CHAPTER 6
6.1
VARIATION IN ORGANISMS
1.
Variation = the phenotypic differences among the individuals of a same species.
2.
Type pf variation:
(a)
Continuous variation
- Continuous variation is a
type of variation in which the differences in a character are not distinct, but
forms a continuum or continuous range of immediate phenotypes between two
extremes.
- Examples: Heights and
weights
(b)
Discontinuous variation
- Discontinuous variation is a type of
variation in which the differences in a character can be divided into two or a
few types of distinct phenotypes, with no intermediate characteristics.
- Example: Blood groups, ability to roll the
tongue, type of finger patterns
Continuous variation
|
Discontinuous variation
|
6.2
CAUSES OF VARIATION
1.
Variation can be the results of:
(a) Genetic factors
(b) Environmental factors
(c) The interaction between the
genetic factors and environmental factors
2. Genetic factors of variation
-
Sources of variation:
(a)
Sexual Reproduction
(b)
Mutation
(a)
Sexual Reproduction
-
Three sources of genetic variation in sexual reproduction:
(i)
Crossing over during meiosis
·
During
prophase 1 of meiosis, nonsister chromatids of homologous chromosomes cross at
chiasma.
·
At
chiasma, the chromatids break and rejoin in such a way that segments of
chromatids are exchanged, causing a genetic recombination.
·
Thus,
the pattern of genes in the chromosomes is altered and gamete with various
combinations of chromosomes are produced.
(ii)
Independent
assortment during meiosis
·
During
metaphase 1 of meiosis, homogolous chromosomes arrange themselves randomly at
the equator.
·
The
random arrangement and the separation of each homologous pair is independent of
one another.
·
Independent assortment produces various
genetic combinations in the gametes.
(iii) Random fertilization
·
Each
gamete has a unique set or combination of genes. A male gamete can fertilise
any of the female gamete.
·
The
fertilization between a male gamete and a female gamete occurs randomly.
·
With
random fertilization, variations occur in the offspring.
(b) Mutation
- Mutation is a
sudden permanent change in the nucleotide sequence of the DNA or in the amount
of DNA.
- Mutation can occur
through the following ways:
(i) Internal
factors: during replication and
segregation of chromosomes in diving cells.
(ii) External
factors: radiation (X-rays, ultraviolet
rays and gamma rays) and chemical (pesticides, nitrous acid, asbestos, benzene,
drugs ext)
- Substances that can
cause mutation are called mutagen. The product of mutation is called a mutant.
- There are two types of mutation:
(i) Chromosomal mutation
(ii) Gene mutation
(i)
Chromosomal mutation
Ø
Chromosomal
mutation may cause a change in the structure of a chromosome or a change in the
number of chromosomes
Ø
Chromosomal
mutation which causes a change in the structure of a chromosome
(1) Deletion: A part of a
chromosome breaks off and then lost
(2) Inversion: Reversal
of a part of a chromosome
(3) Duplication:
Repetition of a part of a chromosome
(4) Translocation: A section of one chromosome breaks and
gets attached to another chromosome
Ø
Chromosomal
mutation can cause a change in the number of a chromosome through
(1)
The
loss of one or more chromosomes
(2)
The
addition of one or more chromosomes
Ø
Examples
of chromosomal mutation:
(1) Down’s Syndrome
-
Caused
by the failure of the 21st homologous chromosomes to separate during
meiosis.
-
When
the gamete (either sperm or ovum) with the extra chromosome is fused, the
zygote with 47 chromosome will develop into a child with a Down’s Syndrome.
-
The
abnormal characteristics of Down’s Syndrome individuals:
«
Mental
retardation
«
Broad
forehead
«
Downward
sloping eyes
«
Protruding
tongue
«
Shorter
nose
«
Short
and sticky body
(2) Turner syndrome
-
A
genetic disorder in women
-
Caused
by the absence of an X chromosome
-
The
individual has only 45 chromosomes (44 autosomes + X chromosome)
«
Underdeveloped
reproductive organ
«
Sterile
«
Lack
of secondary sexual characteristics
(3) Klinefelter’s
syndrome
-
A
genetic disorder in men
-
Caused
by the addition of one X chromosome
-
The
individual has only 45 chromosomes (44 autosomes + XXY)
-
The
abnormal characteristics:
«
Underdeveloped
testes
«
Infertility
«
Possessing
female characteristic, such as breast enlargement
(ii) Gene Mutation
- Gene mutation involves a change in the
structure of genes
- Gene mutation
alters the DNA sequence of the nitrogenous bases and the sequence of amino acid
production
- Changes in the base
sequence of DNA alters the genetic code for protein synthesis
- The resulting
protein shape is altered and will not function as required.
- Examples of gene
mutation:
(1) Sickle-cell
anaemia
(2) Haemophilia
(3) Albinism
3. Environmental factors of variation
-
In
plant, variations can be caused by the following environmental factors:
(a)
Light
intensity
(b)
Fertility
of soil
(c)
Climate
(d)
Water
supply
(e)
Temperatures
(f)
pH
of soil
-
In
human, variations can be caused by the following environmental factors:
(a)
Type
of food
(b)
Sunlight
(c)
Exercise
(d)
Climate
(e)
Skill
(f)
Experience
Education
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