UNIT 15 VARIATION
UNIT 15: VARIATION
Key Unit Competence
Explain variation and mutation as a source of biodiversity
Learning objectives
At the end of this unit, I should be able to:
– Explain population traits and types of variation.
– Describe the differences between continuous and discontinuous variation.
– Describe the causes of variation.
– Explain the genetic basis of continuous (many additive genes control
characteristics) and discontinuous variation.
– Explain, with, examples, how the environment may affect the phenotype of
plants and animals.
– Explain why genetic variation is important in selection.
– Interpret graphs of variations in blood groups and height.
– Construct genetic diagrams to show how sickle cell anaemia is inherited.
– Use a t-test to compare the variation of two different populations (see
mathematical requirements for the syllabus).
– Appreciate the significance of genetic variation in selection.
– Express that discontinuous variation results in a limited number of phenotypes
with no intermediates e.g. tongue rolling.– Justify the effect of the environment on the phenotype of plants and animals.
Introductory activity
The diagrams below represent the beetles, maize and giraffes. Observe andanalyze them carefully and answer the following questions.
a. Explain why the beetles, maize and giraffes have different colors?
b. How do you call the biological term indicated by the above diagrams?
15.1 Variation
Activity 15.1
Use the school library and search additional information on the internet, read
the information related to variation. Based on the readings answer to the
following questions:
1. Describe what variation concept is, in your own words.2. Describe the mechanism and importance of variation.
The earth is inhabited by billions of organisms, every one of which is unique.
Individuals belonging to different species are usually easy to distinguish. Members
of the same species may differ only in small ways; but even clones (such as identical
twins) show some differences. The differences between individuals of the same
species are called variation. These differences between cells, individual organisms,
or groups of organisms of any species are caused either by genetic differences
(genotypic variation) or by the effect of environmental factors on the expression
of the genetic potentials (phenotypic variation). Variation may be seen in; physical
appearance (phenotype of individuals), metabolism, fertility, mode of reproduction,behavior, learning and mental ability, and other obvious or measurable characters
15.1.1 Origins of variation
Genotypic variations are caused by differences in number or structure of
chromosomes or by differences in the genes carried by the chromosomes. Eye color,
body form, and disease resistance results by genotypic variations. Individuals with
multiple sets of chromosomes are called polyploid. Many common plants have two
or more times the normal number of chromosomes and new species may arise by
this type of variation.
Variation may be due to either environmental factors or genetic disorders. For
example, the action of sunlight on a light- colored skin may result in its becoming
darker. Such changes have little evolutionary significance as they are not passed
from one generation to the next. Much more important to evolution are the inherited
forms of variation which result from genetic changes. These genetic changes may
be the result of the normal and frequent reshuffling of genes which occurs duringsexual reproduction, or as a consequence of mutations.
15.1.2 Importance of variation
Variation plays different roles such as:
– Make some individuals better fitted in the struggle for existence.
– Help the individuals to adapt themselves according to the changing
environment.
– Produce new traits in the organisms.
– Allow breeders to improve races of useful plants and animals for increased
resistance, better yield, quicker growth and lesser input.
– Constitute the raw material for evolution.
– Give each organism a distinct individuality.
– Species do not remain static. Instead, they are slowly getting modified forming
new species with time.
– Pre-adaptations caused by the presence of neutral variations are extremely
useful for survival against sudden changes in environment, e.g., resistanceagainst a new pesticide or antibiotic.
Application 15.1
Explain the origin and the importance of variation
15.2 Types of variation
Activity 15.2
Use the school library and search additional information on the internet,
read the information related to types of variation. Furthermore, get outside
classroom and then observe species day after day for at least 10 days. You need
to collect data every day from what you observe. You can even use a ruler or
a weighing machine where applicable. based on what you have done before
answer to the following questions:
1. Write in your own words the differences that exist between types of
variation2. Using a genetic cross, show that sickle cell anaemia is inherited
Variation does occur into two categories namely; genetic and phenotypic as
described in detailed below.
Genetic variation
Genetic differences reflect the genotype (the genetic make-up of an
individual organism, an individual ‘s genotype functions as a set of instructions for
the growth and development) of an organism, that is, its genetic make-up. A diploid
organism has two sets of chromosomes and two forms (alleles) of each particular
gene. These alleles may be the same (the organism is homozygous for that gene) or
different (the organism is heterozygous for that gene). If different, one of the alleles
(the dominant allele) may mask the other allele (the recessive allele). The dominant
allele is therefore expressed in either the heterozygous or the homozygous condition.
