Mutation

Mutations and deleterious genes****Mutations are sudden unpredictable changes that occur in the chromosome or genes and they may alter the phenotype expression of an organism. Mutations are defined as changes in the amount or structure of DNA of an organism as well as arrangement of DNA. Mutation is a sudden inheritable change of the genotypes. Significance of mutations

1. Mutations are rare events because DNA and chromosomes are stable structures.
2. Genetic Variation: Mutations introduce new genetic variations within a population. These variations are essential for the process of natural selection and evolution, helping species adapt to changing environments.
3. Source of New Traits: Mutations can create new traits in organisms. These new traits can be beneficial, harmful, or neutral, depending on the environment. For example, a mutation could result in an organism being resistant to a particular disease.
4. Evolution: Mutations play a key role in the process of evolution. When beneficial mutations accumulate over time, they can lead to the formation of new species as the organisms evolve to better suit their environments.
5. Genetic Disorders: Some mutations can lead to genetic disorders or diseases. For instance, a mutation in the gene responsible for producing a specific protein can cause diseases like sickle cell anemia or cystic fibrosis.
6. Adaptation to Environmental Changes: Mutations can help organisms adapt to new or changing environments. For example, a mutation might help a plant species survive in a dry environment by altering its ability to conserve water.
7. Increased Biodiversity: Mutations increase the genetic diversity within a population. This diversity is important for the survival of species because it provides a pool of traits that can help populations survive in different environmental conditions.

Causes of mutations:

1. Spontaneous Mutations Mutations can occur naturally without any external cause. These happen due to errors in DNA replication during cell division. Such mutations are random and not influenced by external factors.
2. RadiationExposure to radiation, such as ultraviolet (UV) rays from the sun, X-rays, and gamma rays, can cause mutations. Radiation can damage the DNA in cells, leading to changes in the genetic material.
3. Chemical MutagensCertain chemicals, called mutagens, can cause mutations. These chemicals include substances like tobacco smoke, industrial pollutants, and some pesticides. When organisms are exposed to these chemicals, they can cause changes in the DNA sequence.
4. Viral InfectionsSome viruses can insert their genetic material into the DNA of a host cell, causing mutations. For example, certain viruses like the Human Papillomavirus (HPV) can cause mutations in cells that may lead to cancer.
5. Errors During DNA RepairOur cells have mechanisms to repair damaged DNA. However, sometimes the repair process itself can cause errors, leading to mutations. This happens if the repair system does not fix the DNA correctly.
6. Environmental FactorsEnvironmental factors such as pollution, smoking, and exposure to toxic chemicals can increase the chances of mutations. These factors can damage the DNA, causing changes that might lead to mutations.

Types of mutations: Gene Mutations: These occur in the DNA sequence of a gene and affect the genetic code. There are three main types:

• • Substitution: One base is replaced by another (e.g., adenine replaced by guanine).
  • Insertion: An extra base is added to the DNA sequence.
  • Deletion: A base is removed from the DNA sequence.

Chromosomal Mutations: These mutations affect the structure or number of chromosomes. There are different types:

• • Deletion: Part of a chromosome is lost.
  • Duplication: A part of a chromosome is repeated.
  • Inversion: A section of a chromosome breaks off and reattaches in reverse order.
  • Translocation: A piece of a chromosome breaks off and attaches to another chromosome.

Frameshift Mutations: These happen when bases are inserted or deleted from the DNA sequence. This changes the "reading frame" of the genetic code and can lead to the production of completely different proteins. Point Mutations: A point mutation is a change that affects just one base pair in the DNA. This can lead to a small change in the protein, which may be beneficial, harmful, or have no effect at all. Silent Mutations: These mutations occur when a change in the DNA sequence does not affect the protein produced. Even though the base sequence changes, the protein stays the same because of the redundancy in the genetic code. Conditional Mutations: These mutations only show their effect under specific conditions, such as changes in temperature or environmental factors. For example, some mutations might cause problems in one temperature but not in others. Example of gene mutations: Albinism

• This is a type of gene mutation resulting from base substitution in which the correct base sequence is substituted for by incorrect base sequence.
• The result of this substitution is the failure of the enzyme tyrosinase to convert an amino acid tyrosine into melanin. Hence absence of pigment melanin and hence development of a light coloured skin (albinism).
• Albinism is caused by a recessive allele (a) whose dominant allele (A) produces normal skin colours pigment.

