- a) i) Define the term genetics
- the study of heredity(inheritance) and variation or study of mechanisms by which characteristics are passed from parents to offspring
iii) List some characteristics which are inherited
- size
- height/length
- colour/type
- shape
- yield
iii) State the importance of genetics
- helps to explain differences between organisms of the same species
- helps to explain the transmission of characters from generation to generation
- improvement in livestock
- improvement in crops
- can be used to treat some difficult diseases
b) i) Explain the meaning of the following terms
Heredity
- the resemblance among individuals related by descent
- transmission of traits from parents to offspring
Trait
- also called character
- A character of the organism e.g. type of ear, colour of eyes, height, yield etc.
Gene
- unit of inheritance
- it is the heredity factor which transmits traits from parents to offspring
- genes are located at fixed points on chromosomes
- each point is called a locus (loci)
Allele
- genes can exist in a series of alternative forms at a particular locus
- allele refers to alternative forms of genes controlling a particular characteristic
Chromosomes
- threadlike structures found in nuclei of all plants and animals
- they carry genes which are hereditary materials
- they consist of substances called DNA and proteins called histosones
DNA
- deoxyribonucleic acid
- substances that make up chromosomes
- double helix(strand) molecule that contains genes
- DNA consists of nucleotides
- A nucleotide consists of an inorganic phosphate, ribose sugar and a base
- There are four bases in a DNA molecule i.e. Adenine(A), guanine(G), thymine(T) and cystosine (C)
- Ribose sugar has four bases attached to it i.e. adenine, cystosine, guanine and thymine
- Adenine pairs with thymine while guanine pairs with cystosine
- Nucleotide initiates and controls protein synthesis
ii) List the types of chromosomes
- somatic (body) chromosomes also called autosomes
- sex chromosomes (related to reproduction)
c) i) What is variation?
- sequence of differences occurring among individuals of the same species
ii) State the causes of variation in organisms
- random assortment of genes during meiosis
- crossing over
- fertilization
- doubling of chromosome numbers(mutation)
- environmental conditions
iii) Name the types of variation
- Continuous variation (differences not clear cut) e.g. height, length, weight, skin colour, intelligence etc. They are quantitative and show intermediates
- discontinuous variation(differences are clear cut) e.g. ability to roll tongue, ABO blood grouping system, RH factor, patterns of fingerprints, and ability to taste PTC. They are qualitative and have no intermediates
iv) Explain the following terms
Acquired characteristics
- they are as a result of adaptations due to the environment and are not inherited
Inherited characteristics
- are passed down to offspring during sexual reproduction
Genotype
- genetic constitution of an individual/genetic makeup
Phenotype
- characteristics of an individual observed or discernible by other means i.e. observable character
Dominant gene (character)
- expressed in the phenotype when homozygous or heterozygous
Recessive gene
- only expressed in homozygous state
Homozygous
- when two alleles are identical e.g. LL,ll
Heterozygous
- when two alleles are different at a particular locus e.g. Ll
F1 and F2
- F1 means first filial generation i.e. the first generation produced when two varieties can be crossed
- F2 means second generation i.e. product of offspring or from F1 generation
d) i) Explain Mendels first law of inheritance
- also called law of segregation
- it states that genes are responsible for the development of individual characters
- these characters are transmitted individually without any alterations
- Only one character from a contrasting pair can be carried in a gamete, hence only one character can be inherited.
ii) Give an example of this law
- In an experiment, Drosophila (fruit fly) with long wings were crossed with those having short wings. Assume letter L denotes gene for wing size. The gene for long wings is dominant to that for short wings
- the genes for dominant are LL and for recessive ll.
- State the expected results for the first cross
iii) What is monohybrid inheritance?
- when inheritance of one character is studied one at a time e.g. wing size only
- the F2 generation (when selfed) always gives a phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1 in a complete dominance
- What is complete dominance?
- refers to where only one dominant character is expressed while the other character which is recessive is not expressed in the heterozygous state e.g. the case of wing size above
e) i) What is meant by co dominance?
