Variation in phenotype

Question 1

What is variation?

Answer 1

• individuals within a population of a species often show a wide range of variation in their phenotypes
• this variation is due to both genetic and environmental factors
• the primary source of genetic variation results from mitosis and the random fertilisation of gametes during sexual reproduction
• although variation is the result of genetic factors and environmental influences, it is rarely due to one or the other but rather a combination of both

Question 2

What is variation due to genetic factors?

Answer 2

• within a population, all members have the same genes
• genetic differences, however, occur as members of this population will have different alleles of these genes
• these differences not only occur in living individuals but also change from generation to generation

Question 3

What does genetic variation arise from?

Answer 3

• mutations
• mitosis
• genetic factors
• random fertilisation of gametes

Question 4

How do mutations cause genetic variation?

Answer 4

• these sudden changes due to genes and chromosomes may, or may not, be passed on to the next generation
• mutations are a main source of variation

Question 5

How does mitosis cause genetic variation?

Answer 5

-this special form of nuclear division produces new combinations of alleles before they are passed into the gazettes, all of which are therefore different

Question 6

How does random fertilisation of gametes cause genetic variation?

Answer 6

• in sexual reproduction this produces new combinations of alleles and the offspring are therefore different from parents
• which gamete fuses with which at fertilisation is a random process further adding to the variety of offspring two parents can produce

Question 7

When are there no intermediate types?

Answer 7

• where variation is very largely the result of genetic factors organisms fit into a few distinct forms and there are no intermediate types
• in the ABO blood grouping system, for example, there ae four distinct groups - A, B, AB and O
• a character displaying this type of variation is usually controlled by a single gene
• this variation can be represented on a bar chart or pie graph
• environmental factors have little influence on this type of variation

Question 8

What is variation due largely to environmental influences?

Answer 8

• the environment exerts an influence on all organisms
• these influences affect the way the organism’s genes are expressed
• the genes set limits, but it is largely the environment that determines where, within those limits, an organism lies
• in buttercups, for example one plant may be determined by its genes to grow much taller than other plants
• if however, the seed germinated in an environment of poor light or low soil nitrate, the plant may not grow properly and kit will be short
• environmental influences include climate conditions e.g. temperature, rainfall and sunlight, soil conditions, pH and food availability
• some characteristics of organisms grade into one another, forming a continuum
• in humans, two examples are height and mass
• characters that display this type if variation are not controlled by a single gene, but by many genes (polygenes)
• environmental factors play a major role in determining where on the continuum an organism actually lies
• for example, individuals who are genetically predetermined to be the same height actually grow to different heights due to variations in environmental factors, such as diet
• this type of variation is the product of polygenes and the environment
• if we measure the heights of a large population of prop-le and plot the number of individuals against heights on a graph we will most probably obtain a bell-shaped curve known as a normal distribution curve
• in most cases variation is due to the combined effects of genetic differences and environmental influences
• it is very hard to distinguish between the effects of many genetic and environmental influences that combine to produce differences between individuals
• as a result, it is very different to draw conclusions about the causes of variation in any particular case
• any conclusions that are drawn are usually tentative and should be treated with caution