pair is known as the sex chromosomes.
   In Drosophila melanogaster, 2n =8.

 

Chromosome Structure

•  All chromosomes are not of the same size or shape.
   In human beings; each of the twentythree pairs have unique size and structure.
   On this basis they have been numbered 1 to 23.
   The sex chromosomes formthe 23rd pair.

Properties of Chromosomes

•  Chromosomes are very long and thin.
   They are greatly and loosely coiled and fit within the nucleus.
   During cell division they shorten, become thicker and are easily observable.
   Each consists of two chromatids.
   The two chromatids are held at same position along the length, at the centromere.
   Chromatids separate during cell division in mitosis and in the second stage of meiosis.
   Chromosomes take most dyes and stain darker than any other part of the cell.
   This property has earned them the name “chromatin material”
   Each chromosome is made up of the following components:
   Deoxyribonucleic acid (DNA) – this carries the genes.
   It is the major component of the genetic material.
   Protein e.g. histones.
   Ribonucleic acid (RNA) is present in very small amounts.
   Enzymes concerned with DNA and RNA replication – these are DNA and RNA polymerases and ligases.

·         Each chromosome is made up of the following components:

• Deoxyribonucleic acid (DNA) – this carries the
• It is the major component of the genetic
• Protein e.g.
• Ribonucleic acid (RNA) is present in very small
• Enzymes concerned with DNA and RNA replication – these are DNA and RNA polymerases and

Structure of DNA

•  The structure of DNA was first explained in 1953 by Watson and Crick.
   DNA was shown to be a double helix that coils around itself.
   The two strands are parallel and the distance between the two is constant.

Components of DNA

•  DNA is made up of repeating units called nucleotides.
   Each nucleotide is composed of:
   A five-carbon sugar (deoxyribose).
•  Phosphate molecule.
   Nitrogenous base, four types are available i.e,
   Adenine – (A)
   Guanine – (G)
   Cytosine – (C)
   Thymine – (T)
   The bases are represented by their initials as A, G, C and T respectively.
   The sugar alternates with the phosphate, and the two form the backbone of the strands.
   The bases combine in a specific manner, such that Adenine pairs with Thymine and Guanine pairs with Cytosine.
   The bases are held together by hydrogen bonds. A gene is the basic unit of inheritance consisting of a number of bases in linear sequence on the DNA.
   Genes exert their effect through protein synthesis.
   The sequence of bases that make up a gene determine the arrangement of amino acids to make a particular protein.
   The proteins manufactured are used to make cellular structures as well as hormones and enzymes.
   The types of proteins an organism manufactures determines its characteristics.
   For example, albinism is due to failure of the cells of an organism to synthesise the enzyme tyrosine required for the formation of the pigment melanin.

First Law of Heredity

•  It is also known as Law of Segregation (Mendel’s First Law).
   The characters of an organism are controlled by genes occurring in pairs known as Alleles.
   By definition, an allele is an alternative form of a gene controlling a particular characteristic.
   Of a pair of such alleles, only one is carried in each gamete.
   This is explained by first meiotic anaphase stage, when the homologous chromosomes are separated so that each carries one of the allelic genes.

Monohybrid Inheritance

•  This is the study of the inheritance of one character trait that is represented by a pair of genes on homologous chromosomes.
   Gregor Mendel (an Austrian monk) was the first person to show the nature of inheritance.
   He did this through a series of experiments using the garden pea, Pisum sativum.
   As opposed to others before him, the success in his work lay in the fact that:
   He chose to study first a single character at a time (monohybrid inheritance).
   He then proceeded to study two characters at time (dihybrid inheritance) .
   He quantified his results by counting the number of offspring bearing each trait.
   Each character he chose was expressed in two clearly contrasting forms.

Examples

•  Stem length: some plants were tall while others were short.
   Colour of unripe pods: some were green, others yellow.
   There were no intermediates.

Mendel’s Procedure

•  For each character, Mendel chose a plant that bred true.
   A true or pure breed continues to show a particular trait in all the offspring in several successive generations of self-fertilisation.
   He made one plant to act as the female by removing the stamens before the ovary was mature and protecting (e.g. by wrapping with paper).
   The female plant from contact with any stray pollen.
   When the ovary was mature, he carefully dusted pollen from the anthers of the selected male plant and transferred it to the stigma of the female plant.
   Observations were then made on the resulting seeds or on the plants obtained when those seeds were planted.

