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Biology
Variation, Selection, Human Health, Living Organism and their Environment
Genetic Variation arises in different ways
Genetic variation is one reason for the differences between individuals. Tongue rolling, hair colour, eye colour, blood
group and many other characteristics are controlled by genes.
Meiosis
During sexual reproduction, gametes are formed by meiosis. Homologous chromosomes exchange genes (crossing over),
and separate from one another, so the gametes which are formed are not all exactly the same.
Fertilisation
Any two gametes of the opposite types can fuse together at fertilization, so there are many possible combinations of
genes which may be produced in the zygote.
Mutation
Sometimes, a gene may suddenly change and this is called mutation. Mutation is only source of brand-new
characteristics in the gene pool. Most mutations are harmful, but occasionally one may happen which gives the mutant
organism an advantage in the struggle for existence.
Darwin’s Theory of Natural Selection
Variation- Most populations of organisms contain individuals which vary slightly from one to another. Some slight
variations may better adapt some organisms to their environment than others.
Over production- Most organisms produce more young than will survive to adulthood.
Struggle for existence- Because populations do not generally increase rapidly in size, there must therefore be
considerable competition for survival between organisms.
Survival of the fittest- Only the organisms which are really well adapted to their environment will survive.
Advantageous characteristics passed on to offspring- Only these well-adapted organisms will be able to reproduce
successfully, and will pass on their advantageous characteristics to their offspring.
Natural Selection- The theory is often called the theory of natural selection, because it suggests that the best-adapted
organisms are selected to pass on their characteristics to the next generation.
Genetics and Humans
Humans can bring about change in the plants and animals that they grow, using a process called artificial selection.
For example, if a farmer wants to increase the milk yield of his herd of cattle, he will pick out only the best milkproducing cows and allow them to breed with a bull whose mother, sisters or daughters are good milk producers. The
cows which do not produce as much milk will not be allowed to breed. If the farmer continues to do this over many
generations, he can steadily increase the mean milk yield of his herd of cattle.
Genetic Engineering
In recent years, scientists have learned how to take DNA from one organism and put it into another, and make it
work there. This is called gene technology or genetic engineering.
Humans and Health
A disease is something that is wrong with your body.
Nutritional deficiency diseases are caused by an unbalanced diet in which a nutrient is missing.
Physiological diseases are caused by the body just going wrong by itself. Sometimes it simply wears out when as it gets
old (arthritis, cancer, etc).
Hereditary diseases are caused by our genes (sickle cell anaemia).
Pathogenic diseases, also called infectious diseases, are caused by microorganisms that get into the body and breed
(tuberculosis, HIV/AIDS).
Insect Vectors
Malaria is an infection caused by a protozoan called Plasmodium. This infection is transmitted from one person to
another by mosquitoes. When a mosquito bites, it injects fluid from her salivary gland to prevent the victim’s blood from
clotting. If the mosquito bites some infected with malaria or dengue, this bacteria is housed in the saliva of the
mosquito.
An organism like this, which transmits a pathogen from one person to another is called a vector.
Houseflies spread the bacteria that cause gastroenteritis. This illness is caused by pathogenic bacteria that enters the
body in food or drink. When houseflies land on feaces, dead animals or spoiled things. They pick up bacteria on their
feet and later on walk on your foot transferring them.
Below is the life cycle of the Mosquito and House Fly:
Diabetes
We know hormones insulin and glucagon control blood glucose levels. When this control breaks down, a person has
diabetes.
Type I diabetes- This usually develops when a person is very young. The pancreas does not make enough insulin.
Type II diabetes- This develops later in life. The pancreas still makes insulin, but the liver does not respond to it.
Pathogenic Diseases
Microorgamisms that can cause disease are known as pathogens and the disease they cause are pathogenic diseases.
These particular diseases are also called infectious diseases, transmissible or communicable diseases. These diseases
spread in a variety of ways:
Through the skin- Some bacteria/ viruses can get into the body through the skin, even when it is undamaged (wart virus)
Through the respiratory system- Cold and influenza viruses are carried in the air in small droplets of moisture. If you
breathe these in you may become infected and this is called droplet infection.
In food or water- Bacteria (salmonella, cholera, polio) can cause food poising if eaten and taken into the alimentary
canal.
By vectors- A vector is an organism that transmits a pathogen to its host. For example, malaria is caused by a protozoan
called plasmodium. It lives in the salivary gland of some types of mosquito, and is injected into a person’s blood when
the mosquito feeds. The parasite, has two hosts, but causes disease only in one. (Another example- houseflies with
gastroenteritis).
Through sexual intercourse- Sexually transmitted disease are passed on by sexual intercourse.
Immunity and Vaccination
Having a disease and recovering from it is one way of becoming immune to it. This sort of immunity is called active
immunity, because your white blood cells make the antibodies themselves.
You can also acquire immunity by having a vaccination. For example, the BCG vaccination for tuberculosis contain
bacteria which has been weakened. When they are injected into your body, they are too week to reproduce. The white
blood cells recognize them as foreign and learn to make the antibodies to destroy them.
Another type of immunity is called passive immunity. Here your white blood cells do not make the antibodies. Instead,
the antibodies are put into your blood ready-made such as milk for breast-fed babies, some vaccinations contains
antibodies and antibiotics.
Ecosystems
Ecology is the study of organisms in their environment. The type of place where an organism lives is called its
habitat. The habitat of a tadpole might be a pond. There will be many tadpoles in the pond, forming a population of
tadpoles. A population is a group of organisms of the same species, living in the same place at the same time, and able
to interbreed with each other. Tadpoles will not be the only organisms living in the pond. There will be many other kinds
of plants and animals, making up the pond community. A community is all the organisms, of all the different species,
living in the same place at the same time.
An ecosystem consists of a community and its environment; they interact with one another and affect each other.
Within the ecosystem, each species of organism has its own role to play and this is called its niche.
Abiotic factor
These include climate factors such as sunlight, rainfall, humidity, and temperature.
Biotic factors
Biotic factors are caused by other living things. Every living organism is affected by others in some way. There are many
kinds of relationships between living organisms, which may be close or casual, beneficial or harmful. They include
relationships between:
Competitors- organisms that need the same things, so they compete with each other if these things are in short supply.
Mutualistic Partners- organisms that live together for mutual benefit.
Predator and their Prey,
Parasites and their Host