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Biology Highschool Notes
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This notes provide all the topics and sub-topics taught in biology [a science subject] in highschool syllabus
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BIOLOGYFORM 4
NOTES
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?
, 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
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
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