Assignment of Animal Breeding By 16th Batch of Bsc.Ag 6th Semester Roll No. 21-40 on Variation and Causes of Variation

1. Internal Assessment
on the topic
“Variation and causes of
Variation”
SUBMITTED TO
Dr. Ajeet Kumar Jha
SUBMITTED BY
Roll no. Name Roll no. Name
21 Dipa Sharma 31 Kalyani Bhandari
22 Gaurab Neupane 32 Khemraj Subedi
23 Gresha Suwal 33 Khusbu Adhikari
24 Hemant Kumar Sahani 34 Krita Karki
25 Himani Chand 35 Madhukar Chaudhary
26 Ichchha Neupane 36 Mahesh Kumar Bhandari
27 Janak Bhandari 37 Manoj Singh Yadav
28 Jitendra Hamal 38 MD Mumtaz Aalam
29 Jiwan Bhatta 39 Meena Pandey
30 Kalyan Bhattarai 40 Meena Sunar

2. INTRODUCTION
Variation is defined as any difference between cells, individual organisms, or groups of
organisms of any species caused either by genetic differences (genotypic variation) or by the
effect of -environmental factors on the expression of the genetic potentials (phenotypic
variation). In genetics, variation refers to an individual that possesses characteristics different
from the others of the same kind. Genetic variation usually arises as a mutation in a gene that
encodes a protein or an RNA. Variations may be reflected in the phenotype of the individual,
e.g., a difference in size, color or pattern, or may be detectable only by DNA or protein
sequencing. A given variant may be the result of changes in a single gene, or the consequence
of interactions among multiple genes. Populations of the same species living in different parts
of the world may be characterized by specific sets of variations.
TYPES OF VARIATION
S.
N
Continuous variation Discontinuous variation
1. Continuous variation is the combined effect of
the many genes (known as polygenic
inheritance) and is often significantly affected
by environmental influences.
A characteristic of any species with only a
limited number of possible values shows
discontinuous variation. Discontinuous
variation is controlled by alleles of a single
gene or a small number of gene.
2 E.g.: milk yield in cow is determined not only
by their genetic makeup but is also
significantly affected by environmental
factors such as pasture quality and diet
weather and comfort of their surroundings
E.g.: blood group, sex, eye color.
Blood group: In the ABO blood group
system, only four blood groups are possible
(A, B, AB or O). There are no values in
between, so this is discontinuous variation
3.
CAUSES OF VARIATION:
CAUSES
Genetic
Phenotypic
Major
Others Randommating
DNA Mutation
Gene Flow
Sexual Reproduction
Non- Randommating
CrossingOver
Randomfertilization
Environmental variance
Heredity(G)
Environment(E)
G×E

3. Genetic variation: It refers to differences in the genetic make-up of individuals in a population.
Genetic variation results in different forms, or alleles of genes. For example, if we look at eye
colour, people with blue eyes have one allele of the gene for eye color, whereas people with
brown eyes will have a different allele of the gene. Different causes of genetic variation are:
a. DNA Mutation: DNA mutation causes genetic variation by altering the genes of individuals
in a population. They may impact an individual gene or an entire chromosome. Mutation is the
primary source of genetic variation, which is the raw material for evolution through natural
selection. By modifying genes and alleles in a population, mutation results in genetic variation.
Although mutations change an organism's genotype, they may not necessarily change an
organism's phenotype.
b. Gene flow: Gene flow leads to genetic variation as new individuals with different gene
combinations migrate into a population. An important evolutionary force is gene flow: the flow
of alleles in and out of a population due to the migration of individuals or gametes. While some
populations are fairly stable, others experience more movement and fluctuation.
c. Sexual reproduction: Sexual reproduction promotes variable gene combinations in a
population leading to genetic variation. Genetic variation occurs as alleles in gametes are
separated and randomly united upon fertilization. The genetic recombination of genes also
occurs during crossing over or the swapping of gene segments in homologous chromosomes
during meiosis
Phenotypic variation: Phenotypic variation, then, is the variability in phenotypes that exists
in a population. Different causes of phenotypic variation are:
a. Heredity(H): The genetic make-up of an individual is fixed at the time of fertilization and
remains the same for the rest of an individual's life. An individual's genetic make-up is
determined by the genes that are passed down from one generation to the next. Thousands of
genes exist in both individuals and their parents whose exact number is unknown. That’s why,
only the identical twins produced by a single fertilized egg are genetically alike.
b. Environment(E): Environment here includes sum total of all external factors affecting the
individual like feeding, housing, husbandry practices etc. Environment does not directly
change the genetic make-up, but determine the degree of expression, i.e., these factors modify
the development and expression of inherent qualities.
c. H*E interaction: Animals of a certain genotype may perform better in one environment than
in other, i.e., one environment permits the expression of genetic character in a breed or strain
while the other does not. This shows the importance of having the right breed or strain for a
particular environment in order to obtain optimum performance.
Causes of variation can be also classified on the basis of population level which is given below:

4. Literature review
Variation may be shown in physical appearance, metabolism, fertility, mode of reproduction,
behavior, learning and mental ability, and other obvious or measurable characters (Britannica,
2019) for which mutation is the ultimate source, but mechanisms such as sexual reproduction
and gene flow contribute to it as well. The translation of genotype to phenotype is complicated
by gene interactions observed as epistasis, canalization, robustness, or buffering. Analysis of
double mutations in inbred experimental organisms suggests some principles for gene
interaction that may apply to natural variation as well (Hartman et. al., 2001). An individual’s
environment can also cause variation (BBC, 2020). For example, human height is genetically
controlled but actual height reached will depend on the nutrition available. However, fact has
been proven that phenotypic expression of variant alleles might be influenced differentially by
environment (Imasheva et. al., 1999), stochastic events (Adams & Arkin, 1999), and
interactions with multiple other genetic loci. Causes and the effects of variation in traits let
breeders to know about the performance, behaviour, longevity, and fertility of individuals in
their natural habitats, and helps to detect evolutionary processes operating at a variety of
ecological levels: within random-mating populations; within and among subpopulations
distributed over a species’ geographic range; and even among multispecies associations (Mazer
& Damuth, 2001).
Reference
BBC. (2020). Variation and natural selection (CCEA). Biology (Single Science) Genetics.
Retrieved from https://www.bbc.co.uk/bitesize/guides/zgdjk2p/revision/2
Britannica. (2019). variation. Britannica, The Editors of Encyclopaedia. Retrieved from
https://www.britannica.com/science/variation-biology
Hartman, J. L., Garvik, B., & Hartwell, L. (2001). Principles for the buffering of genetic
variation. science, 291(5506), 1001-1004.
Imasheva, A. G., Bosenko, D. V., & Bubli, O. A. (1999). Variation in morphological traits of
Drosophila melanogaster (fruit fly) under nutritional stress. Heredity, 82(2), 187-192.
Mazer, S. J., & Damuth, J. (2001). Nature and causes of variation. In Evolutionary Ecology:
Oxford University Press.
McAdams, H., & Arkin, A. J. T. G. (1999). Genetic regulation at the nanomolar scale: it’sa
noisy business. 15(2), 65-69.
Causes of Variation
Mutation
Withinpopulation
Meiosis
Withinindividual
Gamete
combinations
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Natural
Selection
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Random
geneticdrift
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Gene flow
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Independent
assortment
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Crossingover
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Emigration
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Immigration
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