Variation and Sexual Reproduction

Question 1

What is sexual reproduction?

Answer 1

production of new living organisms by combining genetic information from two individuals of different types (sexes)

Question 2

What is asexual production?

Answer 2

a mode of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only.

Question 3

What does evolution favour?

Answer 3

the individuals that are able to produce the maximum number of surviving offspring using least amount of energy

Question 4

What are the benefits of sexual reproduction?

Answer 4

• Maintains greater genetic variation

- Fertilisation is a random process resulting in new combinations of alleles.

Question 5

What is variation?

Answer 5

the differences that exist between individuals in a population determined by the alleles that are inherited.

Question 6

What does genetic variation allow?

Answer 6

allows a species to adapt and survive in a changing environment and enables long-term evolutionary change. This is a benefit of sexual reproduction.

Question 7

What are the costs of sexual reproduction?

Answer 7

1. Half of the population (males) are unable to produce offspring.
2. Each parent is only able to pass on half of their genetic material rather than the full 100%.
3. Large energy expenditure in courtship and mating.

Question 8

What can asexual reproduction be a successful reproductive strategy in?

Answer 8

Asexual reproduction can be a successful reproductive strategy, particularly in very narrow stable niches or when recolonising disturbed habitats.

Question 9

Give examples of vegetative cloning in plants (asexual reproduction).

Answer 9

 Plantlets on runners e.g. spider plant, strawberry plant.
 Leaf edge plantlets e.g. Mexican Hat Plant.
 Tubers e.g. potato plant.
 Offsets e.g. water lettuce, bulbs such as onions.

Question 10

By what processes may animals reproduce asexually?

Answer 10

through fission, budding, fragmentation, or parthenogenesis.

Question 11

Describe parthenogenesis in detail.

Answer 11

• type of asexual reproduction in which the offspring develop from unfertilised eggs.
• An egg develops into a complete individual without being fertilised.
• Resulting offspring can be either haploid or diploid, depending on the process and the species.
• Occurs in invertebrates such as water fleas, stick insects and bees. Occurs in some vertebrates such as certain reptiles and fish.
• Parthenogenesis is more common in cooler climates that are disadvantageous to parasites.
• It is also common in regions of low parasite density/diversity.

Question 12

What is meiosis?

Answer 12

Meiosis is the process of nuclear division that results in the production of haploid gametes.

Question 13

What doesn’t occur during asexual reproduction?

Answer 13

meiosis

Question 14

What does meiosis do?

Answer 14

Meiosis increases variation.

Question 15

How does genetic variation arise during meiosis?

Answer 15

During meiosis, new combinations of alleles arise by independent assortment and crossing over allowing genetic variation to arise.

Question 16

What are the only diploid cells capable of carrying out meiosis and where are they located?

Answer 16

The only diploid cells capable of carrying out meiosis are the gamete mother cells located in the sex organs.

Question 17

What are homologous chromosomes?

Answer 17

Homologous chromosomes are pairs of chromosomes of the
 Same size
 Same centromere position
 Same genes at the same loci

Question 18

Describe the different between homologous chromosomes.

Answer 18

Although the genes are the same on homologous chromosomes the alleles of the genes of homologous chromosomes may be different.

Question 19

Describe interphase I.

Answer 19

1. Interphase I – chromosomes duplicate to form sister chromatids attached at centromeres.

Question 20

Describe prophase I.

Answer 20

As the chromosomes condense the homologous chromosomes are arranged to form a tetrad during the removal of the nuclear membrane. As the homologous pairs align there is the first main possibility for increased variation as chiasmata form. Each chiasma is a potential site for crossing-over leading to recombinant strands.

Question 21

Describe metaphase I.

Answer 21

The chromosomes line up on the metaphase plate arranged their pairs. This allows the second main increase in variation with random assortment. The Kinetochores of each chromatids in a linked pair are aligned so that each homologous chromosome is linked to each pole.

Question 22

Describe anaphase I.

Answer 22

Spindle fibres pull each of the homologous chromosomes (two joined sister chromatids) to the opposite poles of the cell.

Question 23

Describe telophase I and cytokinesis.

Answer 23

As the chromosomes reach the poles the cell divides. Each pole has a haploid chromosome set, but each chromosome still has 2 sister chromatids. Depending on species there may be nuclear membranes formed. 2 daughter cells formed – cleavage furrows formed in animals whereas cell plates appear in plants. The chromosomes do not decondense.

Question 24

Describe prophase II.

Answer 24

If nuclear membranes formed, then they are removed. Centriole pairs are duplicated again, and spindle apparatus is formed.

Question 25

Describe metaphase II.

Answer 25

The chromosomes are positioned on the metaphase plate. Spindle fibres attach to the kinetochore of sister chromatids.

Question 26

Describe anaphase II.

Answer 26

The sister chromatids are separated and drawn to opposing poles. Chromatids now individual chromosomes.

Question 27

Describe telophase II and cytokinesis.

Answer 27

Nuclei form at the poles and cytokinesis starts. 4 genetically varied haploid daughter cells (gametes) formed.

Question 28

Describe independent assortment.

Answer 28

 Homologous chromosomes pair up along the equator of the cells in a random and independent manner.
 Independent assortment results in the production of gametes with varying combinations of maternal and paternal chromosomes.

Question 29

How can the number of different combinations produce by independent assortment be calculated?

Answer 29

using the formula 2n where n is the haploid number.

Question 30

Describe crossing over.

Answer 30

During meiosis I when homologous chromosomes pair up, the inner non-sister chromatids can cross over at points called chiasma.
The inner chromatids break at the chiasma and swap a section of their genetic material – resulting in the recombination of alleles.
Random crossing over at the chiasmata results in an exchange of DNA between homologous pairs and recombination of alleles of linked genes.

Question 31

What are genes on the same chromosome said to be?

Answer 31

linked

Question 32

Describe the effect of distance between linked genes on the frequency of recombination.

Answer 32

The greater the distance between linked genes, the greater the chance of crossing-over and the greater the frequency of recombination.

Question 33

What does the higher the number of recombinants suggest?

Answer 33

the further apart the genes are.

Question 34

What are many organisms, usually invertebrates said to be?

Answer 34

hermaphroditic

Question 35

What are simultaneous hermaphrodites?

Answer 35

Simultaneous hermaphrodites are organisms with both male and female reproductive organs, e.g. earthworms and slugs.

Question 36

What are sequential hermaphrodites?

Answer 36

Sequential hermaphrodites are born as one sex and may change to the other sex at some point during their life, e.g. some fish and jellyfish. This usually happens if reproductive success is likely to be much greater by being the other sex.

Question 37

Other than genetic control, what can also influence the sex of an organism? Give an example.

Answer 37

Sometimes, sex determination is under environmental rather than genetic control. Crocodiles and alligators are a good example of this. Sex determination in reptiles is controlled by environmental temperature of egg incubation.

Question 38

What three things can cause the sex of an organism to change?

Answer 38

Change in size
Social interactions including competition
Parasitic infection

Question 39

Using Clownfish as an example, explain how social interactions can affect sex determination.

Answer 39

Clownfish have a very structured society, there are zero to four individuals excluded from breeding and a breeding air living in a sea anemone.
Dominance is based on size, the female being the largest and the male being the second largest.
The rest of the group is made up of progressively smaller non-breeders, which have no functional gonads.
If the female dies, the male gains weight and become the female for that group.
The largest non-breeding fish then sexually matures and becomes the male of the group.