Research this Some diseases are caused by the absence of genes that are normally present. Find out what diseases are caused in this way. For one of these diseases outline how the study of genetics helps us to understand the nature of the disease and how it may be inherited.
Genetic variation All individuals within a species share certain traits. For example, with a few rare exceptions, all humans are born with 10 fingers. Similarly, all cats have claws and all ducks have beaks. Although all individuals in a species share similarities, they are different. The differences between individuals of the same species are called variations. Variation within a species is caused by both genetic factors and environmental factors. For example, your skin colour is influenced by genes inherited from your parents but is also influenced by exposure to sunlight. Scientists explore the patterns of variation in a number of ways.
Variations The differences between
individuals of the same species.
Fig. 12 Skin colour is influenced by genes and sunlight exposure.
When scientists examine the height of adult humans they find that most people are close to the average height. The average height of Irish adult males is about 177 cm. Nearly 70% of Irish adult males are within 7 cm of the average, between 170 cm and 184 cm. 95% of men are within 14 cm of the average. The shape of the curve on the graph in Fig. 13 is sometimes called bell-shaped. Scientists
have found that when graphs like this are drawn for other characteristics they often form a bell- shaped curve. Recognising patterns of variation in this way helps scientists to compare different individuals and groups within a species. Scientists can also examine trends of change for a species as a whole.
