This document discusses five major factors that affect genetic equilibrium: 1) Mutations, which create genetic variations, 2) Recombinations during sexual reproduction, which reshuffle genes, 3) Genetic drift, or random changes in allele frequencies influenced by population size, 4) Gene migration between populations, and 5) Natural selection, which causes populations to adapt over generations as traits better suited to the environment are selected for. It provides details on each factor, including types of mutations, the mechanisms of recombination, how genetic drift is influenced by population bottlenecks and founder effects, and gives an example of natural selection producing giraffe's long neck.

1. By: Syeda Laraib Zahra
Roll # 1238(563480)

2. FACTORS AFFECTING GENE EQUILIBRIUM
Some of the major factors which affect the genetic equilibrium and induce
the variability in population are as follows:
1.Mutations
2.Recombinations during Sexual Reproduction
3.Genetic Drift
4.Gene Migration (Gene Flow)
5.Natural Selection
According to Hardy-Weinberg Equilibrium law, the relative frequency of
alleles in the population remains constant from generation to generation in a
population of sexually reproducing organisms when:
(i) The population is large enough so that accident of sampling may be
ignored.
(ii) Mating takes place at random.
(iii) Mutation does not take place or if it does, the rate is same in both
directions.

3. (iv) All the members of the population survive and have equal
reproduction rates.
Factors affecting the Hardy-Weinberg equilibrium:
There are five factors which affect the genetic equilibrium and induce the
variability in the population. These factors are called evolutionary agents.
1.Mutation:
“ These are sudden, large and inheritable changes in the
genetic material.”
These are characterized by:
(1)Mutations are random (indiscriminate) and occur in all directions.
(2) Most mutations are harmful or neutral. It is estimated that only one out of
1,000 mutations is useful.
(3) Rate of mutation is very low, i.e. one per million or one per several
million genie loci. But rate of mutation is sufficient to produce considerable
genetic variability.

4. (4) Certain mutations are preadaptive and appear even without exposure to a
specific environment. These express and become advantageous only when
after exposure to new environment which only selects the preadaptive
mutations that occurred earlier.
Existence of preadaptive mutations in Escherichia coli was experimentally
demonstrated by Esther Lederberg (1952) in replica plating experiment
(Explained in Neo-Darwinism).
On the basis of amount of genetic material involved, mutations are of three
types:

5. Three types of mutation
Gene mutations
(change in sequence
of nucleotide in DNA)
3 types
1.Deletion
2.Addition or insertion
3.Subsitution or replacement
(a)Transition
(b)Transversion
(c)Tautomerism
Chromosomal aberrations
(change in chromosomes
structure)
4 types
1.Deletion
(a)Terminal
(b)Intercalary
2.Duplication
(a)Tandem
(b)Reverse
3.Translocation
(a)Simple
(b)Reciprocal
4.Inversion
(a)Paracentric
(b)Pericentric
Chromosomal numerical
mutation
(change in number of
chromosomes)
Aneuploidy
(Gain or loss of
one or two
chromosomes)
1.Hypoploidy
(a)Monosomy
(b)Nullisomy
2.Hyperploidy
(a)Trisomy
(b)Tetrasomy
Euploidy
(Gain or loss of
one or more
genome)
1.Haploidy
2.Polyploidy
(a)Auto
(b)Allo

6. On the basis of their origin, mutations are of two types:
Differences between Spontaneous and Induced mutations
Characters Spontaneous
mutation
Induced mutation
1.Caused by
2.Frequency of
mutations
3. Causes
By natural agents,so also
called natural mutations or
background mutations.
Very low (about one per
million genes or even
more).
Not certain, many cellular
products e.g. formaldehyde,
nitrous acid, peroxides, etc.
act as mutagens.
By man
Faster
Certain physical {e.g.
radiations temperature,
etc.) and chemical
agents called mutagens

7. Significance of mutations:
1.Mutations create and maintain variations
within a population.
2.These also introduce new genes and alleles in a gene pool

8. 2. Recombinations during Sexual Reproduction:
“Recombination involves reshuffling of genes of chromosomes. Chances of
recombination are more in those organisms which undergo sexual
reproduction which involves gametogenesis followed by fertilization”
Sexual reproduction involves recombinations during three stages:
1.Crossing over
2.By independent assortment of chromosomes
3.By random fertilization
Crossing Over:
“It involves the exchange of genetic material between the
non-sister chromatids of homologous chromosomes”
Mechanism of crossing over involves following stages:

