Topics 8, 14-15: Genetic Basis for Variation
Cell & Nuclear Division
Cell Division consists of:
1. Nuclear Division
a. Process of separating nuclear DNA to daughter cells
b. Can refer either to mitosis or meiosis
2. Cytoplasmic Division
a. the process of separating cytoplasm and organelles to daughter cells
Roles of Mitosis:
1. Unicellular Organisms: Cell division results in genetically identical daughter cells by
asexual reproduction
2. Multicellular Organisms
Cell division enables sexually reproducing organism to grow and develop from a single cell such
as the zygote. After an organism is fully grown, cell division continues to function in renewal and
repair, replacing cells that die from normal wear and tear or accidents.
Ploidy: number of sets of chromosomes within the nucleus of a cell
n: the number of chromosomes in a set
Chromatin: the complex of nucleic acid and associated histone and non-histone proteins; in an
uncoiled and diffused state present during interphase of the cell cycle, or in non-dividing cells
Chromosome: the condensed form of chromatin; additional proteins known as scaffolding
proteins are associated with chromosomes and aid in their condensation; most visible during
mitosis and meiosis
Genes: hereditary units located at specific physical locations along each chromosome i.e. locus
Homologous chromosomes: a pair of chromosomes which are structurally, but not genetically
identical; similar in size, shape, centromere position and sequence of gene loci
Homolog: each chromosome of a pair of homologous chromosomes
Sister chromatids: replicated forms of a single chromosome joined together by the centromere;
structurally and genetically identical; same alleles due to semi-conservative replication
Centromere: the specialised region where two sister chromatids join; associated with
kinetochores for attachment of spindle fibres and is the last place to separate in cell division
Kinetochore: structure formed by proteins on specific sections of the centromere, to which
microtubules of the spindle are attached to, playing an active part in the movement of
chromosomes to the opposite poles
Topics 8, 14-15: Genetic Basis for Variation
Centriole: a barrel-shaped organelle which is found only in animal cells and exists as a cylindrical
pair in the cytoplasm; each member of the pair is composed of nine triplets of microtubules
arranged in a ring; members of the pair are perpendicular to each other, located in the
centrosome. At the onset of mitosis, Centriole pairs duplicate and each pair moves to the opposite
poles of the cell, establishing the two poles of the cell.
Centrosome: a specialised region of the cell that includes a pair of centrioles and the surrounding
cytoplasm, which contains proteins that aid in the assembly of spindle microtubules; aka
microtubule organising centre
Spindle fibres: an organised system of microtubules that attaches to the centromeric regions of
duplicated chromosomes and draws them to opposite poles during eukaryotic cell division:
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Astral spindle fibres – radiate from the centriole towards the peripheral regions of the cells;
only present in cells that contain centrioles; serve as a brace for the functioning of the spindle
fibres
Kinetochore spindle fibres – fibres attached to the kinetochore of the chromatids; pull the
sister chromatids towards the opposite poles of the cell during anaphase
Polar/non-kinetochore spindle fibres – fibres running from pole to pole overlapping at the
equator of the spindle; responsible for elongating the whole cell along the polar axis during
anaphase
Mitosis Cell Cycle (let x be the relative DNA amount)
Interphase
Cytokinesis
Telophase
Prophase
Metaphase
G1 Phase (2n, x): begins after cytokinesis of the
previous cell division. Cells are thus small in size
and low in ATP. Hence, cells increase in size and
acquire ATP during this phase. Intensive cellular
gene expression and synthesis of appropriate
organelles and proteins
S Phase (2n, 2x): Each DNA molecule undergoes
DNA replication, resulting in the production of
Anaphase
two identical molecules of DNA. Each replicated
chromosome now consists of 2 genetically identical sister chromatids held together at the
centromere
G2 Phase (2n, 2x): since the formation of new DNA is an energy-consuming process, the cell
undergoes a second growth and energy acquisition stage. Further synthesis of appropriate
organelles and proteins occurs. Centrioles replicate and mitotic spindle begins to form
Changes during interphase:
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DNA is highly condensed from chromatin to visible chromosomes
Nuclear envelope starts to disintegrated and chromosomes are released into the cytoplasm
Nucleolus no longer present, cell is now transcriptionally inactive
Topics 8, 14-15: Genetic Basis for Variation
Prophase (2n, 2x):
In the nucleus,
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Nuclear envelope disintegrates (breaks up into small vesicles which disperse)
Nucleolus gradually disappears
Chromatin fibres condense by supercoiling to become tightly coiled and folded into