 

Results

•  For each pair of contrasting characters he studied, Mendel obtained the same results.
   For example, when he crossed pure breeding tall plants with pure breeding short plants, the first offspring, known as the first filial generation (FI)were all tall.
   When these were selfed i.e. self-fertilisation allowed to take place, the second generation offspring also know as the second filial generation or F2 occurred in the ratio of 3 tall: 1 short.
   The same ratio was obtained for each of the other characters studied.
   From this it is clear that one character i.e. tall is dominant over the short character.
   A dominant character is that which is expressed alone in the offspring even when the opposite character is represented in the genotype.
   The unexpressed character is said to be recessive.
   From these results and others obtained when he studied two characters at the same time, Mendel concluded that gametes carry factors that are expressed in the offspring.
   These factors are what we know today as genes.
   Mendel put forward the following laws of inheritance:
   Of a pair of contrasting characters, only one can be represented in a gamete.
   For two or more pairs of such contrasting characters, each factor (gene) in the gamete acts independently of the others and may combine randomly with either of the factors of another pair during fertilisation.
   Genetic experiments carried out to date confirm Mendel’s Laws of inheritance
  e. g. T.H. Morgan’s work on inheritance in the fruit fly Drosophila melanogaster.

Terms used in Genetics Genotype:

• The genes present in an individual. The genetic constitution of an individual.It is expressed in alphabetical notation.e.g TT,Tt

Phenotype:

•  The observed character or appearance i.e. the expression of the genes in the structure and physiology of the organism.
   In some cases the phenotype is the product of the genotype and the environment. Phenotype is expressed in words.eg TALL,SHORT,RED WHITE .etc.

Alleles:

•  These are alternative forms of the same gene that control a pair of contrasting characters e.g. tall and short.
   They are found at the same position or gene-locus on each chromosome in a homologous pair.

Homozygous:

 

• This is a state where the alleles in an individual are similar e.g. TT (for tall)

Heterozygous:

• This is a state where the alleles are dissimilar i.e. each of the two genes responsible for a pair of contrasting characters are present
• g. Tt. (T for tall; t for short)

Hybrid:

• This is the offspring resulting from crossing of two individuals with contrasting

Hybrid vigour or Heterosis:

• The hybrid develops the best characteristics from both parents
• e. it is stronger or healthier, or yields more than either parent.

 

Use of Symbols

• To represent genes in the chromosomes, letters are
• It is customary to use a capital letter for the dominant characteristic and small letter for the recessive
• The gametes are
• For example,a cross between a tall and a short pea plant is illustrated as follows;
  • Let –T- represent gene for
  • Let –t- represent gene for

Fertilization-using checker board or Punnet square F1 genotype Tt

F1 Phenotypic ratio =All tall.

F2 Genotype TT,2Tt,tt

F2 Phenotypic ratio;3 Tall;1 short

Test Cross or Back Cross

• This is a eras made between the F 1 bearing the dominant trait with the homozygous recessive
• It is called a back cross because of using the first

It is also a test cross because it tests the genotype of the individual.

 

Complete Dominance

• Mendel happened to choose characters that showed complete dominance,
• e. the dominant trait completely masked the recessive one in the F1generation.
• In man, certain characters are inherited in the same way
• g. colour of the skin; normal colour is dominant to albinism (lack of skin pigment).

 

• The children are all normal but have the gene for
• Such individuals are referred to as

Other characters that show complete dominance in humans are:

• Ability to roll the
• Polydactyly (having more than 5 digits in one limb).
• Brachydactyly – having short
• Achondroplasia – dwarf with bow legs.

Incomplete Dominance

• In this kind of inheritance there is no dominant or recessive gene but the two are expressed equally in the offspring,
• Resulting in blending of the
• The gene for red colour (R) in cattle and the gene for white colour(W) show incomplete dominance or co-dominance.

 

• The offspring are neither red nor write but are intermediate between the
• They are said to be
• In humans, the sickle cell gene and the normal gene are co-dominant.

 

Inheritance of ABO blood groups in humans

• Blood groups in human are determined by three alleles, A, B, and
• An individual can have only two of these
• Genes A and Bare codominant, while gene 0 is recessive to A and
• These are referred to as multiple

The ABO Blood Group System Rhesus Factor

 The Rhesus factor is responsible for the presence of a protein (Antigen D) in the red blood cells.
 If blood from a Rhesus positive (Rh+) person is transferred into a person without the Rhesus factor (Rh-);
 The recipients’ body produces antibodies against the Rhesus factor.
 This causes agglutination of red blood cells which can be fatal if subsequent transfusion with Rh+ blood is done.

Sex Determination in Humans

 XY type e.g. human male
 In males, two types of sperms are produced.
 Half of then containing X chromosomes and half Y chromosomes.
 During fertilisation only one sperm fuses with the egg.
 If it is an X-carrying sperm then a female zygote is formed;
 If it is a Y-carrying sperm then a male zygote is formed.
 It follows then that the chances of getting a boy or girl are half or fifty-fifty.
 Note also that it is essentially the type of sperm that fertilises the egg that determines the sex.

Linkage

 The term linkage describe the situation where genes or certain characters are located on the same chromosome.
 Offspring produced by sexual reproduction show only the parental characteristics and only sometimes few new recombinants.
 i.e. offspring with combinations of characteristics not found in either of the parents due to crossing over in first prophase of meiosis.
 Genes are said to be linked when they are located close together on the same chromosome such that they are always inherited together.

