Describe the various mechanisms that bring about variation within a
species.
Sources of genetic variation:
Meiosis
• Gametes produced by meiosis are different because of:
• crossing over during first prophase
• random arrangement of chromosomes
• during first metaphase
• and second metaphase
Random mating as a source of variation
• In humans and some other mammals, mating takes place between individuals
who select mating partners of a particular kind.
• In most other species, mating takes place randomly amongst all individuals
within
the population / more than one mating partner
Chance fertilisation of egg cells by sperm cells
• Usually more than one egg cell and sperm cells are produced
• and these are all different.
• Fusion of many types of sperm cells and egg cells
• can produce many different types of offspring.
Mutations
• Mutations are sudden, random changes in the genetic code of an organism
• A gene mutation occurs
• as a result of a change in the sequence of nitrogen bases in the DNA molecule
• can result in the formation of a different protein
• leading to new/different characteristics
• A chromosome
• mutation occurs
• as a result of a change in the structure / number of chromosomes
• Mutations occurring in sex cells
• are passed on to offspring and new generations
• resulting in new characteristics being formed
Content 17
Synthesis - relevance 1
- Logical sequence 1
- Comprehensiveness 1
Total = 20
species.
Sources of genetic variation:
Meiosis
• Gametes produced by meiosis are different because of:
• crossing over during first prophase
• random arrangement of chromosomes
• during first metaphase
• and second metaphase
Random mating as a source of variation
• In humans and some other mammals, mating takes place between individuals
who select mating partners of a particular kind.
• In most other species, mating takes place randomly amongst all individuals
within
the population / more than one mating partner
Chance fertilisation of egg cells by sperm cells
• Usually more than one egg cell and sperm cells are produced
• and these are all different.
• Fusion of many types of sperm cells and egg cells
• can produce many different types of offspring.
Mutations
• Mutations are sudden, random changes in the genetic code of an organism
• A gene mutation occurs
• as a result of a change in the sequence of nitrogen bases in the DNA molecule
• can result in the formation of a different protein
• leading to new/different characteristics
• A chromosome
• mutation occurs
• as a result of a change in the structure / number of chromosomes
• Mutations occurring in sex cells
• are passed on to offspring and new generations
• resulting in new characteristics being formed
Content 17
Synthesis - relevance 1
- Logical sequence 1
- Comprehensiveness 1
Total = 20
