Variations are the differences in characteristics found within the same population or species. There are no two equal organisms in the bio-life, even if they are identical twins. This enables us to differentiate different individuals within a population.
Variations usually refer to physical characteristics observed in phenotypic differences caused by changes in structures, physiology and biochemistry. Variations are important in evolution in bio-life because:
- Enable natural selection to continue selecting beneficial genes and removes the non-beneficial ones;
- Enable the species to adapt to their surrounding for survival;
- Enable the cross-breeding among species to form new species.
What are the 2 types of variations?
There are two types of variations:
Continuous Variation
- The differences in the characteristics are not distinct.
- The characteristics are shown by environmental factors such as weight, height and skin colour.
- The characteristic is quantitative and can be measured.
Discontinuous Variation
- Shows distinct differences in the characteristics.
- The characteristic is qualitative and cannot be measured.
- The occurrences of discontinuous variation are due to genetic factors and can be inherited.
- The characteristics are determined by a single gene with two or three alleles and can easily be seen, for example, eye colour and fingerprint pattern.
What are the factors of variation?
The causes of variations vary. In Biology form 5, you will learn that there are two factors of variation.
For continuous variation, it depends on environmental factors, such as temperature, light and pH. It is also known as environmental variation.
For example, environmental factors like sunlight can alter skin colour. Ultraviolet rays in sunlight can destroy the melanin pigment of exposed skin. More melanin pigment will be produced to protect the skin. This will cause the skin to be darker.
However, the effect of the environment on the variation is small because it only involves phenotypic differences and not genotypic differences. Thus, environmental variation cannot be inherited from the next generation.
On the other hand, discontinuous variation is caused by genetic factors like crossing over, an independent assortment of chromosomes, random fertilization, and mutation.
Crossing over
Crossing over happens between non-sister chromatids of the homologous chromosomes during prophase I of meiosis. The recombination produces a new combination of genes. Sister chromatids that separate during anaphase II of meiosis form gametes with different genetic materials at the end of meiosis.
Independent assortment of chromosomes
During metaphase I of meiosis, homologous chromosome pair is arranged randomly on the equatorial plane of the cell. At the end of meiosis, different gametes are produced with different combinations of the paternal and maternal chromosomes. This results in the genetic contents of each gamete being different from the other.
Random fertilization
Fertilization between sperm and secondary oocyte is random. Genetic recombination, which occurs during crossing over and random arrangement of homologous chromosomes in meiosis, results in gametes with different contents from their parents. Therefore, a diploid zygote that is formed after fertilization will have a new genetic combination.
Mutation
The mutation is a permanent change that happens spontaneously on genes or chromosomes. It creates new genotypes. If the mutation appears in the gametes, the characteristics determined by mutated genetic materials can be inherited. Mutation of the somatic cell can cause variation too, but the characteristics cannot be passed to the next generations.
Can environmental variation interact with genetic variation?
Environmental factors can interact with the genetic factor to cause the variation. In this case, the environmental factors will determine the phenotypes. Characteristics that are inherited from the parents, such as intelligence and skin colour, can be influenced by environmental factors.
For example, two girls who are a pair of identical twins have similar genotypes for obesity because they are originated from the same embryo. However, their eating habits and lifestyle affect their phenotype.
One may end up being obese because her food preference is more likely on the high-fat diet and does not like physical activity. On the other hand, the other twin may have an ideal body weight because she prefers a low-fat diet and exercises daily.
What is the relation of variation with human inheritance?
In the inheritance topic, you should already know that human traits are controlled by a pair of alleles. In this case, an allele can be dominant or recessive. This genetic information causes variation in humans.
Dominant Traits
Dominant traits are observed when both dominant alleles are present or when one dominant allele is paired with the recessive allele.
Recessive Traits
Recessive traits are shown when both recessive alleles are present.
The inheritance topics are universal. You can even find form 3 English textbook talking about genes, genealogy and inheritance. Non-science stream students will also learn about variation in the form 4 science textbook.
How does mutation affect humans?
Mutation is a spontaneous and random change of genetic material in the DNA of a cell in an organism. Mutagen is a substance that causes mutation or increases the mutation rate to a dangerous one. Mutant is a new genetic material produced by mutation. It can exist as a mutant gene, mutant cell, mutant organelle or mutant individual.
