12. Variation and Polygenic Influence

How is variation produced by genetic factors?

1. Crossing over

2. Mutations

3. Random assortment

How is variation produced by environmental factors?

• Climate Altitude

• Water availability

• Temperature

• Light Competition

• Diet

• Stress levels

What is polygenic inheritance?

Interaction of a number of genes at different loci, which have a cumulative effect on the phenotype.

How do environmental and genetic factors interact?

Environmental factors determine the achievement of genetic potential

EQ: Explain how the height of an adult human demonstrates how environmental factors with genotype (3)

• malnutrition (1)

• nutrient required for specific growth (1)

• environmental factor determining achievement of genetic potential (1)

Which case studies show the complex interaction between genotype and the environment?

1. Animal fur colour

2. Sex determination

3. Monoamine oxidase A

Suggest why siamese cats have this particular colouration pattern?

At low temperatures the gene that codes for the enzyme tyrosine can be expressed.
The extremities (ears, nose, paws and tail) of the cat are less than normal body temperature.
This results in gene expression of tyrosinase in these areas so melatonin can be produced.
→ Therefore the phenotype of black fur is observed.

Describe Sex Determination case study:

• Interactions between the genotype and temperature.

• In reptiles, the temperature that the eggs are incubated determines the sex of the offspring

• At warm temperature there will be low proportions of males.
  → This can have negative impact on population size

Describe monomise oxidase A case study:

What are monoamines?

• Monoamines are a group of neurotransmitters with a wide range of functions in the human brain and body. They include dopamine, noradrenaline and serotonin

What is the function of monoamine oxidase A?

• Monoamine oxidase A is an enzyme which breaks down monoamines after used in synaptic transmission Abnormally high or low levels of MAOA is problematic. Levels are affected by genotype and environment

Describe behaviour and MAO A:

Low levels of MAO A has been linked to:

• Depression

• Addiction

High levels of MAO A have been linked to:

• Risk Taking Behavior

• Criminality

• Aggression

How can the environment affect phenotype?

• DNA and histones have chemical markers attached to their surface. This is known as the epigenome.

• These markers dictate whether or not a gene is activated or inactivated.

• Environmental factors can cause epigenetic changes which alter and DNA and histones.

• This affects the phenotype as different proteins will be expressed.
  → Methylation of DNA bases affects gene expression
  → Histone modification affects how other proteins bind to DNA

• Therefore genes can be activated and repressed

What environmental stimulus could cause there to be an increase in melanin production?

MSH hormone

Suggest what this stimulus does in order to increase melanin production:

MSH binds to the receptor on the surface of melanocyte cells.
→ This causes genes to be activated in melanin to  form mRNA.

Explain how the stimulus brings about these changes?

1. Chemical stimulus (MSH) binds to melanocyte receptors

2. Melanin gene activates

3. Transcribes to mRNA

4. Translates to protein on ribosome

5. Protein permanently modifies cell