9. (a)Synapsis involves pairing of homologous chromosomes during
zygotene stage of Prophase 1 of meiosis to form bivalents.
(b) Tetrad formation as each bivalent is formed of four chromatids
during pachytene stage of meiosis I.
(c) Chiasma formation due to coiling of non-sister chromatids of
homologous chromosomes to form X-shaped points, called chiasma.
(d) Crossing over involves exchange of genetic material.
(ii) By independent assortment of chromosomes:
During metaphase-I, the bivalents arrange at the equator of the spindle in
two equatorial or metaphase plates. During anaphase I, homologous
chromosomes move towards the opposite poles. This is called disjunction and
results in reduction of chromosome number. Variations occur during the
chance arrangement of bivalents during metaphase I of meiosis. Number of
recombination’s produced depends upon the number of bivalents in the
organism and is given by the formula 2n (where n represents the number of
bivalents) e.g. in human being, number of possible combinations of the
gametes will be 8.6 x 106(223).

10. (iii) By random fertilization:
It involves the chance fusion of gametes e.g., in human being, number of
possible types of zygote formed is 70 x 1012. It is so as any sperm with any
combination of genes can fuse with any ovum with any combination of genes.
Significance:
Due to recombination’s, though only reshuffling of already existing
characters takes place and no new genes are produced but it leads to
redistribution of different traits to different individuals of a population.
Different combinations bring diversity in genotype and phenotype of different
organisms. So recombination is an agent of evolution.
3.Genetic Drift:
“It is the random change in the frequency of alleles
occurring by chance fluctuations”
It is characterized by:
(i) It is a binomial sampling error of the gene pool, i.e. that alleles which
form the gene pool of the next generation are a sample of the alleles of
present population.

11. (ii) Genetic drift always influences frequencies of alleles and is inversely
proportional to the size of population. So genetic drift is most important in
very small populations in which there are increased chances of inbreeding
which increases the frequency of individuals homozygous for recessive
alleles, many of which maybe deleterious.
(iii) Genetic drift occurs when a small group separates from a larger
population and may not have all the alleles or may differ from the parental
population in the frequencies of certain genes. This explains for the
difference between island populations and mainland population.
(iv) In a small population, a chance event (e.g. snow storm) may increase
the frequency of a character having little adaptive value.
(v) Genetic drift can also operate through founder effect. In this, genetic
drift can cause dramatic changes in the allele frequencies in a population
derived from small groups of colonisers, called founders, to a new habitat.
These founders do not have all of the alleles found in their source population.
These founders become quickly different from the parental population and
may form a new species, e.g. evolution of Darwin finches on Galapagos
Islands which were probably derived from a few initial founders.

12. Population Bottleneck
It is reduction in allele frequencies caused by drastic reduction in population
size called population crash e.g. decrease in cheetah population in Africa due
to over-hunting. As the given gene pool is limited, population bottleneck
often prevents the species to reestablish its former richness so new
population has a much restricted gene pool than the larger parent
population.

13. Founder Effect :
A founder effect, as related to genetics, refers to the reduction in genomic
variability that occurs when a small group of individuals becomes separated
from a larger population. Over time, the resulting new subpopulation will
have genotypes and physical traits resembling the initial small, separated
group, and these may be very different from the original larger population. A
founder effect can also explain why certain inherited diseases are found more
frequently in some limited population groups. In some cases, a founder effect
can play a role in the emergence of new species.

14. 4.Gene Migration (Gene Flow):
Most populations are only partially isolated from other populations of same
species. Usually some migration-emigration (moving out of some individuals
out of a population) or immigration (entry of some members of a population
into another population of same species) occurs between the populations.
• This addition or removal of alleles when individuals enter or leave a
population from another locality is called gene flow.
5.Natural Selection:
Definition:
“ Process through which populations of living organisms adapt and
change”.Individual in population are naturally variable,meaning that they are
all different in some ways.This variation means that some organisms have
traits better suited to environment than others.
Example:
A classical example of natural selection at work is the origion of
giraffes’long necks.The ancestors of natural giraffes were animals similar to
deer with neck of ordinary length.

15. References
• http://www.biologydicussion.com
• A book of Genetics-I by P.S Verma & V.K Agarwal from page number 975-992.