- When genes produce independent effects when heterozygous/none of the genes is dominant over the other/where two or more alleles does not show complete dominance/recessiveness due to the failure of any allele to be dominate in a heterozygous condition.
ii) Give an example of co dominance
In a certain plant species, some individual plants may have only white, red or pink flowers. In an experiment a plant with white flowers was crossed with a parent with red flowers. Show results of F1 generation. Use letter R for red gene and W for white gene.
If the plants form F1 were selfed, work out the phenotype ratio for the F2 generation
Phenotypic ratio 1red:2pink:1white
Genotypic ratio 1:2:1
f) i) What is a test cross?
– A cross between an individual showing a character for a dominant gene(that is homozygous or heterozygous) with a homozygous recessive individual
OR
- a cross between individual(organism) of unknown genotype with a homozygous recessive individual
ii) State the importance of a test cross in genetics
- helps in determining the genetic constitution/genotype of an organism
iii) What are multiple alleles?
- a set of more than two alleles that may determine a character
- example is blood group which can be determined by any two of three alleles i.e. A,B and O
iv) Explain the inheritance of ABO blood groups
- in humans blood groups are determined by three alleles i.e. A,B and O
- it is only possible to have two genes at a time
- genes A and B are co-dominant while gene O is recessive to genes A and B
Give a worked example using parents with heterozygous blood groups AO and BO
- Explain the inheritance of Rhesus factor (Rh) in human beings
- in humans blood is either Rh positive or Rh negative
- people who have Rh antigen are Rh(+ve) while those without Rh antigen in their blood are Rh(-ve)
- Rh(+ve) is due to a dominant gene while the recessive gene causes lack of Rh factor. When a person who is homozygous dominant marries a person who is homozygous recessive the result is as shown below
Let the gene for dominant Rh factor be R while gene for recessive be r
- How is sex determined in human beings?
- there are two sex chromosomes in humans, x and y
- males are xy and females are xx
- in females all ova have x chromosome
- in males 50% of sperms contain x chromosomes while 50% of sperms contain y chromosome
- when a sperm containing x chromosome fuses with an ovum this results into a girl
- when a sperm containing y chromosome fuses with an ovum the result is a boy
- an example is given below
g) i) What does the term linkage mean?
– These are genes which occur together on a chromosome and are passed to offspring without being separated
ii) Define the term sex-linked genes
- genes carried in the sex chromosome that are transmitted along with genes that determine sex
iii) What is meant by the term sex linkage?
- genes are located on the sex chromosome
- they are transmitted along with those that determine sex
iv) Name the sex-linked traits in humans
- colour blindness
- haemophilia
- Hairy ears. pinna, nose
- Baldness
- Duchene muscular dystrophy (DMD) muscular wasting
v) Give an example of a sex linked trait in humans on:
Y Chromosome
- tuft of hair sprouting from pinna/baldness
X Chromosome
- colour blindness/haemophilia
vi) In humans red-green colour blindness is caused by a recessive gene C, which is sex-linked. A normal man married to a carrier woman transmits the trait to his children. Show the possible genotypes of the children.
Let C represent the gene for normal colour vision (dominant)
Let c represent the gene for colour blindness
Parental phenotype Norman man x carrier woman
- State the importance of sex linkage
- possible to determine sex of day old chicks
- Haemophilia is due to a recessive gene. The gene is sex-linked and located on the x chromosome. The figure below shows sworn offspring from phenotypically normal parents
What are the parental genotypes?
- XY and XhX
Work out the genotypes of the offspring
h) i) What is mutation?