Consider a cross between two normal individuals producing an albino. Mutation occurs during meiosis where a chromosome/gene may be Deleted, Duplicated, inverted or substituted, in the presence of mutagens.     Sickle cell anaemia

1. This is a bases substitution type of gene mutation.
2. It results into development of an individual with abnormal haemoglobin which causes sickling of the red blood cells.
3. In sickle cell anaemia, the replacement of a base in the DNA molecule results in the wrong amino acid being incorporated into two of the polypeptide chains which make up the haemoglobin molecule. The abnormal haemoglobin makes the red blood cells to become sickle shaped, resulting in anemia and possible death.
4. The synthesis of normal haemoglobin in the body is controlled by a pair of gene with the genotype AA (Hb^A^Hb^A^).
5. The mutated gene known as haemoglobin S (Hb^s^Hb^s^) is recessive and is the one which causes sickle cell anaemia.
6. An individual with the genotype Hb^A^Hb^s^(heterozygous stickler, a condition known as sickle cell trait) has no effect, rather the genotype gives an advantage such as a genotype produces normal shaped red blood cells: However, they are likely to lose their shape when the tension of O2 get lowered.
7. When plasmodium enters such a cell, the tension of O2in the cell get lowered consequently the cell sickles up.
8. These mis-shaped cells are cleared from the blood system by the spleen together with the parasites contained in them. In this way the person is said to be resistant to Malaria and the situation is referred to as heterozygous advantage.

Sickle cell anaemia is characterized by the following features;

1. Sickling of the red blood cells.
2. Severe and eventually lethal anaemia as the Hb is inefficient at carrying O2.
3. Abnormal joint pains.
4. Enlarged spleen.
5. Resistance to Malaria for sickle cell trait.
6. Blocked blood vessels depriving organs of O2 and permanently damaging.

Consider a cross below:   Forms of polyploidyThere are two forms namely;

1. Auto polyploidy.
2. Allopolyploidy.

Auto polyploidyThis condition may arise naturally or artificially as a result of an increase in number of chromosomes within the same species AllopolyploidThis condition arises when the chromosome number in a sterile hybrid become doubled and produces fertile hybrids. F1 organisms are sterile as they cannot form homologous chromosome pairs during meiosis. This is called hybrid sterility. However, the multiples of the original number of chromosomes are fertile. Fusion of either of these gametes with a normal haploid gamete produces a zygote with an odd number of chromosomes. They are usually abnormal. Non disjunction in gamete cell formation   Consequence of non- disjunction in humans (Genetic disorders) Down’s syndrome(Mongolism) In this case 21^st^ chromosome fails to segregate and the gamete produced possesses chromosomes. The fusion of this gamete with a normal one with 23 chromosomes results in the offspring having 47 (2n + 1) chromosomes. This leads to a presentation of three copies of chromosomes, a condition known as trisomy, hence down syndrome is also known as trisomy 21.

1. A Mongol is characterized by the following feature:
2. Big head, protruding tongue, flat facial features, puffy eyes, mental retardation, sterility and short life expectancy.
3. Non disjunction in the case of Down’s syndrome appears in the production of ova rather than sperm.
4. The chances to have a mongol child increases with an increased age of the mother. At the teenage, the chance is one in many thousands at age of 40 – 45, the chances are 35% and above.

Kilinefelter’s syndrome (in feminized males) This is a male genetic disorder in which the victim has got 47 (2n + 1) chromosomes instead of 46.

