FORM IV TOPICS
1. 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?
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, • 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
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, • 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
i) 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
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, 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
ii) 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
iii) 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
www.arena.co.ke 0713779527
1. 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?
www.arena.co.ke 0713779527
, • 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
www.arena.co.ke 0713779527
, • 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
i) 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
www.arena.co.ke 0713779527
, 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
ii) 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
iii) 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
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