- sudden change in the structure of DNA at a particular locus/chromosome/gen
ii) Describe how mutations arise
- mutations arise due to alterations in normal number of chromosomes
- change in a portion of a chromosome affecting one or more genes
- by chromosomal aberration e.g. dleltion/duplication/substitution/inversion/translocation/crossing over
- caused by mutagenic agents e.g. radiation (x-rays, ultra violet light, gamma rays) and chemicals e.g. mustard gas/colchicines
iii) State the factors that may cause mutation
- these are chemicals and radiations
Radiations Effects
X-rays gene/chromosome alteration
Ultra violet rays structural distortion of DNA
Chemicals Effect
– colchicines prevents spindle formation
Cyclamate chromosome aberrations
Mustard gas chromosomes aberrations
Nitrous acid adenine in DNA is deaminated so behaves like guanine
Acridone orange addition and removal of bases of DNA
Formaldehyde
iv) State the characteristics of mutations
- arise suddenly
- are unpredictable
- random
- generally rare
- may breed true
- some are desirable while others are lethal
v) Explain chromosomal mutation
– Change in nature, structure or number of chromosomes
vi) Explain how the following types of chromosomal mutations occur
Duplication
- a section of a chromosome is repeated/replicates
- therefore genes are repeated
Inversion
- occurs when chromatids break at 2 places and when rejoining the middle piece rotates and joins in an inverted position
Deletion
- portion of a chromosome is left out after it breaks off
- alters number and sequence of genes
Translocation
- occurs when a section of a chromatid breaks off and becomes attached to another chromatid of another chromosome
Non-disjuntion
Failure of homologous chromosomes/sister chromatids to separate/segregate during meiosis
Polyploidy
- where number of chromosomes double or triple
- beneficial in plants due to the following
- increased yields/hybrid vigour//heterosis
- resistance to pests
- early maturity
- resistance to drought
- resistance to diseases
vii) What are gene mutations?
- an alteration in the structure of a gene
- Explain how the following occur during gene mutation
Deletion
- some bases/nucleotides of a gene are removed
Inversion
- the order of some bases/nucleotides of a gene is reversed
Insertion
- addition of a base between two existing bases
Substitution
- a portion of a gene is replaced by a new portion
- Name the disorders in humans caused by gene mutation
- albinism
- sickle cell anaemia
- achondroplasia/chondordystorphic dwarfism
- haemophilia
- colour blindness
- phenylketonuria
- State the practical applications of genetics
- Breeding programmes (research)
- high yielding/hybrid vigour/heterosis
- resistance to diseases
- resistance to drought/salinity
- early maturing
- Genetic engineering
- genetic manipulation to produce desired characteristics
- Law
– legal questions of paternity knowledge of blood groups or blood transfusion
iv) Genetic counseling
- aimed at reducing harmful traits e.g. albinism, congenital idiots, colour blindness e.t.c
v) Others
– Pre-sex determination
Understanding human evolution and origin of other species.
- a) i) Explain the meaning of evolution
- a gradual change in living organisms from simple life forms to more complex forms over a long period of time.
ii) Differentiate organic evolution from chemical evolution as theories of origin of life
- organic evolution refers to the emergence of present forms of organisms gradually from pre-existing forms (some of which no longer exist)
- chemical evolution explains the origin of life as having occurred when simple chemical compounds reacted to form the simplest life forms
iii) What is special creation?
- maintains that the whole universe and all living organisms came into being due to the act of a supernatural being
b) Discuss the various kinds of evidence for evolution
- Fossils
- fossils are remains of organisms preserved in naturally occurring materials for many years
- they give evidence of types of plants/animals that existed at certain geological age/long ago/millions of years ago
- gives evidence of morphological/anatomical/structural changes that have taken place over a long period of time e.g. human skull, leg of horse
- Comparative anatomy
- gives evidence of relationship among organisms/gives evidence of a common ancestry of a group of organisms
- organisms have similar structures/organs performing the same function e.g. digestive system/ urinary system/nervous system/vestigial structures and vertebrate heart
- Divergence where the basic structural form is modified to serve different functions e.g. vertebrate forelimb/beak structure in birds/birds feet/parts of a flower. These are called homologous structures
- homologous structures have a common embryonic origin but are modified to perform different functions e.g. the pentadactyl limb
- adaptive radiation is a situation where organism have a homologous structure with common embryonic origin which is modified to perform different functions to adapt organisms to different ecological niches/habitats e.g. beaks of Darwinian finches(birds)
- Convergence is where different structures are modified to perform a similar function e.g. wings of birds and insects/eyes of humans and octopuses. These are called analogous structures
- Vestigial structures are greatly reduced in size and have ceased to function e.g. human appendix/caecium/coccix in humans, wings of kiwi (flightless bird), presence of hind limb pad in python, halters in insects, human hair nictitating membrane in human eye, human ear muscle, pelvic girdle in whale and third digit of wing of bird.