Sex linked genes

 These are genes that are located on the sex chromosomes.
 Sex-linkage – refers to carrying of the genes on the sex-chromosome.
 Gene for a trait may be present, yet offspring does not show the trait.
 This happens in human females (XX) where a gene for the trait is recessive.
 The female acts as a carrier.

In human, sex linked characters found on the X chromosome include:
Haemophilia:

 This is a disease that affects the rate of clotting of blood, leading to excessive bleeding even from a minor cut.
 Haemophilia is more common in males than in females.
 A female my have the gene for haemophilia and not show the trait because the normal gene is dominant over the gene for haemophilia.
 Such females are referred to as carriers.
 If the carrier female offspring will be carriers while the other half will be normal.
 Half the males will be normal and the other heamophilic.

Red-green colour-blindness

 Red-green colour-blindness is caused by a recessive gene found on the X chromosome.
 It is inherited in the same way as haemophilia.
 More males 1:10,000, less female 1: 100 million afflicted.
 It is the inability to distinguish between red and green colours in humans.

Genes found on y-chromosome include:

 Hairy pinna and hairy nose are carried on the Y – chromosome.
 Premature balding.

Mutations

 Mutations are sudden changes in the genotype that are inherited.
 Mutations are rare in nature and mutated genes are usually recessive to the normal (wild type) genes.
 Most mutations are generally harmful and some are lethal.
 A somatic mutation is a genetic change in somatic cells.
 Somatic mutations are only inherited if asexual reproduction takes place e.g. as in plants and unicellular animals.
 A gene mutation is a change in genes of reproductive cells and is always inherited.
 The resultant individual is called a mutant.
 The mutant has different characteristics from the rest of the population.

Types of Mutations

 Chromosomal mutations – are changes in number or structure of chromosomes.
 Gene mutations – also called point mutations – are changes in the chemical nature of the gene.

Mutagens:

 These are agents that cause mutations.
 The include ultra-violet light, Gamma rays., x-rays and cosmic rays.
 Certain chemicals e.g. mustard gas and colchicines also induce mutations.

Causes and consequences of chromosomal mutations

 There are three main types of chromosomal mutations.
 Changes in the diploid number of chromosomes (allopolyploidy).
 The diploid number changes to 3n (triploid) or 4n (tetraploid) and so on.
 This results from the doubling of the chromosome number in the gamete (2n).
 This is due to failure of the chromosome sets to separate during meiosis.
 The phenomenon is known as polyploidy.
 It is common in plant’s and has been employed artificially to produce varieties of crops with hybrid vigour e.g. bread wheat is hexaploid (6n). This is allopolyploidy).
 Change in the total number of chromosomes involving the addition or loss of individual chromosomes (autopolyploidy).
 This is due to failure of individual chromosomes to separate during meiosis.
 One gamete gains an extra chromosome while the other loses a chromosome.
 The term non-disjunction is used to describe the failure of chromosomes to separate.

Non-disjunction results in several disorders in humans:
Down’s syndrome
• The individual has 47 chromosomes due to non-disjunction of chromosome 21.
• It is also known as trisomy 21.
• The individual has slanted eyes with flat and rounded face, mental retardation and large tongue and weak muscles.
Turner’s Syndrome

• This brings about to a sterile and abnormally short female.
• It is due to loss of one of the sex chromosomes
• i.e. the individual has one X chromosome (44 + X) instead of two (44 + XX).

Klinefelter’s Syndrome
• This results in a sterile male who may be mentally retarded.
• It is due to an additional X chromosome
• i.e. the individual i.e. 47 chromosomes (44 + XXY) instead of 46 (44 + XY).

Changes in the structure of a chromosome during meiosis.
• A portion of a chromosome may break off and fail to unite again or it may be joined in the wrong way or to the wrong chromosome.

These mutations are described as follows:
Deletion:

• This is the loss of a portion of a chromosome,
• Deletion results in individuals born with missing body parts .
• e.g. limbs in the extreme of cases.

Inversion:
• A portion may break from a chromosome and then rejoin to it after turning though an angle-of 1800.
Translocation:
• This is when a portion is joined to a non-homologous chromosome.

Duplication:
• A certain section of an intact chromosome replicates such that the genes are repeated.

Gene Mutations
• A gene mutation is a change in the structure of a gene.
• It may involve only a change in one base, e.g. adenine in place of thyamine yet the effect on the individual is profound e.g. sickle cell anemia .
• There are two main type of gene mutations:
 Due to insertion or deletion of one or more (base) pairs.
 Substitution of base pairs e.g. purine for pyrimidine.

Genetically inherited disorders in humans
• Albinism is a mutation that alters the gene responsible for synthesis of skin pigment (melanin).

• The gene for albinism is recessive.
• Sickle cell anemia is a common condition in Kenya.
• Individuals with the sickle-cell gene produce abnormal haemoglobin.
• It is due to gene mutation caused by substitution of the base adenine for thymine.
• The result is the inclusion of the amino acid valine (in place of glutamic

acid) in the haemoglobin synthesised.