discrete, observable chromosomes
In the cytoplasm,
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In animal cells, centriole pairs migrate to opposite poles of the cell
Kinetochore spindle fibres begin to assemble
Astral spindle fibres are seem radiating from the centrioles
Polar spindle fibres extend from each pole toward the equator of the cell
Metaphase (2n, 2x):
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Centriole pairs are positioned at opposite poles of the cell
Shortening and thickening of the chromosomes is at its maximum, and the two sister
chromatids joined at the centromeres of each chromosome are clearly visible
Kinetochores migrate and align singly at the metaphase plate, which is the plane
equidistant from the spindle poles
They are pulled to the metaphase plate by the action of kinetochore spindle fibres
There is no pairing of homologous chromosomes at the metaphase plate
Anaphase (4n, 2x)
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Begins with the separation of the centromeres of sister chromatids
Chromatids are now known as daughter chromosomes
Daughter chromosomes are pulled to opposite poles of the cell as their kinetochore
microtubules shorten
At the same time, the poles of the cell move further apart as the polar spindle fibres slide
past each other, hence elongating the cell
Special motor proteins are involved in the rapid and abrupt movement of chromosomes
towards the poles of the cell during anaphase
At the end of anaphase, the two ends of the cell have equal and complete set of
chromosomes
Telophase (2n, x)
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Begins when the daughter chromosomes reach the poles of their respective spindles
Condensed chromosomes decondense and uncoil into the chromatin form
Nucleolus and nuclear envelope reform
Results in the two nuclei taking on the granular appearance of interphase
Spindle fibres disassemble
Topics 8, 14-15: Genetic Basis for Variation
Cytokinesis (2n, x)
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Division of cytoplasm to produce two completely separated daughter cells
Cell organelles become evenly distributed towards 2 poles of the parent cell, along with
the chromosomes, during Telophase
Two smaller, genetically identical cells result
Generally begins simultaneously with Telophase
Animal cell: form cleavage furrow, which pinches cell in two. On the cytoplasmic side of the
furrow is a contractile ring of microfilaments. As the ring contracts, the cleavage furrow
deepens until the parent cell pinches into two daughter cells, each with a complete nucleus
and share of cytosol, organelles and other subcellular structures
Plant cell: no cleavage furrow formed. A cell plate grows across the metaphase plate.
Vesicles from the golgi apparatus move to the middle of the cell, where they fuse,
producing a cell plate.
Roles of Meiosis:
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Result in the production four haploid daughter cells, each containing half the number of
chromosomes of the original parent cell
Production of gametes with haploid sets of chromosomes
Maintain the constancy of chromosome number from generation of generation by
preventing the doubling of chromosomal number with each generation
Meiosis Cell Cycle
Interphase
Metaphase
II
Anaphase II
Prophase I
Prophase II
Telophase II
Metaphase
I
Cytokinesis
I
Anaphase I
Telophase I
Meiosis I: known as reduction division where
chromosome number and ploidy level are
reduced by half
 Amount of DNA is the same as that of the
parent cell before replication
Cytokinesis
 Synapsis and crossing-over of homologous
II
chromosomes have occurred and their
subsequent segregation to different daughter cells
Interphase (2n, x -> 2n, 2x)
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Precedes meiosis I and includes DNA replication
This process of replication is similar to the DNA replication preceding mitosis
The chromosomes only replicate once throughout meiosis
Pair of centrioles also replicate during interphase
Prophase I (2n, 2x)
Topics 8, 14-15: Genetic Basis for Variation
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Nucleolus disappears and nuclear membrane disintegrates
Spindle fibres begin to form and kinetochore spindle fibres attach to the centromeres of
chromosomes
Chromatin condenses and thickens until the chromosomes become distinct
Homologous chromosomes pair up and form a bivalent
Four chromatids in each bivalent are collectively known as a tetrad
The physical pairing is known as synapsis, where the homologues are bridged by a
synaptonemal complex
This process is precise and brings the genes on each chromosome into precise alignment
Chiasmata is formed between non-sister chromatids of homologous chromosomes at one
or more points
Crossing-over takes place where non-sister chromatids undergo exchange of genetic
material. As a result, sister chromatids are now genetically non-identical and are known
as recombinant chromatids
Metaphase I (2n, 2x)
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Kinetochore spindle fibres from one pole of the cell attach to one chromosome
