Objectives
The reader will be able to:
o Explain what variation is
o Distinguish between continuous and discontinuous variation
o Distinguish between heritable and non-heritable variation.
o Explain the causes of variation and state their source.
o Explain the consequences of variation.
o Explain the term Recombinant DNA technology and state their applications.
VARIATION
This is referring
to the differences in characteristics among individuals of the same species.
This is due to inherited genes from parents or acquired through the
environment. There is variation in skin color, hair color, hair curliness,
eye color and sex.
Types of Variation
There are two main
types of variation: Continuous variation and Discontinuous variation.
Discontinuous Variation
Discontinuous variation is when there exists a clear-cut difference
between the characters with no intermediate forms. Human blood group is an
example of discontinuous variation. There are only 4 types of blood group (A,
B, AB or O). There are no other possibilities and there are no values in
between. So, this is discontinuous variation. Other examples are Rhesus
factors, sex (male or female), red-green color blindness, hemophilia, sickle
cell anemia.
Continuous Variation
Continuous variation is the type of variation within a species with
intermediate forms between two extremes. Human height is an example of
continuous variation. Height ranges from that of the shortest person in the
world to that of the tallest person. Any height is possible between these
values. Other examples include weight, skin color, intelligence, age, body
size, pattern of fingerprint.
Difference between Continuous Variation and Discontinuous Variation
|
Continuous
variation |
Discontinuous
variation |
|
Intermediate forms
present |
No intermediate forms |
|
Controlled by the gene (polygene); polygenic |
Controlled by
major genes (alternate or multiple alleles); not polygenic |
|
Controlled by the
genes and environment |
Entirely genetic (not
affected by environmental conditions) |
|
Caused by co-dominance |
Caused by
complete dominance |
|
Follow normal
distribution curve. i.e frequency of characteristics evenly distributed. |
Not evenly
distributed |
Inherited and Non-Heritable Variation
Some variation within a species is inherited, and some variation is due to the environment.
v Inheritable variations are variations that are
inherited. They are mainly cause by genetic factors. Examples of inherited
variations: eye color, lobed or lobeless ears, blood group, sickle cell,
albinism, rolling of tongue, ability to taste PTC paper, gender (male or
female) etc.
v Non-heritable variations are acquired through the environmental influences and are therefore
not inherited. Examples goiter, river blindness, scars, knowledge and language.
Causes or Sources of Variation
There are two causes of variation. Environmental factors and genetic factors.
Environmental causes of variation
o
These are external or
non-inheritable factors that affect or influence life processes and cause
difference among organism of the same species. Examples of external factors are
climate, diet, accidents, light intensity, altitude, culture and lifestyle. For
example, if you eat too much you will become heavier, and if you eat too little
you will become lighter. A plant in the shade of a big tree will grow taller as
it tries to reach more light.
Genetic causes of variation
§ Mutation
§ Epistasis
§ Co-dominance
§ Cross over
§ Polyploidy
§ Hybridization
§ Polygenic character
§ Segregation and recombination
§ Independent assortment
§ Incomplete Dominance
§ Random fusion of gametes
§ Segregation is the separation of each pair of homologous chromosomes during
meiosis in the formation of gametes and their subsequent recombination during
fertilization. This introduces variation in the offspring. Crossing over also
causes variation.
§ Independent Assortment: this is the independent assortment of genes to produce gametes at
meiosis where the genes segregate so that only one of a pair is found in any
one gamete. The separation of parental chromosomes at meiosis and their
recombination at fertilization introduces possibility of new combinations of
genes.
§ Co-dominance: refer to inheritance patterns when both alleles in a heterozygous organism independently and equally expressed. OR a condition in which both members of a pair of alleles equally express in the phenotype. E.g. blood group AB, the A and B alleles have equal dominance and are both expressed in the phenotype.
§ Incomplete Dominance: it refers to the situation where the action of one allele does not completely musk the action of the other and neither allele has dominant control over the trait. E.g. Sickle cell anemia where the carrier produces 70% normal RBC and 30% abnormal RBC.
§ Epistasis: it sometimes happens that the effects of one gene is suppressed by another; even though they occupy different loci and hence not alleles. This phenomenon is known as epistasis. The gene that prevents the expression of another is said to be epistatic and suppressed gene is known as hypostatic.
§ Polygenic character: if two or more genes are responsible for a single trait, the phenotypic trait is said to be governed by polygenic factors. For example, genes that control growth hormone have a large effect on body size. Likewise, genes that control sex steroids like testosterone have some effect on body size, especially during maturation phases of growth.
Mutation
The ultimate source of genetic variation in populations is via
Mutation. It is spontaneous change in a gene or chromosome of an individual.
