GENETICS

By Oba Mike

                         image credit: biology.mit.edu

IMPORTANCE OF STUDYING GENETICS

The following are some of the reasons nurses need to study genetics:

1.   All nurses have roles in the delivery of genetic services and management of genetic information.

2.   Nurses require genetic knowledge to identify, care or refer persons affected by or at risked of genetic disorders.

3.   Nurses can offer care that protects patients and families from the risk associated with genetic information, including addressing family issues.

4.   Genetic nursing is practiced in different environments such as maternity, paediatrics, psychiatry, and community health nursing.

5.   Nurses require genetic knowledge to solve the physical, spiritual, ethical, and psychological aspects of patients and families with genetic issues. 

GENE

   Gene is the basic unit of inheritance.

   It is a segment of DNA that codes for a trait. For example, a segment of DNA that codes for eye colour.

   Genes usually occur in pairs. Each member of a pair is called allele e.g. TT, Tt, AS etc.

   One allele is inherited from the father while the other, comes from the mother.

   A pair of genes can be homozygous e.g. AA or heterozygous e.g. AS for a trait.

   Gene expression normally occurs in a dominant-recessive order. However, they are also cases of co-dominance and incomplete dominance.

  Genes contains information needed in producing proteins. Proteins form the structure of our body and influences function and behavior.

   The human genome is estimated to contain about 20,000-25000 genes. Note that every individual has two copies of each gene gotten from father and mother.

             DEOXYRIBONUCLEIC ACID

                   image credit: biologydiscussion.com

Deoxyribonucleic acid (DNA) is mainly found in the cell nucleus. A little of it is also located in the mitochondria called mitochondrial DNA.

The DNA molecule is double stranded and shaped like a twisted ladder.

 Each strand is composed of four nitrogenous bases; Adenine (A), Guanine (G), Thymine (T) and Cytosine (C).

Adenine and guanine are called purines while thymine and cytosine are called pyrimidines.

The bases on one strand of DNA pair with complimentary bases on the opposite strand to form the rungs (steps) of the ladder.

The bases always pair together in the same way, A-T and G-C. Each pair is held together by hydrogen bonds.

Each DNA strand has a beginning and an end called 5’ (five prime) and 3’ (three prime) respectively.

The two strands run in opposite direction to each other so that one runs 5’-3’ while the other runs 3’-5’. They are called sense and antisense strands respectively.

The strands are separated during DNA replication.

The double stranded structure of DNA was first described in 1953 by Francis Crick and James Watson.

The entire DNA of an organism is called genome. Each human cell with a nucleus has a copy of the human genome.

It took scientists 13 years (1990-2003) to sequence the entire human genome, which contains about 3 billion base pairs.

                                                   

DNA Replication

It is the process by which a cell makes an identical copy of its genome before it divides. The double stranded structure of DNA provides a simple mechanism for replication. Here, the two strands are separated then each strand is recreated by an enzyme called DNA polymerase. This enzyme makes the complimentary strand by finding the correct base and bonding it with the original strand. The old strand usually dictates which base appears on the new strand. Errors in DNA replication can lead to mutation in the affected gene.

                  CHROMOSOME

Image credit:biology corner

Chromosome is a threadlike structure found in the cell nucleus. Each chromosome consist of DNA coiled round proteins called histones.

Chromosomes can only be seen using a microscope during the prophase of cell division.

Read more from the note given earlier.

             

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   SEX DETERMINATION

Sex of the offspring is determined at the time of fertilization. It cannot be changed latter. In humans, there are 23 pairs (46) chromosomes. From this, 22 pairs are called  autosomes and have no direct effect on determining the sex of an individual. One pair is directly concerned with sex determination and is called sex chromosome or allosome. 

In the male, the two sex chromosomes are different and represented as X and Y or XY. Each male gamete carries either an X or a Y chromosome. In females however, both chromosomes are the same and represented as XX.

During fertilization, the combination of an egg with a sperm carrying either an X or a Y chromosome occurs by a 50:50 chance. If a sperm with an X chromosome combines with an egg, the resulting zygote will be XX (females). But if a sperm with a Y chromosome combines with an egg, the zygote becomes XY (male).


                GENE MUTATION

A gene mutation is a permanent change in the DNA base sequence that maked up a gene. Mutations range in size from a single DNA building block (DNA base pair) to a large segment of a chromosome.

Features of Gene Mutation
Will be read in class.
            
                Types of Gene Mutation

Gene mutations are of two main types:

1. Germ line or inherited Mutation

2. Somatic or acquired mutation

Germ line or inherited mutation

These are mutations that occur in gametes and are passed from parents to offspring. This type of mutation is present throughout a person's life in virtually all the cells in the body. The affected offspring can as well  pass it to the next generation.

Somatic or acquired mutation 

These forms of mutation are acquired during a person's lifetime and occurs in body cells. Somatic mutations occur due to environmental factors such as high temperatures. It can also occur following a mistake in DNA replication. Acquired mutations cannot be passed on to offsprings because they do not occur in sex chromosomes.