If an organism is haploid (that is, it has only one set of chromosomes), all its alleles
will be expressed and will be reflected in its observable or measurable characters(the features or traits transmitted from parent to offspring).
There are three primary sources of genetic variation:
1. Mutations are changes in the DNA. A single mutation can have a large effect,
but in many cases, evolutionary change is based on the accumulation of
many mutations.
2. Gene flow is any movement of genes from one population to another and
is an important source of genetic variation.
3. Sex can introduce new gene combinations into a population. This geneticshuffling is another important source of genetic variation.
Why is genetic variation important for evolution?
Variation is one of the main things that drive evolution. First, there are limited
resources available, and there is just not enough; food, water, shelter, etc. available
for all organisms. Second, to make matters worse, most species have many offspring
that can possibly survive. Just think of how many insect eggs are laid compared
to the number that make it to adulthood. This leads to competition for the limitedresources.
Not all individuals in a species are the same. There are variations in; size, speed,
coloration, etc. These small variations can help or hinder individuals in their survival.
These variations are caused by small differences in genes. Organisms that have
helpful variations are more likely to survive. On average, they get more food, get
better shelter, etc. Coloration can help a predator get closer to prey and eat better.
Or, for the prey species, coloration can make it harder for predators to find and eat
it. So, organisms that have helpful variations tend to survive better, and reproduce
more. As they reproduce, their genes (including the helpful genes) become more
common in the gene pool, and these variations spread out more and more.
Phenotypic variation
The measurable physical and biochemical characteristics of an organism, whether
observable or not, make up its phenotype (observable physical or biochemical characteristics
of an individual organism, determined by both genetic make-up and environmental
influences, for example, height, weight and skin color). The phenotype
results from the interaction of the genotype and the environment. The genotype
determines the potential of an organism, whereas the environment factors to which
it is exposed determine to what extent this potential is fulfilled. For example, in humans
the potential height of a person is genetically determined, but a person cannot
reach this height without an adequate diet. Phenotypic variation is of two maintypes: continuous and discontinuous.
a. Continuous variation
Continuous variation is variation which does not show clear cut differences i.e.
it shows a gradual change from one extreme to another. Characteristics such as;
human height and weight show continuous variation, and are usually determined
by a large number of genes (i.e. polygenic) and/ or considerable environmental
influence. Some examples of continuous variation are: Height, weight, heart rate,
finger length, and leaf length. They are also called fluctuating variations because
they fluctuate on either side (both plus and minus) of a mean or average for the
species. Continuous variations are typical of quantitative characteristics. They
show differences from the average which are connected with it through small
intermediate forms. If plotted as a graph, the mean or normal characteristic will
be found to be possessed by maximum number of individuals. The number of
individuals will decrease with the increase in degree of fluctuation. The graph (figure
15.1) will appear to be bell shaped. The variations are already present in differentorganisms or races of a species.
Figure 15.1: Continuous variations or fluctuations in the height of adult human beings
Continuous variations are produced by:
– Segregation of chromosomes at the time of gamete or spore formation.
– Crossing over or exchange of segments between homologous chromosomes
during meiosis.
– Chance combination of chromosomes during fertilization.
Therefore, these variations are also known by the name of re-combinations. They
make an organism better fitted to struggle for existence in a particular environment.
They also enable human beings to improve the races of important plants and
animals. However, they are unable to form a new species.
Continuous variations are of two types:
a. Substantive: They influence appearance including; shape, size, weight and
color of a part or whole of the organism, for example., height, shape of nose,
skin color, color of eyes, hair, length of fingers or toes, yield of milk, eggs, etc.
b. Meristic: They influence the number of parts, for example, number of grains in
an ear of wheat, number of epicalyx segments in Althaea, tentacles in Hydra or
segments in earthworm, etc.
c. Discontinuous variation
Discontinuous variation is variation where there is a clear cut difference with no
intermediates between individuals e.g. blood groups (A, B, AB, or O), Rhesus factor
(+ve or –ve), mice coat colour, gender, eye colour in drosophila, haemophilia, tongue
rolling, flower colour, seed shape, pawpaw tree sex (male or female) etc. Such
variations are represented in a bar graph as shown in Figure 15.2. Such variations
are controlled by a single gene or many alleles of the same gene. Continuous
variations are usually quantitative (they can be measured) whereas discontinuous
variations are qualitative (they tend to be defined subjectively in descriptive terms).
Thus height in humans is a continuous variation given a value in meters, whereas
height in sweet peas is a discontinuous variation described as tall or dwarf. Such
discontinuous variations are not changeable and neither can environment changethem.
