Y12 MW - Genetic Variation (Complete)

Question 1

What is intraspecific variation?

Answer 1

Differences in organisms of the same species

Question 2

What is interspecific variation?

Answer 2

Differences in organisms between species

Question 3

What is a species?

Answer 3

Organisms which can breed together and have fertile offspring

Question 4

What does it mean if an organism has an odd number of chromosomes?

Answer 4

They are infertile
This is because an odd number means gametes cannot be made through meiosis

Question 5

What are the differences between continuous and discontinuous variation?

Answer 5

Continuous variation (e.g height)
No distinct categories - usually quantitative
Controlled by many genes
Strongly influenced by the environment

Discontinuous variation (e.g eye colour)
Distinct categories - usually qualitative
Controlled by few genes
Unaffected by the environment

Question 6

What is crossing over during meiosis and what is the result of this?

Answer 6

The process whereby a homologous chromosomes are in close proximity of each other and non-sister chromatids entangle together during meiosis 1. Sections of the chromatids may break and rejoin to the chromatid of the other homologous chromosome so that their alleles are exchanged

The result of this is genetic variation

Question 7

What is meiosis?

Answer 7

The production of four genetically diverse haploid cells (gametes) over two divisions

Question 8

What are homologous chromosomes?

Answer 8

A chromosome pair, one paternal and one maternal, with the same gene loci

Question 9

What is the process of meiosis?

Answer 9

S phase - semi-conservative replication of DNA (producing a chromosome made up of two sister chromatids joined at the centromere)
Prophase 1 - chromosomes condense around histones and become visible. Homologous chromosomes closely associate with each other forming a bivalent structure. Crossing over can take place where a small section of DNA can be swapped from one homologous chromosome to the other
Metaphase 1 - homologous chromosome pairs line up at the equator of the cell. The spindle fibres form and attach to the centromere of each homologous chromosome
Anaphase 1 - within the homologous chromosome pair, each chromosome is pulled to opposite poles of the cell by the spindle fibres
Telophase 1 - cell membrane forms separating each pole of the cell. Two cells are formed, each with one chromosome from each pair

Prophase 2 - chromosomes condense around histones
Metaphase 2 - chromosomes line up at the equator of the cell. Spindle fibres form and attach to the centromere and the centromere divides
Anaphase 2 - sister chromatids are pulled to opposite poles of the cell by the spindle fibres
Telophase 2 - membrane forms, separating the opposite poles of the cell and forming two different cells

Overall, four genetically different daughter cells are produced containing 23 chromosomes

Question 10

What does it mean that gametes are haploid cells?

Answer 10

They have 23 chromosomes. Therefore, when a sperm and egg cell fuse during fertilisation a diploid cell can be produced

Question 11

What is a zygote?

Answer 11

The cell produced when a egg and sperm cell fuse together during fertlisation

Question 12

What is the point called where crossing over takes place?

Answer 12

Chiasmata

Question 13

Does semi-conservative replication take place during meiosis 2?

Answer 13

No

Question 14

What is non-dysjunction?

Answer 14

When homologous pairs of chromosomes fail to separate during meiosis so are pulled to the same pole of the cell

Question 15

What is polyploidy?

Answer 15

When an organism has three or more sets of chromosomes, usually occurs in plants

Question 16

What is hybridization?

Answer 16

Combining the genes of different varieties of species to produce a hybrid

Question 17

What is a type of chromosome mutation?

Answer 17

Non-dysjunction

Question 18

Give an example of directional selection

Answer 18

Antibiotic resistance in bacteria

Variation in bacteria due to mutations
Variation of bacteria is spread due to the transfer of plasmids containing the gene for an enzyme which hydrolyses the antibiotic
The patient takes the antibiotic, bacteria with antibiotic resistance survive and increase in number by binary fission while the bacteria without the antibiotic resistance die

Question 19

Give an example of stabilising selection

Answer 19

Human birth weight

Variation in the birth weight of babies
Low and high-weight babies are at a disadvantage and die
Medium-weight babies survive and reproduce, passing on beneficial alleles to the next generation
Change in allele frequency
Birth weight remains at a constant

Question 20

What is natural selection

Answer 20

Variation due to mutation
Individuals with beneficial characteristics survive and reproduce, passing on advantageous alleles
Change in allele frequency over a long period of time
Initial and new populations are no longer able to interbreed

Question 21

What type of selection does this show?

Answer 21

Directional selection

Question 22

What type of selection does this show?

Answer 22

Stabilising selection

Question 23

Why is courtship required?

Answer 23

• Enables potential mates to recognise their own species
• Avoids aggression between individuals
• Enables sexually mature organisms to recognise each other
• Results in making between two organisms of the opposite sex