Variation And Sexual Reproduction

Question 1

State some of the costs involved in sexual reproduction.

Answer 1

• males are unable to produce offspring

- only half of each parent’s genome is passed on to offspring

Question 2

Why is the fact that only half of each parents genome is passed on a cost of sexual reproduction?

Answer 2

It disrupts successful parental genomes.

Question 3

Which is higher in sexual reproduction, costs or benefits and why?

Answer 3

The benefits outweigh costs due to an increase in genetic variation in populations.

Question 4

What is genetic variation and why does it benefit species?

Answer 4

Genetic variation provides the raw materials required for adaptation, giving sexually reproducing species a better chance of survival under changing selection pressures.

Question 5

What hypothesis can be used to explain the persistence of sexual reproduction?

Answer 5

The Red Queen Hypothesis

Question 6

Explain why co-evolutionary interactions between parasites and hosts may select for sexually reproducing hosts.

Answer 6

• hosts better able to resist and tolerate parasites have greater fitness
• parasites better able to feed, reproduce and find new hosts also have greater fitness
• if hosts reproduce sexually, the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites.

Question 7

Explain the Red Queen Hypothesis in terms of parasites and hosts.

Answer 7

• host evolves and becomes more resistant to parasitic infection
• this increases selection pressure on parasite infection
• which causes parasites to evolve and become more virulent towards it’s host
• this increases selection pressure on host
• the cycle starts agin

Question 8

Explain the benefits of asexual reproduction.

Answer 8

• whole genomes are passed on from parent to offspring
• only on parent is needed
• offspring can be reproduced more often and in larger numbers

Question 9

When is maintaining the genome of the parent an advantage?

Answer 9

• In very narrow, stable niches or when recolonising disturbed habitats.

Question 10

State some examples of asexual reproduction.

Answer 10

• vegetative cloning and parthenogenesis in lower plants and animals that lack fertilisation.

Question 11

What is parthenogenesis and where is it more common?

Answer 11

Parthenogenesis is reproduction from a female without fertilisation and is more common in cooler climates, which are disadvantageous to parasites, or regions of low parasite density or diversity.

Question 12

How can asexually reproducing species provide some degree of variation?

Answer 12

• mutation

- horizontal gene transfer

Question 13

Give an example of an asexual horizontal gene transfer.

Answer 13

The plasmids of bacteria and yeast.

Question 14

What is horizontal gene transfer?

Answer 14

When prokaryotes exchange genetic material horizontally (same generation) resulting in faster evolutionary change than in organisms that only use vertical transfer.

Question 15

What is Meiosis?

Answer 15

Meiosis is the division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte.

Question 16

What are homologous chromosomes?

Answer 16

Chromosomes of the same size, same centromere position and with the same sequence of genes at the same loci.

Question 17

Explain the process of meiosis.

Answer 17

• chromosomes are replicated prior to meiosis 1 and consist of two genetically identical sister chromatids attached at the centromere
• at meiosis 1, chromosomes condense, the homologous chromosomes pair up and chiasmata form at points of contact between non-sister chromatids of a homologous pair and sections of DNA are exchanged
• spindle fibres attach to the homologous pairs and line them up at the equator
• the orientation of the pairs of homologous at the equator is random, and each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin- this is independent assortment
• the chromosomes of each homologous pair are separated and move towards opposite poles
• cytokinesis occurs and only two daughter cells form
• each daughter cell from meiosis 1 undergo a second division called meiosis 2 which the sister chromatids are pulled apart
• a total number of 4 haploid cells are produced as a result of meiosis 2.

Question 18

State the stages in order of meiosis.

Answer 18

• interphase
• prophase
• metaphase
• anaphase
• telophase
• cytokinesis

Question 19

How is the sex of birds, mammals and some insects determined?

Answer 19

By the presence of sex chromosomes.

Question 20

What is the name of the gene in mammals that encodes testes determining factor (TDF) and determines development of male characteristics?

Answer 20

SRY gene

Question 21

What is the term that describes the (X,Y) characteristics in males?

Answer 21

Heterogametic

Question 22

What is the term that describes the (X,X) characteristics in females?

Answer 22

Homogametic

Question 23

What is X chromosomes inactivation and why is it useful?

Answer 23

X chromosome inactivation is when one of the two X chromosomes present in each cell is randomly inactivated at an early stage of development. It prevents a double dose of gene products which could be harmful to cells. Carriers are less likely to be affected by deleterious mutations on one of their X chromosomes because of random inactivation and as it is random, half the cells in any tissue will have a working copy of the gene in question.

Question 24

What are hermaphrodites?

Answer 24

They are species that have functioning male and female reproductive organs and they produce both male and female gametes, and usually need a partner with which to exchange gametes.

Question 25

When is being a hermaphrodite useful and why?

Answer 25

When the chance of encountering of a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex.

Question 26

What other factros can determine sex and sex ratio other than genetic factors?

Answer 26

Environmental factors.