• As a result the red blood cells become sickle shaped when oxygen concentration becomes low i.e. inside tissues.
• This leads to blockage of capillaries.
• Tissues do not get sufficient oxygen.
• Homozygous individuals are seriously anaemic and die in early childhood.
• Heterozygous individuals have a mixed population of normal and sickled red blood cells.
• They are not seriously anaemic and can lead fairly normal lives.

• Haemophila (bleeder’s diseases) is due to lack of gene for production of proteins responsible for blood clotting.

Practical Applications of Genetics

• Study of genetics has been put into a wide variety of uses en-compasing plants and animals and in particular humans.

Blood transfusion

• Blood groups are genetically determined.
• As discussed earlier a person of blood group A can only get blood from another one of A or O.
• In case of emergencies and unavailability of blood, a patient may be given blood group A + when he/she is A-.
• First transfusion is fine since, by the time enough antibodies are produced most of the red blood cells of donor have completed their lifespan but a subsequent transfusion of A+ blood is fatal.

Plant and Animal breeding

• Genetics is applied mostly in plant and animal breeding in order to produce varieties that are most suitable to man’s needs.
• This is done through artificial selection.
• Varieties are developed that are resistant to pests, diseases or harsh climatic conditions.

Genetic counselling

• Genetic counselling involves advising about hereditary diseases and disorders so that they can make informed decisions.

This is done through:

• Taking family history.
• Screening for genotypes e.g. through amniocentesis.

• In amniocentesis, cells are obtained from amniotic fluid during pregnancy.
• Conditions such as Down’s syndrome can be detected using microscopy.

Genetic Engineering

• This is a technology that involves the manipulation of the genotype of an organism to get the desired trait.
• It also involves the transfer of gene coding for the desired trait from one organism to another.

Application of Genetic Engineering

Pharmaceutical industries:

• Making of hormones e.g. Human insulin and human growth hormone.
• Enzymes e.g. Alph-Anti-Trypsin (AAT) used to treat emphysema. (c) Proteins.
• Drugs and vaccines.

Agricultural industries:

• Transgenic animals and plants are produced which are also called Genetically Modified Organisms (GMO’s).
• A variety of tomato with improved paste and a longer shell life.
• Sheep for producing desired proteins in milk.
• Plants resistant to pests and diseases.

Cloning

• This is the making of identical copies of genes, DNA and whole organisms.
• Cloning is used in plants – that is tissue culture e.g. in development of various varieties of bananas and Eucalyptus trees.
• The first mammal to be cloned successfully was Dolly – the sheep.
• A nucleus from the cell obtained from the udder of the sheep was inserted in an unfertilised egg without a nucleus.
• This zygote was introduced into the uterus of a sheep and developed to full term.

Gene therapy

• Involves injecting genes into patients of certain diseases
• e.g. Parkinson’s diseases.
• The injected gene alters metabolism to bring about the cure of the disease.

 

Practical Activities

Todemonstrate Continuous variations

Height of students

• Students should work in pairs, use chalk and metre rule to mark level of top of head onto the wall

• Or door as one student stands straight without shoes, next to the wall or door.
• The height for each student is recorded on chalk board.
• The frequency distribution of height is recording as the height is grouped into various classes.
• A histogram to represent frequency against height is drawn.
• The normal bell shaped curve is observed.

Discontinuous variations – ability to roll tongue

• The number of students who can roll their tongue is recorded as well as the number of non-tongue rollers.
• The ratio of tongue-rollers to non tonguerollers is worked out.
• Gene for the ability to roll the tongue is dominant, therefore is expected more tongue rollers.

Demonstration of Mitosis and Meisosis Mitosis

• Plasticene is used to represent number and shapes of various chromosomes

e.g. 8 in Drosophila melanogaster.

• Each stage of mitosis illustrated e.g. interphase,
• Each is rolled to appear long is and coiled, prophase is each made into a ball and then shaped to the appropriate length; and split into two to represent chromatids.
• Centromeres for different chromosomes can be illustrated in different positions.
• Each stage of mitosis is illustrated and telophase can be illustrated by surrounding the “chromosomes” with a long many drawn plasticene to represent cell membrane.
• It is manipulated to show how telophase takes place.

Meiosis

• The same procedure is followed.
• Plasticine with contrasting colours is used to show clearly gene mixing in crossing over.
• Each pair of homologous chromosomes is represented by plasticene with two different colours e.g. red (paternal) blue for maternal chromosome.
• All the steps in the two stages of meiosis are illustrated up to the production of four haploid gametes.

Human Finger Prints

• The finger prints for each student’s thumb, forefinger and middle fingers of the left hand is imprinted on a white paper.
• A rubber stamp with ink is used to and each finger -tip phalange is rolled onto the inkpad.
• For best results students work in pairs.

