Form 4 Biology – GENETICS AND VARIATION -2

GENETIC DISORDERS.

MUTATION

Mutation are changes in the genetic material in the gametes.

• It includes appearance of new characters that have never been before in that population

• Individuals who undergone mutation are called Mutants

•Mutation can be due to

1. Change in a gene itself
   
2. Change in arrangement of genes
   
3. Loss of chromosomes (due to unbalanced meiosis)
   

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• Mutation can be caused by agents known as Mutagens

• X-rays
  
• Cosmic rays
  
• Heavy metal (lead & mercury)
  

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TYPES OF MUTATION

1. Gene mutation
   
2. Chromosomal mutation
   

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1. GENE MUTATION
   

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Gene mutation occur as a result of altering the chemical structure of genes

• There is a change in the sequence of nucleotides is the segments of DNA corresponding to one gene. This in turn alters the sequence of amino acids required in synthesis of a particular protein.

• The protein formed will be different from the normal ones and produce a profound effects on both the structure and development of an organism Example: sickle cell, dwarfism.

TYPES OF GENE MUTATION

1. Substitution
   
2. Insertion
   
3. Deletion
   
4. Inversion
   

   i. SUBSTITUTION
   

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   This is the replacement of one or more portions of a gene with a new one. E.g. A thymine (T) on ATA on the DNA molecule is replaced by cytocise (C) and result to ACA on the DNA

This is examplified in sickle cell anaemia only one nucleotide is changed. This kind of mutation involving the change of one nucleotide is called Point Mutation.

ii. INSERTION

     This involves adding a new portion of a gene to an existing one. Example: If the base Guanine (G) is inserted between two Adenine result into AGA which does not code for any amino acid.

iii. DELETION

   Deletion is the remove of a portion of a gene Example: -If base Guanine (G) is deleted in a base triplet CGC resulting into alteration of base sequence reducing the number of amino acids.

iv. INVERSION

   A portion of DNA strand cuts and rotates through 180° the inversion results in alteration of the base sequence at this part.

Example: -A base triplet CTA can have its base thymine (T) and Adenine (A) cut and rotated. The result is CAT which is different from amino acid.

2.. CHROMOSOMES MUTATION

Chromosomes mutation involves changes in the structure of the chromosomes. During meiosis homologous chromosomes interwine at several points called chiasmata and create opportunity for various changes on the chromatids leading to mutation.

         TYPES OF CHROMOSOME MUTATION

1. Deletion
   
2. Duplication
   
3. Inversion
   
4. Trans location
   
5. Non-disjunction
   
6. Polypoidy
   

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1. DELETION : This occurs when a portion of the chromosome breaks off and fails to reconnect to any of the chromatids, The result is the loss of genetic materials.
   

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Deletion can be caused by error in chromosomal crossover during meiosis. These causes serious genetic deceases

2. DUPLICATION
This occurs when a portion of the chromosome replicates itself adding extra length. The result is addition of a set of genes which is a duplication

           This may result to over emphasizing of a trait in an organism.

                       

3. INVERSION
This occurs when a middle piece of the chromosomes break and rotates at 180° and rejoins the chromatid. This has the effect of reversing the gene sequence.

              

4. TRANS LOCATION

   This occur when a portion of one chromosome breaks off and becomes attached to another chromatid of non-homologous pair. The result is transfer of genes from one pair of homologous chromosome to another.

5. NON-DISJUNCTION

This kind of chromosomal mutation is caused by addition or loss of one or more chromosomes. This occurs during meiosis where homologous chromosomes fail to separate. This results in some gametes having more chromosomes that others.

Example of non – disjunction

(a) DOWN’S SYNDROME

This is caused by presence of an extra chromosome number 21 individuals with this defect have a total of 47 chromosomes they have

• Resistance to infection
  
• Mentally retarded
  
• Have thick tongue
  
• Short body
  

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Also children of old parents (above 40 years woman and 55 man) have increased chance of Down’s syndrome.

(b) KLINEFELTER’S SYNDROME

   This is caused by failure of X chromosome to separate during the process of egg formation. An individual with this condition has two X chromosome and one Y chromosome (XXY). They are – outwardly male but may also have female characteristics.

(c) TURNER’S SYNDROME

     This is an individual with 45 (44 + x 0) chromosome in a cell instead of 46 (44 +xx).   Individual with this condition have one X and no Y i.e (XO) they individual is sterile and abnormally short female.

6. POLYPLOIDY

Occurs if the whole set of chromomes doubles after fertilization, where the spindle fail to be formed and the cell does not divide.It is rare in animals but common in plants

7. SICKLE CELL ANAEMIA

   Sickle cell anaemia is an example of gene mutation. The normal haemoglobin is entirely replaced by an abnormal haemoglobin known as haemoglobin S

   In sickle cell anaemia, the glutamic acid is replaced by another amino acid, the valine forming a haemoglobin s denoted by Hbs. Normal haemoglobin is denoted by HbA.

Haemoglobin S begins to crystallize when Oxygen concentration falls and causes red blood cell to assume the shape sickle. Half the number of red blood cell is sickle.

   

GENETIC COUNSELLING

Genetic information is used to advice couples who have hereditary disorders about chances of children inheriting the disorders. Genetic information could also be used in choosing marriage partners.

GENETIC ENGINEERING

This is the alteration of the structure of DNA by man.

• Genetic engineering enables man to carry out research.

• Manufacture protein (insulin)
  
• Improve animal and plant breeds
  
• Correct genetic disorders
  

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• Genetic engineering is the technique of changing the genotypes of an organism. It involves inserting genes from one organism into the chromosomes of another organisms. Once inserted the foreign genes work as if they were in the organism they were taken from.

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APPLICATION OF GENETICS

            1. MEDICINE

Genetics engineering has enabled biologists to program and make useful substance. For example the gene in man that produces insulin was inserted into escherichia colia for producing pure insulin in large quantities.

• Human growth hormone ha also been made by using bacteria which the proper gene has been added.

• Also blood clotting factors such as fibrinogen needed by haemophiliacs are produce.

• Vaccines from viruses are produced.
  

  2. Biological warfare
  

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Genetic engineering can help humans to produce biological weapons i.e. Anthrax and Vibrio cholera

3. Agriculture

• It is common for farmers to select and plant seeds from the healthiest and high yielding varies of plants with the aim of improving desirable traits as high fruits and crop production.

• Also genetics has enabled the beginning of selective breeding. Selective breeding is the crossing of animals or plants that have desirable traits to produce offspring that have a connection of the parents’ desirable characteristics

• Also the knowledge of genetics developed in breeding which involves crossing relatively individuals to maintain desirable traits. The various breeds of cattle, dogs, pigeons, chicken and maize, sugarcan and goats are a result of in breeding

4.
Genetic disorder

1. Pregnant women can be informed about the deformation of the fetus
   
2. It can help in the modification of disordered genes
   

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Dangers of genetic engineering

1. The outcome of genetic engineering can be weird out of our imagination
   
2. Production of new pathogens accidentally or deliberately
   

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