Mutation may be spontaneous, that is it may occur naturally. It can also be
induced by exposure to physical and chemical agents in the environment called mutagens.
Example of mutagenic agents are X-rays, gamma rays, ultraviolet rays, cosmic
rays and some chemicals such mustard gas, cigarette smoke and colchicine.
Types of mutations
v Gene mutation: alteration of the sequence of nucleotide of the DNA molecule cause changes in individual genes. An example of gene mutation is point mutations which affect only one or a few nucleotides within a gene. Gene mutations in human may result in albinism, color blindness, hemophilia, cystic fibrosis, dwarfism etc.
v Chromosome mutation: a change in the number of chromosomes or arrangement of genes in
chromosomes. Chromosomal mutations can result from;
ü Changes in the structure of a chromosome
o Deletions - a chromosome segment is lost from a chromosome
o Duplications - a chromosome segment is present more than once in a set of chromosomes.
o Inversions - result when two breaks occur in a chromosome and the broken segment is rotated 180º and join up again.
o Translocations – result from the exchange of segments from two non-homologous chromosomes (two chromosomes that carry different genes)
ü Changes in the number of chromosomes
o Centric fusion - two non-homologous chromosomes fuse into one.
o Centric fission - one chromosome splits into two.
o Aneuploidy - one or more chromosomes of a normal set are lacking or present in excess. E.g., trisomic - occurrence of chromosomes 3 times.
o Polyploidy - the duplication of chromosome sets such that individuals have more than 2 of each chromosome. Diploidy is the normal state (2 of each chromosome, 2N), but some animals are triploid (3N) and tetraploid (4N).
Consequences of Variation
Variation in a given population may lead to
§ Natural selection
§ Artificial selection
Natural Selection
Genetic variation plays a vital role in natural selection. Some
variations are favorable and others are not. Favorable variation gives some
selective advantage over the unfavorable variations. Environmental changes
cause individual with favorable variation to survive whilst the unfavorable
ones die. Individual with superior characteristics produce more offspring and
pass on their genetic variations to their offspring and dominate the
population. These changes take place along period of time, and may result in
the origin of new species, and the extinction of the organism.
Artificial Selection
This involves the application of the knowledge of genetics; by
farmers; by special methods; to produce offspring with desirable
characteristics.
Examples of artificial selection are in maize production; new
varieties include ‘Abelehi’, ‘Okomasa’ and ‘Dobido’. New breed in farm animals
such as cattle, pigs, sheep and goat have been developed.
Methods use in artificial selection
¨ Inbreeding
¨ Outbreeding
¨ Genetic engineering / Recombinant DNA technology
Ø Inbreeding: this involves mating of closely related individuals of the same
species in order to preserve certain desirable characteristics of stock.
Advantages of inbreeding
Ø Maintain traits with the stock
Ø No variation from parent stock
Disadvantages of inbreeding
· Stock later looses reproductive vigor
· Increase susceptibility to disease
· Give rise to homozygous recessives
Ø Outbreeding: this involves the mating of unrelated individuals of the same
species in order to produce offspring; possessing the characteristics of both
parents.
Advantages of outbreeding
o Results in variation
o High vigour
o Produces healthier/more resistance
Disadvantages
§ Desirable characteristics of the parents may be lost
§ New undesirable characteristics may be introduced
Genetic Engineering
Genetic engineering or Recombinant DNA technology is the
manipulation of genes by scientists in order to produce organs or organisms
with desire qualities.
How genetic engineering works
□
The most important tools in
process are restriction enzymes produced by species bacteria.
□ These enzymes can recognize a
particular sequence of DNA molecule (nucleotide bases) and cut the DNA at that
location.
□ The fragments of DNA generated
can be joined using other enzymes called ligases to form a desirable DNA
molecule
□
Also, pieces of desired DNA
inserted into a vector or carrier DNA.
□
The vector in host cell
generates multiple copies of the DNA.
□
The process of engineering a
DNA fragment into a vector is called cloning.
Application of genetic engineering
ü
Diagnosis and treatment of
genetic diseases (gene therapy)
ü
Production of single cell
proteins
ü
Production of interferon
against viral diseases
ü
Mass production of human
insulin
ü
Production of genetically
modified foods (GM foods) such as golden rice, maize
ü
Generation of vaccines
ü
Improve growth rate
ü
Production of organs for human
organ transplant
ü
Introduction of gene into sugar
beet to deactivate the effects of herbicides
Application of variation (Importance of studying genetics)
Ø
Crime detection
Ø
Resolving problem of paternity
Ø
Giving compactible blood
transfusion
Ø
Blood group determination
Ø
Classification of human race
Ø
Production of disease
resistance crops
Ø
Fossil studies /
anthropological studies
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