3.4 Differentiation & Variation

Some rat mothers spend significant amounts of time licking, grooming and nursing their pups. Others spend much less time doing this.

Highly nurtured pups usually grow up to be calm adult rats, whilst pups who receive less attention usually grow up to be anxious.

Scientists have discovered that the difference between a calm and anxious rat is epigenetic, rather than genetic. 

Describe what is meant by the term epigenetics. 

The rat pups in part (a) behave differently because of a change in the glucocorticoid receptor (GR) gene during development.

The gene is highly methylated and inactive at birth.

If a rat mother is attentive towards her pups, the pups' GR gene demethylates making the gene increasingly active. These pups will be more relaxed in response to stress.

The pups that were not given attention, and do not express the GR gene, became anxious more easily. 

Explain what methylation is and how it makes genes inactive.

Name two environmental factors that can affect the methylation of DNA in humans.

A patient with a spinal cord injury was healed using stem cell therapy by the following process:

1. Stem cells were collected from the patient's own tissues

2. Stem cells were cultured in the laboratory for one month

3. Stem cells were injected into the injury site of the patient

4. Stem cells developed into neurones and replaced damaged nerve tissue

Which of the following are two properties of stem cells?

Explain how stem cells, such as those in part (a), develop into nerve cells.

Explain why the stem cells used in part (a) were taken from the patient and not from a stem cell donor.

The use of stem cells poses many issues and can be a controversial subject. 

Evaluate the benefits and issues of stem cell research and use in humans

A geneticist investigated genetic variation in six different breeds of dog. They compared DNA base sequences of the same genes from a large number of dogs from each breed.

Give two other ways in which genetic variation between organisms can be measured.

The geneticist compared the base sequence of a gene in mitochondrial DNA. They compared the gene base sequence of each dog with all of the other six breeds. For each comparison, they calculated the percentage difference in base sequence.

The diagram below shows the base sequences for part of a gene in two of the breeds. 

Species 1   A C C G T T C T C G A

Species 2    A G C T C A C T A C A

Calculate the percentage difference in base sequence for these base sequences.

The geneticist then calculated the mean genetic variation for each breed of dog. The value of this mean was between 0 and 1.

• A mean value of 1 shows maximum genetic variation.
• A mean value of 0 shows no genetic variation.

The results are shown in the table

Breed of dog	Mean genetic variation	Standard deviation
Golden retriever	0.68	0.01
Labrador retriever	0.57	0.03
Flat coated retriever	0.61	0.01
Curly coated retriever	0.45	0.02
Chesapeake Bay retriever	0.32	0.03
Nova Scotia retriever	0.78	0.04

Describe what the data shows about the differences in genetic variation between these breeds of dog.

Another geneticist was studying the pigments, eumelanin and phaeomelanin, which determine coat colour in dogs.  It was found that the loci that determine coat colour behave in a mostly additive manner.  This can be used to explain the deepening of a coat colour as shown in the diagram below.

Describe what is meant by an additive effect on genes.

Variation can be described as continuous or discontinuous.

Which choice below shows two correct statements about continuous and discontinuous variation? 

Cacti are plants that are adapted to live in arid conditions.

They store water in thick stems which are protected from grazing by herbivores through the presence of multiple spines. The number of spines that are present on cacti may vary greatly, depending on the frequency of grazing that they experience.

The graph shows the number of spines present in a species of cactus.

State the type of variation that is represented by the graph and provide two reasons for your answer.

Explain the possible impact of grazing on the variation observed in the graph, with regards to the number of spines on cacti.

Contrast the genetic basis of continuous and discontinuous variation with each other.