Mutagen
There are three types of mutagen agents:
1.Physical Agents
- Ultraviolet rays from the sun
- Ionizing rays, such as X-ray, alpha ray and beta ray
2. Chemical Agents
- Carcinogen in cigarettes smoke
- Food preservatives
- Formaldehyde
- Benzene
3. Biological Agents
- Virus
- Bacteria
Gene Mutation
Gene mutation takes place when there is a change in the nucleotide base sequence of a gene. It is also known as a point mutation.
The change modifies the genetic code that is used to synthesize amino acids. As a result, there is a change in protein structure and this new protein cannot function well.
Chromosome Mutation
Chromosome mutation affects changes to the chromosomal structure or changes to the chromosomal number. It can change the characteristics of an organism.
1. Chromosomal Structure
Changes the gene sequence in a chromosome. This causes abnormality to the chromosome. This change is known as chromosome aberration.
2. Chromosomal Number
A change in chromosomal number causes a diploid to lose one or more chromosomes or gain one or more chromosomes.
This happens when the homologous chromosomes fail to separate during anaphase I in meiosis. It is also caused by the failure of sister-chromatid to separate during the anaphase II in meiosis.
Why does this happen?
Probably because the normal spindle fibres fail to form during meiosis. This phenomenon is known as nondisjunction. The nondisjunction causes abnormality in the number of chromosomes and in a gamete during spermatogenesis or oogenesis.
When both of the abnormal gametes from paternal and maternal fertilized, a zygote that developed into an individual with abnormal characteristics will be produced. This will be explained in detail in the reproduction, development and growth in humans chapter.
There are many diseases caused by mutation. These diseases may occur due to abnormalities in the autosome or abnormalities in the sex chromosome.
During Science form 2, you should already learn about human immunity and diseases that can occur due to chromosome abnormality. You can refer to the science form 2 textbook in chapter 4.
You might also want to keep science form 3 and science form 2 all chapters' notes in your hands because you can refer to them while doing revision on the biology form 4 and form 5 topics.
Otherwise, you can always refer to these textbooks online:
- Textbook science form 2
- Textbook science form 3
Abnormalities in autosome diseases
1. Down Syndrome
An individual with down syndrome has the following characteristics: slant eyes, flat nose, protruding tongue, broad forehead and is usually mentally retarded.
2. Cri du Chat Syndrome
Infants who are affected with Cri du Chat syndrome produce a crying sound that is similar to a meowing cat. They usually show delayed development with mental and physical retardation. Most patients die during childhood.
Abnormalities in sex chromosome diseases
1. Klinefelter Syndrome
A sterile man with small testis that fails to produce sperms. The infected person possesses a voice and chest similar to women and also tends to have long legs and hands.
2. Jacob Syndrome
One of the characteristics of a man affected by this disease is his height - is taller than the average person. The individual will also have pimples on his face, will experience slowness in acquiring speech and learning impairment and will possess weak muscles.
3. Turner Syndrome
A sterile female with a lack of secondary female characteristics, undeveloped breasts and ovaries. She will also have a weblike neck and a low IQ.
Mutation of somatic cells and gametes
Mutation can occur in somatic cells and gametes in order to produce variation in a population. There are a number of differences between a mutation in somatic cells and gametes.
1. Mutation in somatic cells
- Involves somatic cells, such as skin cells and eye cells;
- Cannot be inherited by the next generation;
- The disease is present only in the individual with the mutation;
- Examples of this type of mutation are diseases related to the nervous system.
2. Mutation in gametes
- Involves germ cells that produce gametes (secondary oocyte or sperm);
- Can be inherited by the next generation;
- The disease is present in the individual with the mutation and is also inherited by their descendants;
- An example of this type of mutation is thalassemia.
Variation is one of the most important and hardest topics to be learned in Biology SPM subject. If you face difficulties in learning SPM Biology topics, such as biodiversity and the human digestive system, it is advisable that you seek help from a tutor.
Hop onto Google and start searching for ‘tuition near me’. You can go for a personal tutor or look out for well-known tuition Malaysia.
Alternatively, you can search for a tutor with a Biology background who specialised in SPM examination through a reputable online tutoring website such as Superprof.
Superprof offers you a wide selection of excellent home tutors online for any of your preferred subjects.
With more than 16 million available tutors in over 1,000 subjects, you can be sure to find your ideal tutor on Superprof. Check out the website and start your Biology lessons right away.
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