6.2.2 Variation: Phenotype

Question 1

What is phenotypic variation?

Answer 1

Phenotypic variation is the difference in phenotypes between organisms of the same species

Question 2

What are the two factors that can explain phenotypic variation?

Answer 2

In some cases, phenotypic variation is explained by genetic factors
For example, the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene

In other cases, phenotypic variation is explained by environmental factors
For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions

Phenotypic variation can also be explained by a combination of genetic and environmental factors
For example, the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria

The complete phenotype of an organism is determined by the expression of its genotype and the interaction of the environment on this:

Phenotype = Genotype + Environment

Question 3

How can genes have varying affects on an organism’s phenotype?

Answer 3

Genes can have varying effects on an organism’s phenotype
The phenotype may be affected by a single gene or by several genes
The effect that the gene has on the phenotype may be large or small

Specifically, the different alleles an organism has at a single gene locus can determine the phenotype
Remember - diploid organisms will inherit two alleles of each gene, these alleles can be the same or different

For example, the F8 gene that codes for the blood-clotting protein Factor VIII
The different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia

Question 4

How can environmental factors change an organisms phenotype?

Answer 4

The environment that an organism lives in can also have an impact on its phenotype

Different environments around the globe experience very different conditions in terms of the:
Length of sunlight hours (which may be seasonal)
Supply of nutrients (food)
Availability of water
Temperature range
Oxygen levels

Changes in the factors above can affect how organisms grow and develop
For example, plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics (their phenotype is being affected by their environment)

Variation in phenotype caused solely by environmental pressures or factors cannot be inherited by an organism’s offspring (only alterations to the genetic component of gametes will ever be inherited)

Question 5

What are some examples of how the environment can affect phenotypic variation?

Answer 5

Diet in animals:
The fruit fly (Drosophila melanogaster) is normally grey but there is a genetic mutant that is yellow (a genotypic characteristic that is expressed regardless of the environment)
If the larvae of normal grey flies are given a diet of silver salts, they develop the yellow colour regardless of their genotype
This means that flies that should be grey (according to their genes) can become yellow due to an environmental factor (their diet)

Growing conditions for plants:
Plants that are grown in the dark or that cannot access enough magnesium become yellow even though, genetically, they should be green
Plants that are grown in the dark may also develop long stems with small, curled leaves even though, genetically, they should develop normally
This is known as etiolation