(homologue) of each bivalent, while kinetochore spindle fibres from the opposite pole
attach to the other homologue
Pair of homologous chromosomes or bivalents randomly align at the metaphase plate/
equatorial plate
Independent assortment of homologous chromosomes occurs at this stage: when a
bivalent lines up on the metaphase plate, the orientation of homologues towards the
poles in any one bivalent is random, and is independent of that of any other bivalent
Anaphase I (2n, 2x)
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The two homologous chromosomes of each bivalent separate and move towards the
opposite spindle poles of the cell
Homologous chromosomes are segregated to opposite poles, centromeres first,
producing a characteristic “V”-shaped pattern
The centromeres in anaphase I remain intact and the sister chromatids remain attached to
each other
Their physical segregation is referred to as disjunction, meaning the separation of
chromosomes from one another
Telophase I (n, x)
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Chromosomes arrive at opposite poles of the cell
Spindle fibres usually disassemble
Chromatids usually uncoil and a nuclear envelope reforms around each set of
chromosomes
Nuclei formed are haploid because the chromosome number and ploidy level have been
halved
Topics 8, 14-15: Genetic Basis for Variation
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Each of the chromosomes still exists as two chromatids joined at the centromeres, which
may not be genetically identical due to crossing-over
Cytokinesis I (n, x)
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Occurs simultaneously with Telophase I forming haploid daughter cells
Meiosis II: known as equational division as chromosome number does not change, while amount
of DNA in daughter cells are half of that of the parent cell before replication
 Crossing over may have occurred during prophase I of meiosis I, the chromatids in meiosis II
may not be genetically identical to each other
 Variation due to crossing over, independent assortment and random fertilisation
Prophase II (n, x)
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Nucleoli disperse and nuclear envelopes disintegrate
Chromatin undergoes condensation and thickening to reform distinct chromosomes
In animal cells, centrioles move to opposite poles of the cells at the end of prophase II
New spindle fibres appear and are arranged at right angles to the spindle of meiosis I
Metaphase II (n, x)
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Kinetochore spindle fibres attach to the kinetochores at the centromeres of the
chromosomes
Chromosomes migrate and align singly at the metaphase plate/equatorial plate of the cell
Metaphase plate of meiosis II is perpendicular to that of meiosis I
Anaphase II (2n, x)
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Centromeres segregate and the two chromatids segregate to opposite poles
Chromatids are now called daughter chromosomes
Poles of the cell move further apart as the polar spindle fibres slide past each other,
hence elongating the cell
Telophase II (n, ½ x)
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Chromosomes uncoil and decondense
Spindle fibres disassemble
Nuclear envelope reform around each nucleus
Cytokinesis (n, ½ x)
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4 haploid genetically non-identical gametes are produced
Topics 8, 14-15: Genetic Basis for Variation
Genetic Basis for Variation
Gene: a unit of inheritance located at a particular locus of a chromosome, with a particular locus
of a chromosome; it determines the phenotype of an individual
Locus: the specific location of a gene on a chromosome
Genotype: complete genetic makeup of an organism; term used in reference to the paired alleles
carried by an organism that give rise to a phenotype
Phenotype: physical manifestation of a genetic trait that results from a specific genotype and its
interaction with the environment
Wild type: the phenotype most common in nature
Allele: An alternative form of a gene at a particular gene locus responsible for determining
contrasting traits of the same character; all alleles of a gene determine the same character, but
each has a unique nucleotide sequence, which may result in different phenotype
Dominant Alleles: produce their effects in both the homozygous and heterozygous condition;
mask the influence of the recessive allele
Recessive Alleles: produce their effects only in the homozygous condition
Homozygous: condition in which the alleles of a gene pair in a diploid condition are identical
Heterozygous: condition in which the alleles of a gene pair in a diploid condition are different
Heredity: transmission of genetic characteristics from one generation to the next and the effects
of this transmission
Variation: recognisable differences between individuals of the same species and even between
parents and offspring
Hybridisation: mating or crossing of two true-breeding varieties
Mendel’s First Law of Segregation is based on Mendel’s work involving monohybrid crosses
(Monohybrid inheritance).