              Causes of Gene Mutation

An extracellular factor that causes gene mutation is called mutagen.  These 
1. Physical factors such as temperature and high energy radiations (cosmic rays, ultraviolet rays, x-rays etc)

2. Chemical factors such as nitrogen acids and drugs use for chemotherapy.

    Effects of Gene Mutation on           Health and Development 

By changing a gene’s instruction for making a protein, a mutation can cause the protein to malfunction or to be missing entirely. When mutation alters a protein that plays a critical role in the body, it can disrupt normal development or cause a medical condition. A condition caused by mutation in one or more genes is called a genetic disorder.

In some cases, gene mutations are so severe that they prevent and
 embryo from surviving till birth. Some mutations are useful.

       PATTERNS OF INHERITANCE

Diseases caused by mutation can be inherited in several ways. This has been grouped into:

1. Mendelian patterns of inheritance

2.  Non-Mendelian patterns of inheritance.

          Mendelian Patterns of Inheritance

These include:

Autosomal dominant

Autosomal recessive 

Sex-linked inheritance

Autosomal dominant

Autosomal dominance is a pattern of inheritance in which an affected individual has one copy of a mutant gene and a normal one. This single copy of mutated gene is enough to cause a disorder. 
Features of autosomal dominance include:
 i.  Gene is on autosome
ii.  A copy of mutant gene is enough to cause a disease
iii. Males and females are equally affected
iv.  Affected persons usually have one affected parent.
v.  Disease usually do not begin early
vi. Examples of autosomal dominant disorders include Huntington’s disease, Polycystic kidney disease, Hereditary breast and ovarian cancer.

Autosomal recessive
For an autosomal recessive disorder to occur, two mutated copies of the gene must be present.
Features of autosomal recessive inheritance or disorders include:
i.  Gene is located on autosome
ii. Two copies of the mutant gene are needed for phenotypic manifestation
iii. Males and females are equally affected
iv. Affected persons usually have parents who are unaffected or carriers
Disease begins at early age
v. Examples of autosomal recessive disorders include sickle cell anaemia, cystic fibrosis etc.

                   Sex-linked Inheritance
Some genetic conditions are sex-linked, meaning that the mutant allele is present on the X or Y chromosome. Those on the X chromosome are X-linked while those on the Y chromosome are Y-linked.
Generally, sex-linked disorders are more common in males than females.

X-linked Inheritance
Characteristics whose genes are located on the X chromosome are said to be X-linked. These disorders affect both males and females but more common in males. The reason being that males receive only one copy of the X chromosome which is always dominant. If this chromosome is affected, there's no chance of the male escaping the disease.
However, females receive two copies of the X chromosome (each from father and mother). Therefore, for a female to develop an X-linked disorder, both chromosomes must be affected or in rare cases the affected one dominates the normal (X-linked dominant).
Examples of X-linked disorders include:
Colour blindness
Haemophilia
Baldness
Duchene muscular dystrophy
Hunter syndrome
Ocular albinism.
Analysis of these diseases will be in class.

Y-linked Inheritance
The Y chromosome is short compared with the X. Few genes are located on the Y chromosome hence it has less clinical significance. Most Y-linked genes manifest their effect with one copy and show male to male transmission exclusively. All sons of an affected male will eventually develop the trait.
Y-linked disorders mostly cause inherited infertility in men.

Non-Mendelian Patterns of Inheritance
This describes patterns of inheritance in which genes do not segregate according to Mendel laws. They include:
1. Co-dominance
2. Incomplete dominance
3. Mitochondrial inheritance
4. Multifactorial inheritance

Co-dominance
In co-dominance, both alleles of a gene are dominant and the traits are equally expressed.
An example of co-dominance in humans is the ABO blood grouping. In this system, it is possible for a father with type A blood and a mother with type B blood to produce an offspring with type AB blood(co-dominance).

Incomplete dominance
In incomplete dominance, neither of the allele is dominant rather, both combine (mix) to display a new trait.
For example, a man with curly hair and a woman with straight hair can produce an offspring with wavy hair.
A common disorder that arise from incomplete dominance is Tay-Sachs disease.

Mitochondrial inheritance
This type of inheritance also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondria which are structures in the cell cytoplasm in addition to their normal energy producing role, contain small amount of DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial conditions to their children. Mitochondrial disorders affect both males and females, but fathers do not pass mitochondrial traits to their children.
Example is Leber's hereditary optic neuropathy.

Multifactorial inheritance
Multifactorial inheritance conditions are believe to be the result of multiple mutations and environmental influence that combine to cause birth defects or disease. Genetic disorders with multifactorial cause tend to cluster in families but do not follow the characteristic pattern of inheritance.
Examples include:
Congenital heart disease
Cleft lip/palate
High blood pressure
Diabetes

APPLICATION OF GENETICS

Gene therapy
Gene therapy is the insertion of genes into an individual's cells and tissues to treat a hereditary disease. In this process, a defective or mutant gene is replaced with a functional one.

Genetic testing
This is the analysis of human DNA and other genetic components in order to detect heritable disease related genotypes, mutations, phenotypes or karyotypes for clinical purpose. It include predicting risk of disease, identifying carriers and making clinical diagnosis or prognosis.
Paternity test is a common example.
There are several other applications of genetics.

 

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