Discontinuous variations are caused by:
– Chromosomal aberrations like; deletion, duplication, inversion and
translocation,
– Change in chromosome number through aneuploidy and polyploidy,
– Change in gene structure and expression due to addition, deletion or changein nucleotides.
Figure 15.2: Discontinuous variation of blood groups
Sickle-cell anaemia an example of discontinuous variation
It is caused by the substitution of a single amino acid in molecular structure of RBCs.
When the oxygen content of an affected individual is low (at high altitude or under
physical stress), the sickle cell Hb deforms the RBCs to a sickle shape. Sickling of the
cells, in turn, can lead to other symptoms.
Individuals who are heterozygous (having a single copy of the allele) for the sicklecell
allele are said to have sickle-cell trait. They carry a normal life but suffer some
symptoms of sickle-cell disease when there is an extended reduction of blood
oxygen. Although the sickle-cell anaemia is lethal for homozygous, the sicklecell
trait (heterozygous) is sometimes considered as an advantage. People who
are heterozygous are resistant to malaria. Thus, in tropical Africa, where malaria iscommon, the sickle-cell allele is both beneficial and an afflicition.
Genotype for sickle cell anemia
Most genes, including the β-globin polypeptide gene, have several different alleles.
For the moment, only the two alleles of this gene are considered. For simplicity, the
different alleles of a gene can be represented by symbols. In this case, they can berepresented as follows:
HbA = the allele for the normal β-globin polypeptideHbS = the allele for the sickle cell β-globin polypeptide
The letters Hb stand for the locus of the haemoglobin gene, whereas the superscripts
A and S stand for particular alleles of the gene. In a human cell, which is diploid, there
are two copies of the β-globin polypeptide gene. The two copies might be: HbAHbA
or HbSHbS or HbAHbS. The alleles that an organism has form its genotype. In this
case, where we are considering just two different alleles, there are three possible
genotypes.Table 15.1: Genotype for sickle cell anemia
Inheriting genes
In sexual reproduction, haploid gametes are made, following meiosis, from diploid
body cells. Each gamete contains one of each pair of chromosomes. Therefore, each
gamete contains only one copy of each gene. Think about what happens when
sperm are made in the testes of a man who has the genotype HbAHbS. Each time a
cell divides during meiosis, four gametes are made, two of them with the HbA allele
and two with the HbS allele.
Of all the millions of sperm that are made in his lifetime, half will have the genotype
HbA and half will have the genotype HbS. Similarly, a heterozygous woman will
produce eggs of which half have the genotype HbA and half have the genotype HbS.
This information can be used to predict the possible genotypes of children born to
a couple who are both heterozygous. Each time fertilisation occurs, either an HbAsperm or an HbS sperm may fertilise either an HbA egg or an HbS egg.
Table 15.2: The possible results of genotypes and phenoty
Figure 15.3: Meiosis of a heterozygous cell produces gametes of two different genotypes. Only one pair of
homologous chromosomes is shown.
As there are equal numbers of each type of sperm and each type of egg, the chances
of each of these four possibilities are also equal. Each time a child is conceived, there
is a one in four chance that it will have the genotype HbAHbA, a one in four chance
that it will be HbSHbS and a two in four chance that it will be HbAHbS. Another way of
describing these chances is to say that the probability of a child being HbSHbS is 0.25,
the probability of being HbAHbA is 0.25, and the probability of being HbAHbS is 0.5.
It is important to realize that these are only probabilities. It would not be surprising
if this couple had two children, both of whom had the genotype HbSHbS and so
suffered from sickle cell anaemia.
The major distinctions between continuous and discontinuous variations in
inheritance are as follows:
Continuous variations have the following characteristics:
– The variations fluctuate around an average or mean of species.
– Direction of continuous variations is predictable.
– They are already present in the population.– Continuous variations are formed due to chance segregation of chromosomes
during gamete formation, crossing over and chance pairing during fertilization.
– They can increase adaptability of the race but cannot form new species.
– Continuous variations are connected with the mean or average of the species
by intermediate stages.
– The continuous variations are also called fluctuations.
– When represented graphically, continuous variations give a smooth bell
shaped curve
– They are very common– Continuous variations do not disturb the genetic system.
Discontinuous variations have the following characteristics:
– A mean or average is absent in discontinuous variations.
– The direction of discontinuous variations is unpredictable.
– Discontinuous variations are new variations though similar variations might
have occurred previously.
– Discontinuous variations are produced by changes in genome or genes.
– Discontinuous variations are the fountain head of continuous variations as
well as evolution
– These variations are not connected with the parental type by intermediate
stages.
– Discontinuous variations are also known as mutations or sports.
