Genetic Variation

Question 1

Meiosis

Answer 1

DNA replicates
Chromosomes condense
Chromosomes associate in homologous pairs or bivalents
Crossing-over takes place
Centromeres join to spindle at equator of cell
Homologous chromosomes move to opposite poles
Cytokinesis
Chromosomes move to equator
Centromeres attach to spindles
Chromatids move to opposite poles
Cytokinesis

Answer 2

Mutation
Caused by mutagenic agent
Change in base sequence of DNA Deletion/ substitution
New protein coded for
Crossing over
Different combination of alleles

Independent assortment of homologous chromosomes/chromatids in meiosis I and II
Different combinations of maternal and paternal chromosomes

Random fusion of gametes at fertilisation
New combinations of alleles

Question 3

Directional selection - Antibiotic resistance in bacteria

Answer 3

Variation in bacteria due to mutation
Variation in bacteria due to the tranter of plasmids containing gene for an enzyme that hydrolyses antibiotic
Patient takes antibiotics – selection pressure.
Selection for antibiotic resistant allele
Bacteria with antibiotic resistant allele survive and increase in number by binary fission.
Bacteria without the antibiotic resistant allele die.
Number of antibiotic resistant bacteria increases.

Question 4

Stabilising selection – human birth weight

Answer 4

Variation in the birth weight of babies.
Low weight and high weight babies at disadvantage and die.
Medium weight babies survive and reproduce.
Pass beneficial alleles to the next generation.
Leeds to a change in the allele frequency.
Birth weight remains constant.

Question 5

Natural selection

Answer 5

Variation due to mutation
Individuals with beneficial characteristics survive and reproduce.
Selection for advantageous alleles
Leads to change in allele frequency
Occurs over a long period of time
Populations no longer able to interbreed.