Sex–linked traits are characteristics whose genes are carried on the X chromosome of the sex chromosomes instead of autosomes. Such genes are inherited along with such X chromosomes. They are all controlled by a recessive gene. Examples of Sex-linked traits are: colour blindness, haemophilia, baldness, sickle cell anaemia and albinism.

  1. Colour blindness: A colour blind person cannot distinguish near colours. It is an abnormality of the gene that controls the production of cone cells (light receptors) in the retina of the eye.
  2. Haemophilia: This is a disorder in which bleeding takes an abnormally long time to stop or fails to stop because blood clotting will not occur. In haemophiliac (the victim) small injuries can result to bleeding to death e.g. Queen Victoria’s lineage (gene for haemophilia arose as a mutation in Queen Victoria or one of her parents) in British Royal Family.
  3. Baldness: The recessive gene controlling this trait causes the hair on the upper part of the head to pull out prematurely. It is more common in male human beings.
  4. Albinism: This is the condition in which the skin of an animal is non – pigmented because of lack of the pigment called melanin.
  5. Sickle cell anaemia: The recessive gene controlling this abnormality causes some of the red blood cells to be sickle shaped. The haemoglobin of the affected red blood cells is abnormally shaped thereby making it inefficient in transporting oxygen. In a condition of low oxygen concentration, the haemoglobin breaks down causing the cells to be sickle shaped. This then leads to the blockage of the cavities of the small blood vessels in the body thus hindering free flow of blood. The body part affected receives lower blood, oxygen and nutrients. Therefore, the victim goes into crisis at such periods characterized by pains in the bones and joints.



Probability is usually expressed in units ranging from 0 – 1. Mendel’s works were based on probability.


Probability = No of times an event occurs

Total no of trials

The two guiding principles of probability in genetics are:

  1. The result of one trial of a chanced event does not affect the result of latter trials of the same event.
  2. The chance that two independent events will occur together simultaneously is the product of their chances of occurring separately. 



In agriculture, genetics is relevant and has led to the following:

  1. Cross fertilization &self-fertilization procedures
  2. Development of early maturing varieties of organisms.
  3. Development of disease–resistant varieties of organisms.
  4. Production of crops and animals that can adapt to climatic conditions.
  5. Improvement of quality and quantity of product

In medicine, genetics helps in the following:

  1. Determination of paternity of a child.
  2. Blood transfusion
  3. Diagnosis of diseases
  4. Sex determination
  5. Marriage counseling to avoid cases of genetic disorder.
  6. Knowing and choosing the sex of a baby.
  7. Development of test tubes babies.


NOTE:  All the applications listed above sum up the relevance of biology to life in what is now termed biotechnology. In biotechnology the DNA is now being manipulated to the benefits of humanity i.e. genetic engineering.


See also




Scheme of work


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