– A curve is not produced when discontinuous variations are representedgraphically.
– These variations appear occasionally.
– They disturb the genetic system of the organism
Application 15.2
1. Using a table differentiate between continuous and discontinuous
forms of variation.
2. Draw and interpret graphs of variations in blood groups and height.
15.3 Causes of variation in living things
Activity 15.3
Use the school library and internet to search additional information about
cause of variations. Summarize the information in a table. Share and discuss
with your classmates.
a. Crossing over
Genes are interchanged resulting in new chromosomes (recombinants), different
from the parental combination. Chromosomal crossover (or crossing over) is the
exchange of genetic material between homologous chromosomes that results in
recombinant chromosomes during sexual reproduction. Crossing over and random
segregation during meiosis can result in the production of new alleles or new
combinations of alleles. Portions of paired chromosomes may be exchanged to
form new chromosomal and gene combinations in gametes resulting into new trait
combinations in offspring.
Figure 15.4: Illustration of crossing over
b. Non-disjunction
Non-disjunction results into doubling of the chromosome number due to failure
of chromosomes to segregate during meiosis. This leads to increase in cell size andsubsequent increase in size of various parts of the organism, hence variation.
Figure 15.5: illustration of non-disjunction
c. Random fertilization
Random fertilization that results during the fusion of the gametes also contributes
to variation. Gametes are the egg and sperm, or pollen, produced by meiosis. Each
gamete has a unique set of combination of genes. A male gamete can fertilize any of
the female gametes. The fertilization between a male gamete and a female gamete
occurs randomly in the fallopian tube. As a result, each zygote is unique and hence
variation occurs due to the different combination of genes from the male and female
gamete.
The random fusion of gametes is a source of genetic variation in offspring (with the
same parents). For example, a litter of puppies or kitten (bred) by the same father
will show variation between individuals as shown below.
d. Random mating
Random mating involves individuals pairing by chance, not according to their
genotypes or phenotypes. Random mating is a source of variation in a population.
For example, a population in which mating only occur between organisms of similar
phenotypes, such as red beetles mating with red beetles and yellow beetles mating
with yellow beetles, will tend to show less variation than a population where crosses
are random. For example, red beetles mating with yellow beetles.
e. Mutations
Mutations are sudden and permanent changes in the genes and chromosomes
which are then passed on from cell to cell during mitosis. Such changed genes or
chromosomes will produce offspring that differ from parents.
A mutation is also a change in the amount or the chemical structure of DNA. If the
information contained within the mutated DNA is expressed, it can cause a change
in the characteristics of an individual cell or an organism. Mutations in the gametes
of multicellular organisms can be inherited by offspring. Mutations of the body cells
of multicellular organisms (somatic mutations) are confined to the body cells derived
from the mutated cell; they are not inherited. Mutations can happen spontaneously
as a result of errors in DNA replication or errors during cell division, or they can be
induced by various environmental factors (such as certain chemicals, X-rays, and
viral infection). Factors that induce mutations are called mutagens.
f. Independent assortment of homologous chromosomes
This occurs at the time of gamete formation. At the time of gamete formation during
meiosis, the parental chromosomes separate at random hence forming different
gametes with different chromosomes. This independent assortment gives a wide
variety of different gametes and hence individuals.
g. Environmental factors
These variations are not inherited but are due to environmental factors. The
environmental factors bring about only slight modifications in animals but in plants
the modifications are much more conspicuous. This is due to the environmental
effect on the meristems of various parts. A slight change in the meristematic activity
can have permanent effect on the plant. Environment can also change the amount
of flowering and bring about non- inheritable changes in the floral parts.
1. Light
In the absence of light, the plants remain etiolated. Shade produces elongated
internodes and thinner and broader leaves. It increases the succulence of many
vegetables. Strong light, on the contrary, helps in the production of more mechanical
tissue and smaller and thicker leaves. The effect of light has also been observed
by Cunningham in flat fish Solea. The fish habitually rests on left side. It develops
pigmentation and eyes on right side, the side exposed to sun. If left side is exposed
to sunlight in the young fish, both eyes and pigmentation develop on that side.
2. Temperature
Temperature directly affects the metabolic activity of the organisms and rate of
transpiration in plants. Plants growing in hot area show stunted growth of the aerial
parts and greater growth of the root system. Strong sunlight and high temperature
bring about sun-tanning of human skin by production of more melanin for protectionagainst excessive insulation and ultraviolet radiations.
3. Nutrition
The individual provided with optimum nutrition grows best while the under
nourished shows stunted growth. The abundance or deficiency of a mineral salt
produces various types of deformities in plants. A larva of honey bee fed on royaljelly grows into queen while the one fed on the bee bread develops into worker.
