Variation

Question 1

What is continuous variation?

Answer 1

When individuals in a population vary within a range and do not all fit into specific groups.

Question 2

Give an example of continuous variation.

Answer 2

Height: you could be any height within a range

Question 3

What is discontinuous variation?

Answer 3

When there are two or more distinct categories and every individuals falls into one of them.

Question 4

Give an example of discontinuous variation.

Answer 4

Blood groups: you can only fit into groups A, B, AB or O

Question 5

What causes variation in phenotype?

Answer 5

Variation in genotype.

Question 6

Define the term ‘monogenic’.

Answer 6

Characteristics that are controlled by one gene. They tend to show discontinuous variation.

Question 7

Define the term ‘polygenic’.

Answer 7

Characteristics that are controlled by a number of genes at different loci. They usually show continuous variation.

Question 8

Define the term ‘locus’.

Answer 8

Different alleles for the same gene are found in the same position on chromosomes.

Question 9

Give two examples of characteristics that are controlled by both genes and environmental factors.

Answer 9

1. Height: polygenic and affected by environmental factors, especially nutrition
2. Cancer: the risk of developing cancer is affected by genes and other environmental factors such as diet.

Question 10

Define the term ‘epigenetic control’.

Answer 10

The attachment/removal of chemical groups to or from the DNA. This alters how easy it is for enzymes and other proteins needed for transcription to interact with and transcribe genes. This alters the genes that will be expressed. Therefore, the phenotype that is expressed will be altered too.

Question 11

Give two types of epigenetic control.

Answer 11

1. Methylation of DNA

2. Modification of histones

Question 12

Describe the process of methylation of DNA

Answer 12

1. A methyl group is attached to the DNA. It attaches to the CpG site.
2. Increased methylation of DNA changes the DNA structure so the transcription enzymes and proteins cannot bind to the gene. Therefore, the gene cannot be expressed as it is inactivated.

Question 13

What is a CpG site?

Answer 13

Where Cytosine and Guanine are next to each other in the DNA sequence.

Question 14

What is a histone?

Answer 14

Proteins that DNA wrap around to condense and form chromatin.

Question 15

What happens to gene expression when histones are acetylated?

Answer 15

The chromatin becomes less dense. This means proteins can bind to the DNA, allowing genes to be transcribed and activated.

Question 16

What happens to gene expression when acetyl groups are removed from the histones?

Answer 16

The chromatin becomes highly condensed so the genes cannot be transcribed. This is because the proteins cannot bind to them. Therefore, the genes are repressed/inactivated.

Question 17

How are epigenetic changes passed on?

Answer 17

• When a cell divides and replicates, epigenetic changes may be passed on to daughter cells.
• This means the genes that have been activated or deactivated in the original cell will be the same in the daughter cell
• If epigenetic changes occurred in response to a change in the environment, the daughter cell will be equipped to deal with the changed environment in the same way as the original cell.