- Comparative embryology
- some embryos of different animals appear very similar thus showing relationship and possibility of a common ancestry
- e.g. different classes of vertebrates larvae of annelida and mollusca are similar (tocophere)
- Comparative serology/physiology
- these show biochemical and immunological comparisons of blood groups/components to show immunological similarities of tissues therefore showing relatedness of different organisms
- e.g. antigen antibody reactions, human blood groups/Rh factor reveal some phylogenic relationship among organisms/common ancestry
- Geographical distribution
- organisms differ in various geographical regions
- present continents are thought to have been a large land mass joined together/pangea/Eurasia/Gondwanaland
- present continents drifted apart from one land mass/continental drift
- as a result of continental drift isolation of organisms occurred bring about different patterns of evolution
- organisms in each continent evolved along different lines hence emergence of new species/divergence/convergence
Examples
- marsupials in Australia
- illama, jaguar, panther in S. America
- lion, camel in Africa
- tiger in Asia
- Cell biology (cytology)
- structures and functioning of cells are similar
- occurrence of organelles e.g. mitochondria in all cells/both plant and animal cells
- these point at a common ancestry
c) i) State the evolutionary characteristics that adopt human beings to the environment
– Brain
– Eyes
– Upright posture/bipedal locomotion
– prehensible arm/hand
– Speech
ii) State the ways in which Homo sapiens differs from Homo habilis
- standing upright/erect posture
- intellectual capacity/higher thinking capacity/bigger brain/higher brain capacity
- communication through language/speech
d) i) Explain Larmarck’s theory of evolution
- Inheritance of acquired characteristics/environment induces production of a favorable trait which is then inherited
ii) Explain why Lamarck’s theory of evolution is not accepted by biologists today
- evidence does not support Lamarck’s theory
- acquired characteristics are not inherited/inherited characteristics are found in reproductive cells only
iii) Explain Darwin’s theory of evolution
- inheritance of genetically acquired characteristics
- a character happens to appear spontaneously which gives advantage to an organism therefore adapted then inherited through natural selection
e) i) What is natural selection?
– Organisms with certain characteristics are favoured by the environment
Such organisms tend to survive and produce viable offspring
Others not favored are eliminated from subsequent generations
ii) With examples, explain how natural selection takes place
- organism with certain characteristics are favored by their environment
- such organisms tend to survive and produce viable offspring
- others not favored are eliminated from subsequent generations
- as the environmental conditions change the survival value of a character may alter with time so that characteristics which were favored may no longer have advantage and other characters may then become favorable
- if a favorable character is inherited, then offspring produce generations which are better adapted to survive in a population
- more offspring are produced than can survive which results in struggle for survival
- the fittest survive
iii) State the advantages of natural selection to organisms
- assist to eliminate disadvantageous characteristics/perpetuates advantageous characteristics
- allows better adapted organisms to survive adverse changes in the environment/less adapted organisms are eliminated
iv) State the ways in which sexual reproduction is important in the evolution of plants and animals
- brings about useful variations/desirable characters
- variations make offspring better adapted for survival/more resistant to diseases
- may lead to origin of new species
v) Explain the significance of mutation in evolution
– Mutation bring about variation which can be inherited
– Some of these variations are advantageous to the organism
– Others are disadvantageous
– The advantageous variations favour the organism to compete better in the struggle for survival
– This results into a more adapted organism to its environment or new species/varieties
– Those with disadvantageous characters will be discriminated against therefore eliminated from the population/death/perish
vi) Plain why it is only mutations in genes of gametes that influence evolution
- gametes form the new offspring
vii) How would you prove that evolution is still taking place?