• Observations are made at all forefingers, thumb prints and differences noted.
• The main patterns are noted. It is also noted that no two, fingerprints are exactly similar.

EVOLUTION

Meaning of Evolution and Current Concepts

• Evolution is the development of organisms from pre-existing simple organisms over a long period of time.
• It is based on the similarities in structure and function that is observed in all organisms.
• All are made up of cells, and similar chemical compounds are present.
• This indicates that all organism may have had a common origin.
• Evolution seeks to explain the diversity of life and also to answer the question as to the origin of life, as well as its present state.

The Origin of Life

• Human beings have tried to explain how life began.

Currently held views are listed below:

• Special creation -life was created by a supernatural being within a particular time.
• Spontaneous generation life originated from non-living matter all at once. e.g. maggots arise from decaying meat.
• Steady state – life has no origin.
• Cosmozoan – life on earth originate from elsewhere, outer space.
• Bio-chemical evolution-life originated according to chemical and physical laws.
• Only special creation and chemical evolution will be discussed.

Special Creation

• The earliest idea is that of special creation which is recorded in the old testament (Genesis 1: 1-26).
• It states that God created the world and all living things in six days.
• Some hold the six days literally, while others say it may represent thousands of years.
• According to his theory, the earth and all organisms were created mature.
• Similarities in structure and function denote the stamp of a “common Designer”
• Evidence for this view arises from observations of life itself.
• Faith explains it all.
• By faith we understand that the universe was created by the command of God.
• Several scientists hold this view and their research confirms accounts in the old testament of a universal flood explains the disappearance of dinosaurs as vegetation decreased.

 

 

 

 

 

 

• The bones making the foot and hand are metatarsals and metacarpals respectively.
• The bones of toes and of fingers i.e. phalanges
• Observe the various modifications of these bones in the various animals.
• Limbs of different mammals e.g. rabbit, cow, donkey reveal that the anatomy is adapted to mode or type of movement .
• e.g. the horse has a single digit.
• An outdoor activity to observe various sty les of movement in different mammals can be studied.
• It is noted that some move on tips of toes (donkey) others on the whole leg (rabbit).

Comparision of Wings of bird-and insect

• Wings of birds and insects (grasshopper, butterfly or moth) are obtained.
• A hand lens or a dissecting microscope is used to observe the specimens.
• The differences in their anatomy is noted.
• Insect wings are membranous while those of birds are made up of feathers that interlock.

Education tour to Archeological site/local Museum

• Visits to the local museum yield important information that greatly supplement study of evolution.
• The National museum in Nairobi has many fossils.
• Visit to the various archeological sites that exist in Kenya is recommended.

RECEPTION, RESPONSE AND CO-ORDINATION IN PLANTS AND ANIMALS

Introduction

• The structures involved in detecting the changes may be located far away from the ones that respond.
• There is need for a communication system within the body.
• The nervous system and the endocrine system perform this function,
• i.e. linking the parts of the body that detect changes to those that respond to them.

Irritability

• Living organisms are capable of detecting changes in their internal and external environments and responding to these changes in appropriate ways.
• This characteristic is called irritability, and is of great survival value to the organism.

Stimuli

• A stimulus is a change in the internal or external environment to which an organism responds.
• Examples of stimuli include light, heat, sound, chemicals, pH, water, food, oxygen and other organisms.

Response

• A response is any change shown by an organism in reaction to a stimulus.
• The response involves movements of the whole or part of the body eithertowards the stimulus or away from it.
• It also results in secretion of substances e.g. hormones or enzymes by glands.

Co-ordination

• Co-ordination is the working together of all the parts of the body to bring about appropriate responses to change in the environment.

Reception

• Reception is the detection of changes in the environment through receptors.

 

 

 

 

 

 

• Sensory nerves link the sensory cells (receptors) to the central nervous system and transmit nerve impulses from a sense organ to the CNS.

Structure and Functions of Neurons

• A nerve cell consists of a cell body (centron) where the nucleus is located, and projections called dendrites arise.
• One of the projections is drawn out into an axon i.e. the longest process.
• Each axon contains axoplasm which is continuous with the cytoplasm in the cell body.
• The axon is enclosed in a fatty myelin sheath which is secreted by Schwarm cell.
• The myelin sheath is interrupted at approximately 1 mm intervals by constrictions known as nodes of Ranvier.
• The myelin sheath is enclosed by a thin membrane called the neurilemma,

which is part of the Schwann cell in contact with axon.

• The myelin sheath and nodes of Ranvier enhance transmission of the impulse.

There are three types of neurons:

Sensory neurone

• Also known as afferent neurone.
• Transmits impulses from sensory cells to the CNS.
• The cell body of a sensory nerve cell is located at some distance along the length of the axon outside the CNS.

 

 

Motor neurone

• • Known as efferent or effector neurone
  • Transmit impulses from the CNS to the effectors(muscles and glands)
  • Its cell body is located inside the CNS.