Mendel’s Second Law of Independent Assortment is based on his work involving dihybrid crosses
(Dihybrid inheritance), stating segregation of one pair of alleles is independent of the
segregation of other pairs.
Topics 8, 14-15: Genetic Basis for Variation
Linking Genotype to Phenotype
During gene
expression, an
allele is
transcribed to
form mRNA;
after which the
mRNA is
translated to
form polypeptide
Different alles will
result in the
production of
different
polypeptides and
hence, different
proteins/
enzymes
Different
proteins/enzymes
have different
effects on a
metabolic pathway,
resulting in
different
phenotypes
eg. dominance,
codominance,
incomplete
domicance
Inheritance of genes
DNA replication
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During S phase of interphase, genes on chromosomes are replicated; via semi-conservative
DNA replication
In asexually
reproducing
organisms
• through mitosis, daughter chromosomes are separated into
daughter cells with the same number of chromosomes as their
parents
• In metaphase I, independent assortment of homologous
chromosomes occurs; resulting in new combination of paternal
and maternal chromosomes/ alleles
In sexually
reproducing • At the end of meiosis I, homologous chromosomes are then
separated into daughter cells
organsims
• At the end of meiosis II, daughter chromosomes are separated
into haploid gametes
• In random fertilisation, fusion of gametes forms new individuals
in the next generation and genes are passed on
• this restores the diploid number of chromosomes in the next
generation
Monohybrid Inheritance: the inheritance of a single character of contrasting traits
Incomplete Dominance: condition in which neither of the two alleles is completely dominant to
the other, so that the heterozygote has a phenotype which is intermediate
E.g. Colour of Snapdragon
Postulate that the allele CR allows for the production of a functional enzyme required for the
synthesis of red pigment and CW for a non-functional enzyme. Heterozygotes possess only one
Topics 8, 14-15: Genetic Basis for Variation
copy of the allele per cell and hence produce inadequate enzyme to synthesize enough red
pigment as compared to a homozygote. Consequently, heterozygotes are pink.
Codominance: condition in which both alleles are equally expressed in the phenotype of the
heterozygote. The heterozygote simultaneously expresses the phenotypes of both types of
homozygotes
E.g. coat colour of short horn cattle
Both alleles of a gene code for functional products. Both products appear in the phenotype of the
heterozygote. In the example, the heterozygote has a roan coat that consists of a mixture of red
and white hairs.
**for incomplete dominance, the heterozygote phenotype is intermediate between the two
homozygote phenotype, while for codominance, the heterozygote phenotype is not intermediate,
but equal expression of both parental traits
Lethal Genes: cause death, frequently at an early developmental stage
Multiple Alleles: three or more alleles controlling a characteristic in a population e.g. ABO blood
group in humans
Dihybrid Inheritance: Inheritances of two pairs of contrasting characters at the time in each
dihybrid cross
Monohybrid Crosses
DD x dd
Dd x Dd
Dd x dd (Test cross)
CDCd x CDCd
Lethal Genes
Dihybrid Crosses
AABB x aabb
AaBb x AaBb
AaBb x aabb (Test cross)
Genotypic Ratio
All Dd
1DD : 2Dd : 1dd
2Dd : 2dd
1CDCD : 2CDCd : 1CdCd
2Dd : 1dd
Genotypic Ratio
All AaBb
9A_B_ : 3A_bb : 3aaB_ : 1aabb
4A_B_ : 4A_bb : 4 aaB_ : 4aabb
Phenotypic Ratio
All heterozygous dominant
3:1
1:1
1:2:1
2:1 (1/4 of offspring dies)
Phenotypic Ratio
All heterozygous dominant
9:3:3:1
1:1:1:1
Sex Chromosomes
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the 23rd pair of XY chromosomes
Sex Linkage: the carrying of genes on the sex chromosomes
X chromosome contains many loci that are required in both sexes, whereas the Y
chromosome contains only a few genes
Genes located on the X chromosome are known as sex-linked genes because they follow
the transmission pattern of the X chromosome
E.g. haemophilia, red-green colour blindness and Duchenne muscular dystrophy in humans,
and white eye colour in Drosophila
Topics 8, 14-15: Genetic Basis for Variation
Human X-linked disorders
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Sex-linked inheritance tend to affect males as they are always hemizygous for every sexlinked locus
As males possess only a single X chromosome, every X chromosome allele present is
expressed
Haemophilia: reduced ability of blood clotting, due to deficiency of one of the blood
clotting factors
Haemophilia is more common in males than in females because males need only one copy
of the defective of the defective allele to suffer from haemophilia whereas females require