4. Water
Plants growing in soils deficient in water or in areas with little rainfall show
modifications in order to reduce transpiration and retain water, e.g., succulence,
spines, reduced leaves, thick coating, sunken stomata, etc. Those growing in humidand moist area show luxuriant growth.
Application 15.3
1. Outline and explain in your own words any 3 environmental factors
that cause variation1. Distinguish the random fertilization from random mating.
15.4 t-test
Activity 15.4
Search from books or internet to have more information on t-test?
Collect measurements from populations of organisms in two varying sites
and use t-tests to distinguish whether or not these are likely to represent twodistinct populations.
The t- test is used to test the statistical significance of continuous variables. The t-test
therefore has less application in genetics and far more in other areas of biology,
such as ecology. The t- test is used when a sample size is relatively small, e.g. Under
30 readings/ figures. The mean and standard deviation of these small samples are
prone to error since a single extreme reading will have a disproportionate effect.
The t- test accounts for this error. For the t-test to be of use, the data used have to
conform to certain conditions, namely they must be related to one another, normally
distributed, have similar variances and the sample size must be small. The t-test canbe expressed as:
Where the suffixes 1 and 2 refer to samples 1 and 2 respectively
To take an example. A farmer wishes to decide which of two fertilizers gives the best
yield for her crop of wheat. She divides one of her fields into 16 plots, eight of which
she treats with fertilizer 1 and eight with fertilizer 2. The number of tons of wheat
obtained from each plot is given in this table
The first stage of t-test is to calculate the standard deviation for each sample. It is
calculated from the mean of each sample (see table 15.3), the deviation of each
reading from the mean (see table 15.4) and the square of this deviation and the sumof squares (see table 15.4)
Table 15.4: The mean of each sample
It is now substitute in the equation:
Finally, to discover whether value of 3.68 indicates whether the different readings
are significant, or merely due to chance, we need to look up 3.68 on a statistical table
called t-table. To do this we need to know the degrees of freedom. This is calculatedaccording to the formula:
Degrees of freedom (v) = (n1 + n2) – 2. In our example: v = (8+8) -2 =14
It is found that looking along the row for 14 degrees of freedom values of 3.68
lies between 2.98 and 4, 14, which corresponds to a probability value of between
0.01 and 0.001. This refers to the probability that chance alone is the reason for the
difference between the two sets of data. In this example, the probability that the
different wheat yields when using our two fertilizers was pure.
Application 15.4
1. Explain how to calculate the t- test.2. Why is t-test important in variation?
End unit assessment 15
Multiple choice questions
1. Which of the following gives rise to genetic variation in a population?
a. Crossing over and independent assortment in meiosis
b. Different environmental conditions
c. Random mating and fertilization
d. Mutation
Choose the best answer
a. 1, 2, 3 and 4
b. 1, 2 and 3 only
c. 1, 3 and 4 only
d. 2, 3 and 4 only
2. Inheritance variations could result
a. high energy radiation
b. geographical isolation
c. environmental factors
d. mutation
Questions with short answers
3. If a diploid organism has two different alleles for the same gene, is it
homozygous or heterozygous?
4. Is weight in human an example of continuous variation or discontinuous
variation?
5. What is a mutagen? Give one example.
Essay questions
6. Explain why variation caused by the environment cannot be passed from
an organism to its offspring.
7. Answer the following questions:
a. Distinguish between continuous and discontinuous variation.
b. Explain the genetic basis of continuous variation.8. The histogram shows the height of wheat plants in an experiment plot.
a. What evidence from the data suggests that there were two strains of
wheat growing in the experimental plot?
b. Based on the figure:
i. Which type of variation is shown by the height of each of the strains
of wheat plants? Give the reason for your answer
ii. Explain why the height of the wheat plants varies between 45 cmand 120 cm.
Practical work
9. Body Mass Index (BMI): Group work
Aim: identify one’s BMI and behave accordingly
Requirement: weighing scale, tap ruler, calculators
The BMI helps identify the weight status of individuals. The BMI is calculatedby:
The values of BMI are ranged into five categories as follow:
Record in a table weight and height of all students in your classroom and thereafter
answer the following questions:
a. Calculate the BMI for all the classmates:
b. What health advice would you give to people in each BMI category?
c. Is the BMI genetic or phenotypic variation? Give reason.
d. Is BMI continuous or discontinuous variation? Give reason.
e. Considering your class members’ weight and height, calculate:
i. The standards deviation (S)
ii. The degree of freedom (Df )iii. The t-test (t)