Intermediate or connector neurone

• • Also called relay neurone
  • Found inside the CNS.
  • The connect sensory and motor neurons with each other and with other nerve cells in the CNS.

 

Functions of the neurone

• • The nerve impulse is electrical in nature.
  • Its transmission depends on differences in electrical potential between the inside and the outside of the axion.
  • The outside is positive while the inside is negative.
  • The stimulus triggers a change that affects the permeability of neurone membrane.
  • The result is a change in the composition of ions on either side of the membrane.

 

• • • • The outside becomes negative as the inside becomes positive due to sodium ions rushing in.
      • The above constitutes a nervous impulse which is transmitted along the sensory neurone to the CNS.
      • The speed of transmission is very high.
      • Certain mammalian axions transmit impulses at the rate of 100m/s.
      • The dendrites of neurons do not connect directly to each other, but they leave a small gap called synapse.
      • The transmission of an impulse from one cell to the next takes place through synapse.
      • Synaptic knobs are structures found at the ends of dendrites.
      • Thus the dendrites of one nerve cell make contact with the dendrites of the adjacent nerve cell through the synapses.
      • Impulses are transmitted in the form of a chemical transmitter substance which crosses the gap between one dendrite and the next.
      • The transmitter substance is found within synaptic vesicles.
      • The chemical substance is eitheracetylcholine or noradrnaline.
      • The synaptic vesicles burst and release the transmitter substance when an impulse arrives at the synaptic knob.
      • Impulses in motor neurone s are trans mitted to effectors.
      • The space betweenmotor end dendrite and muscle is known as neuro- muscular Junction.

    Functions of Major Parts of the Human Brain

     

    • • The Central Nervous System (CNS) consists of the brain and the spinal cord.
      • The CNS co-ordinates body activities by receiving impulses from sensory cells from different parts of the body.
      • It then sends the impulses to the appropriate effectors.
      • The brain is enclosed within the cranium or braincase.
      • It is covered and protected by membranes known as meninges.
      • When meninges are infected by bacterial or fungi they cause meningitis.

    The brain consist of the following parts:

    Cerebrum.

    • • This is the largest part of the brain.
      • It consists of two cerebral hemispheres.
      • It is highly folded in order to increase the surface area.
      • The cerebrum controls learning, intelligence, thought, imagination and reasoning.

    The medulla oblongata (brain stem).

    • The medulla oblongata has centres which control breathing (ventilation) rate,
    • heart beat rate (cardiac frequency),
    • swallowing, salivation, blood pressure
    • temperature regulation, hearing, taste and touch.

    The cerebellum

    • Is located in front of the medulla and is a folded dorsal expansion of the hindbrain.
    • It controls posture movement and balance.

    The hypothalamus

    • Controls functions such as body temperature and osmoregulation.

    The pituitary gland

    • Is an endocrine organ that secretes a number of hormones which control osmoregulation, growth, metabolism and sexual development.

    Optic lobes-control the sense of sight. Olfactory lobes-control the sense of smell. Spinal Cord

     

    • • • The spinal cord is located within the vertebral column and consist of the following:
        • The grey matter forms the central part of the spinal cord.
        • It consists of nervecell bodies and intermediate nerve fibres.
        • The white matter of the spinal cord carries sensory nerve fibers while the ventral root carries motor nerve fibers.

    Simple And Conditioned Reflex Actions

     

    Simple Reflex Action

    • • • A simple reflex action is an automatic response to a stimulus.

    The route that is followed by impulses during a reflex action is called a reflex arc.

    

  • • Lung cancer caused by nicotine.
    • Emphysema.
    • Liver cirrhosis -caused by alcohol.
    • Interferes with vision – alcohol.
    • Sterility – khat (rniraa).
    • Sleeplessness – insomnia – khat (miraa).
    • Hallucinations – Canabis sativa (Bang i).
    • Digestive system is upset, nausea.
    • Diarrhoea and vomiting.
    • Headache and double vision.
    • Skin tone changes – e.g. too dark.
    • Appetite is extreme – very poor or very great.
    • Weight loss.
    • Personality changes e.g. irritable and confused.
    • Convulsions, lethargy and depressions due to inhalation of solvents e.g. glue.
      

      A reflex action follows the following sequence:

      • A receptor is stimulated and an impulse is transmitted along a sensory nerve fibre to the spinal cord.
      • The impulse is picked up by an intermediate neurone within the CNS.
      • The intermediate nerve fibre transmit the impulse to a motor nerve fibre which is connected to an effector.
      • The effector responds.

      Examples of reflex action include:

      • • Pulling the hand away from a hot object.
        • The knee jerk.

       

      • • Sneezing.

      Conditioned Reflexes

      • These are learned responses.
      • When two or more stimuli are presented to an animal at the same time and repeatedly, the animal eventually responds to either stimulus.
      • For example, if a hungry animal is presented with food, it will respond by salivating.
      • If a bell is rung at the same time as the food is presented to the animal, the animal will learn to associate the sound of the bell with food.
      • Eventually, the animal can be made to salivate at the sound of the bell alone.
      • This response is called conditioned reflex and is one of the ways by which animals learn.