two copies of the defective allele to be affected
Duchenne Muscular Dystrophy: X-linked, recessive condition affecting muscle
development as allele for DMD codes for an enzyme that induces the replacement of
muscle by fibre
Red-green colour blindness: a father with the recessive X-linked allele will transmit the
allele to all daughters but not to any sons because his son will inherit his Y chromosome
only; chromosome from mother
Reciprocal Cross: a pair of crosses in which the traits of the two parents are reversed
conducted to discern if a trait is carried on a sex chromosome (X-linked) or on an
autosomal chromosome
Results: male transmits the X chromosome only to his female offspring (all normal)
female transmits an X chromosome to both male and female offspring (normal
female, affected male)
Autosomal Recessive Inheritance
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Gene of interest for the trait is carried on the autosomes
A recessive trait only becomes phenotypically apparent when two similar alleles of a gene
are present
If both parents are affected, all children should be affected
Traits often skip generations
Unaffected parents can produce affected individuals
Autosomal Dominant inheritance
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The phenotype it codes for will be expressed even if the individual is heterozygous
Unaffected parents should not have affected children
Traits should not skip generations
Appear in almost equal numbers among both sexes
Eg. Huntington Disease
Linkage (Mendel’s Law does not apply): when genes are situated on the same chromosome
Topics 8, 14-15: Genetic Basis for Variation
Complete Linkage: assumption that 2 genes are located so closely together on the same
chromosome that they tend to be inherited together as one unit because no chiasma can be
formed in between them. As a result, no reshuffling of alleles to form new combinations of alleles
Incomplete Linkage: genes that are located some distance apart from each other on the same
chromosome can be separated when crossing over during meiosis
*as crossing over is a random process, the separation of coupled alleles will occur in some cases
but not in other. Therefore, offspring produced show a majority of parental allele combinations
and hence phenotypes and a minority of recombinant allele combinants and hence phenotypes
Detection of Linkage
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test cross with a double homozygous recessive individual can be performed to detect if
genes are unlinked, completely linked or incompletely linked
if genes are unlinked, 4 different phenotypes in the ratio of 1:1:1:1 are produced
if genes are completely linked, 2 phenotypes in the ratio of 1:1 are produced
if genes are incompletely linked, 4 phenotypes with a larger percentage of parental
phenotypes and smaller percentage of recombinant phenotypes are produced
Coupling: two dominant alleles are on one chromosome and two recessive ones are on the
homologous partner
Repulsion: dominant allele is linked with recessive on one chromosome
Chromosome Mapping
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chance of crossing over occurring between two linked genes on the chromosome is
proportional to the distance between them
the further apart two linked genes are, the greater the statistical chance that crossing over
will separate them than if they were closer, and therefore the greater the proportion of
recombinants that will be formed
Cross-over value: recombination frequency
COV = (number of individuals showing recombination)/(total number of offspring) x 100%
Causes of Genotypic/Genetic Variation
Process
Meiosis &
Sexual
Reproduction
Mechanism
Crossing over, independent assortment and
separation of homologous chromosomes of
independent arrangement and separation of
chromatids and random fertilisation
Gene Mutation Deletion, insertion and substitution
Chromosomal Deletion, duplication, inversion and translocation;
mutation/
change in number of chromosomes due to nonaberration
disjunction
Effects
Reshuffles existing alleles
to form new genetic
combination; does not
result in new alleles
May result in new alleles
May or may not results
in new alleles
Topics 8, 14-15: Genetic Basis for Variation
Effect of Environment on Phenotype
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Degree of expression of genes may be influenced by the environment in which the
organism develops
Effect of Temperature: Coat colour in Himalayan rabbits
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All Himalayan rabbits are homozygous for the Ch allele of the gene coding for a heatsensitive form of an enzyme, tyrosinase, which is needed for melanin production
Tyrosinase is active then the air temperature is below 33oC, thus there is growth of black
fur