1. It is due to an extra X chromosome. The genotype is therefore XXY instead of normal XY. It is like Mongolism, an example of trisomy.
2. It may occur during spermatogenesis or during oogenesis.

Symptoms:

1. Infertility – sperm are never produced.
2. Usually taller than average.
3. Enlarged breasts.
4. Enlarged hipbone.
5. Very small testes.
6. Low intelligency.
7. Little facial hair.
8. Smooth skin texture.
9. Voice pitched higher than normal.

Treatment – Male hormones can be given. Breast then returns to normal size and the condition is diagnosed only after puberty. Turner’s syndrome (XO)

1. This is a female genetic disorder in which there are only 45 chromosomes. Patients can be described as incompletely developed females.
2. The sex constitution is said to be XO.

Symptoms:

1. Infertility – ovaries as are absent.
2. Small uterus.
3. Shortness of stature.
4. Broad chest with widely spaced nipples.
5. Under developed breasts.

TreatmentFrom the age of puberty, a woman is given female sex hormones to make female develop breasts & have periods. Though this does not cure infertility. Explanation of Klinefelter’s syndrome and Turner’s syndrome as a result Non – disjunction of the father’s sex chromosomes Non – disjunction of the mother’s sex chromosomes

VARIATIONVariations are differences among the individuals of the same species. Those variations which can be inherited are determined by genes. These are called genetic or inheritable variations. Some variations are determined by the individual’s environment and are known as acquired characteristics. Acquired characteristics such as big muscles developed from training and exercise are not inherited. Inheritable variations may be caused by mutation or by new combination of genes in the zygote. Non inheritable variations arise and disappear from a species when the individuals die.

In genetics we are concerned with inheritable variations. Many variations are controlled by genes. There are two types of inheritable phenotypic variations.

1. Continuous variation and
2. Discontinuous variation.

Discontinuous VariationThis occurs when an organism must either have or not have a certain character. There is no gradual change between the two extreme. This case of variation produces organisms with a clear cut differences between them and with no intermediate between them. Such characteristics include sex where an individual is male or female, eye colour, blood group, finger prints, tongue rollers, non-tongue rollers.

Characteristics showing discontinuous variation are usually controlled by one major gene which may have two or more allelic forms. Discontinuous variation cannot be altered by environment. For example you cannot change your blood group by altering your diet.

Continuous VariationContinuous variation occurs when every member of species shows a certain characteristic but not to the same extent. Some examples of such characteristics are hand span in humans, length of tail in other animals, number of leaves per plant, body weight and height of the people of the same age. These characteristics vary continuously in the population. Characteristics which show continuous variation are controlled by the combining effect of a number of genes called polygenes and any character which results from the interaction of many genes is called polygenic characters.

HISTOGRAMS

Environmental influenceOne of the reasons for continuous variation is that all phenotypic characters are influenced by the effects of the environment. Many continuously variable characteristics are affected by environment or by what happens during individual’s life time. For example a genotypically tall organism may be dwarfed by not getting enough food or balanced diet and therefore appear similar to a child whose genotype is for shortness. No character of any organism can be said to be completely due to effects of heredity (nature), or due to environment (nurture). Environment and heredity always interact in producing the phenotype. **Origins of variation:**Variation may be due to,

1. Environment effect e.g.: Diseases and Nutritional standards.For example: The action of sunlight on a light coloured skin may result in it becoming darker. Such changes have little evolutionary significance as they are not passed from one generation to the next.
2. Genetic factors. These are much more important to evolution as they are inherited. These genetic changes may be the result of the normal and frequent reshuffling of genes which occur during sexual reproduction, or as a consequence of mutations.

**Reshuffling of genes:**The sexual process in organisms has three inbuilt methods of creating variety.

1. The mixing of two different parental genotypes where cross-fertilization occurs (Fertilization)
2. The random distribution of chromosomes during metaphase I of meiosis (Independent assortment).
3. The crossing over between homologous chromosomes during prophase I of meiosis.