      The Role of Endocrine System in Human Beings

      • Endocrine system consists of glands that secrete hormones.
      • The glands have no ducts and are known as endocrine glands.
      • Other glands are known as exocrine glands because they have ducts.
      • The pancreas has an outer exocrine portion and an inner endocrine portion.
      • Hormones are chemical substances, protein in nature which are secreted at one part of the body and have effects on other parts not necessarily near the point of secretion.
      • They are secreted directly into blood and transported by blood.
      • Each hormone either has a generalised co-ordinating effect on the body or brings about a specific response in a particular target organ.

      Hormones produced in humans and the in effects on the body.

      Endocrine gland	Hormone(s) produced	Role of hormone	Effect of deficiency	Effect of excess
      1. Pituitary	Trophic Hormones (i) Somatotropin (Growth Hormone)	Controls growth	Dwarfism	Gigantism
      	(ii) Thyrotrophic Hormone	Controls production of thyroxin by thyroid gland	Same as for deficiency of thyroxin	Same as for excess of tyroxine
      	(iii) Adrenocorticotrophic Hormone (ACTH	Stimulates the activity of adrenal cortex
      	(iv) Follicle stimulating Hormone (FSH)	Development of Graafian follicles in the ovary
      	(v) Luteinising Hormone

       

      	(L.H)
      2. Thyroid	Thyroxine	Regulates the metabolic rate;	Retardation of physical and mental development (cretinism)	High metabolic rate, rapid heartbeat, general wasting of the body: protrusion of eyeballs (exopthalrnic goitre)
      3.1slets of Langerhans in pancreas	(i) Insulin	Regulates blood sugar by causing conversion of glucose to glycogen	Hyperglycaemia (high blood sugar); diabetes mellitus	Hypoglycaemia (low blood sugar)
      	(ii) Glucagon	Regulates blood sugar by causing conversion of gylcogen into glucose	Hypoglycaemia (low blood sugar)	Hyperglycemia (high blood sugar)
      4. Gonads Testis and ovaries	Androgens and	Development of secondary sexual characteristics.	Secondary sexual characteristics fail to develop	In females leads to development of male.? In males leads to development of female characteristics.
      (i) Ovaries	Oestrogen	Repair of uterine wall
      	Progestrone	Causes thickening of wall of uterus; inhibits ovulation during pregnancy; prevents contraction of uterus	Miscarriage when level falls during pregnancy
      (ii)Testis (interstitial cells)	Testosterone	Promote spermatogenesis; and male secondary sexual characteristics.	Male sterility
      5.Adrenal glands’	(i) Adrenaline	Changes in response to fear, stress or shock; increased heartbeat, conversion of glycogen to glucose; dilation of pupils; increased blood flow to skeletal muscles
      	(ii) Hydrocortisone	Metabolism of carbohydrates, lipids and proteins	Less glycogen stored in the liver and muscles
      	(iii) Aldosterone	Promotes retention of sodium chloride and bicarbonate ions	Kidneys excrete too much sodium, chloride and bicarbonate ions

       

       

      Adrenaline

      • Enhance activity of sympathetic nervous system.

      Over secretion

      • Increased heartbeat
      • High blood pressure
      • Thin toneless muscles.

      Under secretion

      • Low blood pressure
      • Inability to withstand stress
      • Muscular weakness

      Thyroxine

      Over secretion is termed hyperthyroidism this causes:

      • Increased Basal Metabolic Rate (BMR) hence increased temperature.
      • Person becomes very angry, nervous and hands may shake.
      • Increased heartbeat which lead to cardiac failure.

      Under secretion is termed hypothyroidism:

      • Poor growth and mental retardation (cretinism).
      • Reduced metabolic rate hence decreased temperature.
      • Person becomes inactive and slothful.
      • Eyes and face become puffy as fluid gets stored under skin.
      • In extreme cases the tongue is swollen and skin becomes rough.
      • Enlarged thyroid gland.

      Comparison between endocrine and nervous system

      	Nervous communication	Endocrine communication
      Speed of Transmission	Usually rapid	Usually slower
      Response	Usually specific in a given Effector	Usually widespread; in some cases in particular target organ.
      Nature of impulse	Electrochemical along Axion; chemical across synapses	Chemical; in the form of a hormone that travels in blood
      Duration of	Usually short lived	Usually long-lasting.e.g.

       

      response

      growth

       

      Similarities

      • Both endocrine and nervous system are involved in the coordination of body functions.
      • Both have target organs.
      • Both are controlled via a negative feedback mechanism, i.e too high production results in a reduced production.

      Effects of drugs abuse on the human health.

      • Drug abuse can be defined as misuse of drugs.
      • Drugs are chemical compounds that affect the working of body or kill disease causing microorganisms.