The fur-producing cells will not produce the melanin pigment when exposed to higher
temperatures, thus hair of the rabbits appear light/white
Heat from environment prevents the development of black fur
Effect of soil acidity on Hydrangea Macrophylla
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Hydrangea may have different flower colours, despite carrying the same alleles
The soil acidity in which the plants grow affects the plants’ ability to take up aluminium
In acidic soils (pH 5.5 or lower), aluminium assumes a form that is easily absorbed by plant
roots, and thus flowers are predominantly blue
In alkaline soils (pH 6.5 or higher), aluminium is unavailable and flower colour is pink
purple
Sometimes a single plant has both blue and pink flowers due to varying soil condition
around the plant
Gene interactions: the idea that two or more genes influence one particular character
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Various gene products function in a metabolic pathway that contributes to development of
one particular phenotype
Two independently assorting genes may interact to influence a single character
Biochemical basis of comb shape in chickens: different combinations of alleles from the
two genes result in different phenotypes of a single character, presumably due to the
interaction of their gene products, each of which contributes to the comb shape at the
biochemical or cellular level
Epistasis: when the expression of an allele of one gene suppresses/inhibits the expression of
alleles of a different gene at a different locus; suppressed gene is termed the hypostatic gene
*Dominant/recessive/duplicate recessive epistasis
Topics 8, 14-15: Genetic Basis for Variation
Type of interaction
Non-epistatic
Recessive epistasis
Dominant epistasis
Duplicate recessive
epistasis
Duplicate interaction
Duplicate dominant
epistasis
Dominant and
recessive epistasis in
dihybrid cross
Phenotypic
Ratio
A_B_
9:3:3:1
9
9:3:4
9
12:3:1
9:7
Genotype
A_bb aaB_ aabb
3
3
1
3
4
12
3
7
9
9:6:1
15:1
9
6
13:3
13
(including
aabb)
1
Example
Comb shape in chickens
Coat colour in Labrador
retrievers/mice
Colour in summer squash
Flower colour in sweet
pea
1
1
15
3
Variation describes the recognisable differences in characteristics between organisms of the same
natural population or species
Discontinuous variation
Observable  Definite and clear cut; can be
phenotype
divided into discrete phenotypic
classes
 Intermediates are not observed
No. of genes  Single or a few genes with 2 or
controlling
more alleles
phenotypic
variation
Effect of
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environment
on phenotype
Mode of
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phenotypic
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measurement
Examples
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Continuous variation
 Not clear cut and cannot be divided into
distinct contrasting groups
 Range of phenotypes observed
 Intermediates are observed
 Controlled by the combined effect of
multiple additive genes and are thus known
as polygenic inheritance
 Genes act on phenotype in an additive
manner, producing combined effects
Little to no environmental
 Phenotypes can be modified by the
effect on phenotypic expression
cumulative effect of varying environmental
eg. ABO blood group
factors acting on the different genotype
 Degree of expression allowed to genetic
potential hinges on environmental factors
eg. height
Bar graphs
 Normal distribution curve
Qualitative: counts and ratios
 Quantitative: population parameters such
as mean and standard deviation
Inheritance of height in pea
 Height, weight, and intelligence in humans
plants
Chi-square test: statistical test for the significance of data that consists of discontinuous/discrete
variables; assumes:
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Random fertilisation
Equal opportunity of survival among offspring
Large number of offspring produced
Topics 8, 14-15: Genetic Basis for Variation
Null Hypothesis: there is no significant difference between the observed and expected results. It
assumes that any differences are due to chance
Alternative Hypothesis: there is significant difference between the observed and expected results.
It assumes that differences are not due to chance
χ2calc
(O − E)2
=∑
E
If 𝛘𝟐𝐜𝐚𝐥𝐜 > 𝛘𝟐𝐜𝐫𝐢𝐭 , the probability that chance alone is the reason for the difference of the observed
from the expected results/ratio is less than 5%. The deviation is significant; hence reject the null
hypothesis in favour of alternative hypothesis.
If 𝛘𝟐𝐜𝐚𝐥𝐜 < 𝛘𝟐𝐜𝐫𝐢𝐭 , the probability that chance alone is the reason for the difference of the observed
from the expected results/ratio is more than 5%. The deviation is not significant; hence do not
reject the null hypothesis.