Biology – Key Topic 6
4.6 Inheritance, variation and evolution
4.6.1 Reproduction
4.6.1.1 Sexual and asexual reproduction
Sexual reproduction involves the joining (fusion) of male and female gametes, to
produce offspring that are genetically different:
- In humans, each gamete contains 23 chromosomes – half the number in a normal
cell (haploid)
- The egg and the sperm cell then fuse together (fertilisation) to form a cell with 46
chromosomes (diploid)
- Sperm and egg cells in animals
- Pollen and egg cells in flowering plants – pollination
In sexual reproduction there is mixing of genetic information which leads to variety in
offspring – introduces variation into a population. The formation of gametes involves
meiosis
- Used by larger more complex organisms
Asexual reproduction involves only one parents and no fusion of gametes. There is no
mixing of genetic information or variation. This leads to genetically identical offspring
(clones). Only mitosis is involved
- used by small simple organisms – bacteria and microscopic animals and plants
4.6.1.2 Meiosis
Cells in reproductive organs divide by meiosis to form gametes
- In sexual reproduction, the mother and father produce gametes by meiosis
- To make gametes which only have half the original number of chromosomes, cells
divide by meiosis involving two cell division. It humans it only happens in the
reproductive organs
Meiosis produces cells that have half the normal number of chromosomes
When a cell divides to form gametes:
- Before the cell starts to divide, it duplicates its genetic information, forming two
armed chromosomes – one arm of each chromosome is an exact copy of the other
arm. After replication, the chromosomes arrange themselves into pairs
- In the first division in meiosis, the chromosome pairs line up in the centre of the
cell
- The pairs are then pulled apart so each new cell only has one copy of each
chromosome. Some of the father’s chromosomes and some of the mother’s
chromosomes go into each new cell
, - In the second division, the chromosomes line up in the centre of the cell and the
arms of the chromosomes are pulled apart
- You get four gametes with only a single set of chromosomes in it. Each of the
gametes is genetically different from the other because the chromosomes all get
shuffles up during meiosis and each gamete gets only half of them, at random
Gametes join at fertilisation to restore the normal number of chromosomes. The new
cell divides by mitosis. The number of cells increases. As the embryo develops cells
differentiate
- mitosis – asexual reproduction, repair and growth
Asexual Sexual
No of parents 1 2
Speed Quick Slow
Type of cell division Mitosis Meiosis
Variation No Yes
Compare to parent Same Different
4.6.1.3 Advantages and disadvantage of sexual and asexual reproduction
Advantages of sexual reproduction:
- Offspring have a mixture of two sets of chromosomes – the organism inherits
genes from both parents which produces variation in the offspring
- If the environment changes variation gives a survival advantage by natural
selection
- Individuals with certain characteristics are better adapted to the environment so
have a better chance of survival – they are more likely to breed successfully and
pass the genes and characteristics on
- Natural selection can be sped up by humans in selective breeding to increase food
production
Advantages of asexual reproduction:
- Only one parent needed
- More time and energy efficient as don’t need to find a mate – lower risk of failure
- Faster than sexual reproduction
- Many identical offspring can be produced when conditions are favourable
Some organisms can reproduce by both methods depending on the circumstances
- Malarial parasites reproduce asexually in the human host, but sexually in the
mosquito
- Many fungi reproduce asexually by spores but also reproduce sexually to give
variation and are often produced in response to an unfavourable change in the
environment, increasing the chance that the population will survive the change
- Many plants produce seeds sexually, but also reproduce asexually by runners such
as strawberry plants
4.6 Inheritance, variation and evolution
4.6.1 Reproduction
4.6.1.1 Sexual and asexual reproduction
Sexual reproduction involves the joining (fusion) of male and female gametes, to
produce offspring that are genetically different:
- In humans, each gamete contains 23 chromosomes – half the number in a normal
cell (haploid)
- The egg and the sperm cell then fuse together (fertilisation) to form a cell with 46
chromosomes (diploid)
- Sperm and egg cells in animals
- Pollen and egg cells in flowering plants – pollination
In sexual reproduction there is mixing of genetic information which leads to variety in
offspring – introduces variation into a population. The formation of gametes involves
meiosis
- Used by larger more complex organisms
Asexual reproduction involves only one parents and no fusion of gametes. There is no
mixing of genetic information or variation. This leads to genetically identical offspring
(clones). Only mitosis is involved
- used by small simple organisms – bacteria and microscopic animals and plants
4.6.1.2 Meiosis
Cells in reproductive organs divide by meiosis to form gametes
- In sexual reproduction, the mother and father produce gametes by meiosis
- To make gametes which only have half the original number of chromosomes, cells
divide by meiosis involving two cell division. It humans it only happens in the
reproductive organs
Meiosis produces cells that have half the normal number of chromosomes
When a cell divides to form gametes:
- Before the cell starts to divide, it duplicates its genetic information, forming two
armed chromosomes – one arm of each chromosome is an exact copy of the other
arm. After replication, the chromosomes arrange themselves into pairs
- In the first division in meiosis, the chromosome pairs line up in the centre of the
cell
- The pairs are then pulled apart so each new cell only has one copy of each
chromosome. Some of the father’s chromosomes and some of the mother’s
chromosomes go into each new cell
, - In the second division, the chromosomes line up in the centre of the cell and the
arms of the chromosomes are pulled apart
- You get four gametes with only a single set of chromosomes in it. Each of the
gametes is genetically different from the other because the chromosomes all get
shuffles up during meiosis and each gamete gets only half of them, at random
Gametes join at fertilisation to restore the normal number of chromosomes. The new
cell divides by mitosis. The number of cells increases. As the embryo develops cells
differentiate
- mitosis – asexual reproduction, repair and growth
Asexual Sexual
No of parents 1 2
Speed Quick Slow
Type of cell division Mitosis Meiosis
Variation No Yes
Compare to parent Same Different
4.6.1.3 Advantages and disadvantage of sexual and asexual reproduction
Advantages of sexual reproduction:
- Offspring have a mixture of two sets of chromosomes – the organism inherits
genes from both parents which produces variation in the offspring
- If the environment changes variation gives a survival advantage by natural
selection
- Individuals with certain characteristics are better adapted to the environment so
have a better chance of survival – they are more likely to breed successfully and
pass the genes and characteristics on
- Natural selection can be sped up by humans in selective breeding to increase food
production
Advantages of asexual reproduction:
- Only one parent needed
- More time and energy efficient as don’t need to find a mate – lower risk of failure
- Faster than sexual reproduction
- Many identical offspring can be produced when conditions are favourable
Some organisms can reproduce by both methods depending on the circumstances
- Malarial parasites reproduce asexually in the human host, but sexually in the
mosquito
- Many fungi reproduce asexually by spores but also reproduce sexually to give
variation and are often produced in response to an unfavourable change in the
environment, increasing the chance that the population will survive the change
- Many plants produce seeds sexually, but also reproduce asexually by runners such
as strawberry plants