      Prescription drugs

      • Are drugs prescribed by a doctor.
      • Prescribed drugs can be abused through taking overdose which may cause death.

      Over the counter drugs(OCD)

      • Are self prescribed drugs.
      • These have harmful effects and may lead to tolerance such that higher doses are needed.

      Below is a list of effects of hard drugs on human health

      • Lung cancer caused by nicotine.
      • Emphysema.
      • Liver cirrhosis -caused by alcohol.
      • Interferes with vision – alcohol.
      • Sterility – khat (rniraa).
      • Sleeplessness – insomnia – khat (miraa).
      • Hallucinations – Canabis sativa (Bang i).
      • Digestive system is upset, nausea.
      • Diarrhoea and vomiting.
      • Headache and double vision.
      • Skin tone changes – e.g. too dark.
      • Appetite is extreme – very poor or very great.
      • Weight loss.
      • Personality changes e.g. irritable and confused.
      • Convulsions, lethargy and depressions due to inhalation of solvents e.g. glue.

Structure and Function of Parts of the Human Eye Structure

 

• The human eye is spherical in shape and situated within a socket or orbit in the skull.
• It is attached to the skull by three pairs of muscle, which also control its movement.
• It is made up of three main layers; sclerotic layer, choroid and the light sensitive retina.

Sclerotic layer

• Outermost white part situated at the sides and back of the eye.
• Made up of collagen fibres.
• It protects the eye and gives its shape.

Cornea

• This is the transparent front part of the sclera that allows light to pass through.
• It is curved, bulging at the front. It thus reflects light rays hence helps to focus light rays onto the retina.

Choroid

• The second or middle layer.
• It has many blood vessels that supply nutrients to the eye and remove metabolic wastes from the eye.

 

 

 

 

 

 

 

 

• Closing the left eye, the paper is slowly moved towards the face as the right eye is fixed on the cross.
• At 50 cm distance the cross and the dot are seen clearly.
• As-the paper is moved closer to the face, the dot disappears.
• The distance at which the dot disappears is measured.
• This is the distance of the blind spot.
• When the light rays from the dot are focused on the blind spot it disappears hence the dot is not seen.

The Knee Jerk Experiment

• Students work in pairs, one student sits on the table, high stool or bench with one leg crossed over the other.
• The other student chops the crossed knee just below the knee cap with the edge of palm or wooden ruler.
• It is observed that the crossed knee jerks.
• This is a spinal reflex. END

Support and Movement in Plants and Animals

Movement is a characteristic of all living organisms.

• Most animals move from place to place but some are sessile (i.e. fixed to the substratum).
• However, though not easily observed all living protoplasm shows movement of one type or another.

Necessity for support and movement in plants

• They enable plants to be held upright to trap maximum light for photosynthesis and gaseous exchange.
• It enables animals and plants to adjust to their environment.
• To hold flowers and fruits in appropriate position for pollination and dispersal respectively.
• To enable plants to grow to great heights and withstand forces of environment e.g. strong winds.
• Movement of male gametes to effect fertilization and ensure perpetuation of a species.
• Plant parts move in response to certain stimuli in the environment of tropisms.

Tissue distribution in Monocotyledonous and Dicotyledonous plants

• Vascular bundles are the main support tissues in plants.
• In monocotyledonous stem they are scattered all over the stem.
• while in dicotyledonous stem they are found in a ring or rings.
• In monocots the xylem and phloem alternate around with pith in the centre.
• In dicots of the xylem forms a star in the centre – there is no pith.
• Phloem is found in between the arms of xylem.
• Dicotyledonous plants have cambium which brings about secondary growth resulting in thickening of the stem and root hence providing support.
• Secondary xylem becomes wood, providing more support to the plant.

 

Role of support tissues in young and old plant

Plants are held upright by strengthening tissues ;

• parenchyma,
• collenchyma,
• sclerenchyma
• xylem tissue.
• Parenchyma and collenchyma are the main support tissues in young plants.

 

 

 

 

 

 

 

 

 

To examine bones of the axial skeleton of a rabbit.

• Bones of the vertebra column are obtained.
• These are cervical, thoracic, lumbar and sacral.
• For each of the bones the distinguishing features are listed down.
• Labelled drawings of the anterior and lateral views is made.

To observe bones of appendicular skeleton.

• Bones of pectoral girdle and fore limb are obtained i.e., scapula, humerus, ulna and radius.
• Labelled drawing of each bone is made.
• Observations on how the bones articulate with one another is made.
• Bones of pelvic girdle and hind limbs are obtained i.e., pelvic girdle, femur, tibia and fibula.
• Labelled drawings of each, bone is made.
• The distinguishing features of each bone is noted.
• Observations on how the bones articulate with one another is made.

revision notes biology form 1

revision notes biology form 2

Form 3 Biology Notes

2019 mokasa biology paper 2

More Free KCSE Top National Schools Mock Papers