- resistance of organism to antibiotics, pesticides and drugs
- new varieties of bacteria are resistant to certain antibiotics such as penicillin
- houseflies and mosquitoes are resistant to DDT
- Explain why some bacteria develop resistance to a drug after they have bee subjected to it for some time
- bacteria mutates/develops a new strain/chemical composition is altered hence is able to produce enzymes/chemicals which degrade the drug rendering it non-susceptible to the drug
- the new strain is favoured by selection pressure/ natural selection
f) How has industrial melaninism i.e. peppered moth contributed towards the mechanism of evolution
– This is an example of natural selection
– The peppered moth exists in two distinct forms, the speckled white form (normal form) and a melanic form (the black/dark)
– They usually rest on leaves and barks of trees that offer camouflage for protection
– Originally the “speckled white” form predominated the unpolluted area of England
– This colouration offered protection against predatory birds
– Due to industrial pollution tree barks have blackened with soot
– The white form underwent mutation
– A black variety/mutant emerged suddenly by mutation
– It had selective advantage over the white forms that were predated upon in the industrial areas
– The speckled white form is abundant in areas without soot/smoke
3. a) i) Define irritability, stimulus and response irritability
-also called sensitivity
– Responsiveness to change in environment
Stimulus
A change in the environment of organism which causes change in organism’s activity
Response
- change in activity of an organism caused by a stimulus
ii) State importance of irritability to living organisms
- Adjusting to environmental conditions. Sensitive/defect/responding
- List the examples of external stimuli to organisms
- air/oxygen (aero)
- light(photo)
- osmotic pressure (osmo)
- current (Rheo)
- chemical concentration (chemo)
- \water/moisture (hydro)
- Touch/contact (hapto/thigmo)
- Gravity/soil (geo)
- Temperature (thermo)
b) i) What are tactic responses?
- response in which whole organism or its motile parts move e.g. gamete
ii) What causes tactic responses?
- caused by unidirectional stimulus
- usually doesn’t involve growth
- response is either positive or negative
- named according to source of stimulus
- e.g phototaxis, aerotaxis, chemotaxis
iii) State the importance of tactic response to:
Members of kingdom protista
- move towards favorable environment/move away from unfavorable environment
- move towards their prey/food
Microscopic plants
– escape injurious stimuli/seek favorable habitats
- Name the type of response exhibited by:
Euglena when they swim towards the source of light
- phototaxis
- sperms when they swim towards the ovum
- chemotaxis
- State the advantages of tactic responses to organisms
- to avoid unfavorable environment/injurious stimuli
- escape from predators
- to seek favorable environment
- to seek for food/prey
c) i) Define the term tropism
- growth movement of plants in response to external unilateral/unidirectional stimuli
ii) Explain the various types of tropism in plants
Phototropism
- growth movements of plant shoots in response to unilateral sources of light
- the tip of the shoots produce auxins down the shoot
- light causes auxins to migrate to outer side/darker side causing growth on the side away from light hence growth curvature towards source of light roots are negatively phototrophic
Geotropism
- response of roots/parts of a plant to the direction of force of gravity
- auxins grow towards the direction of force of gravity causing positive geotropism in roots while shoot grows away from force of gravity (negatively geotrophic)
Thimotropism/Haptotropism
- growth response of plant when in contact with an object
- contact with support causes migration of auxins to outer side causing faster growth on the side away from contact surface
- this causes tendrils/stem to twin around a support
Hydrotropism
- growth movement of roots in response to unilateral source of water/moisture
- the root grows towards the source of water/ positively hydrotropic while leaves are negatively hydrotropic
chemotropism
- growth movement of parts of plant to unilateral source of chemicals
- the chemicals form a gradient between two regions e.g. pollen tube growing towards the ovary through the style
iii) State the ways in which tropisms are important to plants
- expose leaves/shoots in positions for maximum absorption of sunlight for photosynthesis
- enables roots of plants to seek/look/search for water
- enables plant stems/tendrils to obtain mechanical support especially those that lack woody stems
- enables roots to grow deep into the soil for anchorage
- enables pollen tube grow to embryo sac to facilitate fertilization
iv) Explain the differences between tropic and tactic responses
| Tropisms | Taxes |
| growth curvature in responseslowinfluenced by hormones | locomotory responsefastexternal influence |
