Variation and Sexual Reproduction

Question 1

What are the costs of sexual reproduction?

Answer 1

males are unable to produce offspring, only half of each parents gamete is passed onto offspring and it disrupts successful parental genomes.

Question 2

What are the benefits of sexual reproduction?

Answer 2

Theres an increase in genetic variation.

Question 3

What does genetic variation provide?

Answer 3

Genetic variation provides the raw material required for adaption, giving sexually reproducing organisms a better chance of survival under changing selection pressures.

Question 4

Use the Red Queen Hypothesis to explain the persistence of sexual reproduction.

Answer 4

Without such genetic variety, the Red Queen Arms race would stop. In other words, this variation can drive selection and evolution should the different genes and alleles provide the individual with an advantage and increased chance of survival.

Question 5

What may select for sexually reproducing hosts?

Answer 5

Co-evolutionary interactions between parasites and hosts may select for sexually reproducing hosts.

Question 6

What does it mean for parasites when a host reproduces sexually?

Answer 6

If hosts reproduce sexually, the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites.

Question 7

What are the benefits of asexual reproduction?

Answer 7

Asexual reproduction can be a successful reproductive strategy as whole genomes are passed on from parent to offspring.

Question 8

In what niche is maintaining the genome of a parent is an advantage?

Answer 8

Maintaining the genome of the parent is an advantage in very narrow, stable niches or when re-colonising disturbed habitats.

Question 9

What are examples of asexual reproduction in eukaryotes?

Answer 9

Vegetative cloning in plants and parthenogenesis in lower plants and animals that fertilisation are examples of asexual reproduction in eukaryotes.

Question 10

How often and how much offspring are produced during asexual reproduction.

Answer 10

Offspring can be reproduced more often and in larger numbers with asexual reproduction.

Question 11

Where is parthenogenesis more common?

Answer 11

Parthenogenesis is more common in cooler climates, which are disadvantageous to parasites, or regions of low parasite density or diversity.

Question 12

How can mutations help organisms and plants that reproduce asexually.

Answer 12

Asexually reproducing populations are not able to adapt easily to changes in their environment, but mutations can occur that provide some degree of variation and enable some natural selection and evolution to occur

Question 13

How do organisms that reproduce principally by asexual reproduction have variation?

Answer 13

Organisms that reproduce principally by asexual reproduction also often have mechanisms for horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts.

Question 14

What is meiosis?

Answer 14

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

Question 15

What do chromosomes appear as in diploid cells?

Answer 15

In diploid cells, chromosomes typically appear as homologous pairs.

Question 16

What happens in meiosis 1?

Answer 16

The chromosomes, which have replicated prior to meiosis 1, each consist of two genetically identical chromatids attached to the centromere.
The chromosomes condense and the homologous chromosomes pair up
Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged.
The crossing over of DNA is random and produces genetically different recombinant chromosomes.
Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle.
The orientation of the pairs of homologous chromosomes at the equator is random.
The chromosomes of each homologous pair are separated and move towards opposite poles.
Cytokinesis occurs and two daughter cells form.

Question 17

What happens in meiosis 2?

Answer 17

Each of the two cells produced in meiosis 1 undergoes a further division during which the sister chromatids of each chromosome are separated.

Question 18

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

Answer 18

The sex of birds, mammals and some insects is determined by the presence of sex chromosomes.

Question 19

What does a SRY gene on the Y chromosome determine?

Answer 19

In most mammals the SRY gene on the Y chromosome determines development of male characteristics.

Question 20

Where do heterogametic males lack corresponding homologous alleles?

Answer 20

Heterogametic males lack most of the corresponding homologous alleles on the shorter Y chromosome.

Question 21

What does lack in corresponding homologous alleles result in?

Answer 21

This can result in sex-linked patterns of inhertence as seen with carrier females and affected males.

Question 22

What inactivates in early stages of development?

Answer 22

In homogametic females one of the two X chromosomes present in each cell is randomly inactivated at an early stage of development.

Question 23

What does X chromosome inactivation prevent?

Answer 23

X chromosome inactivation prevents a double dose of gene products, which could be harmful to cells.

Question 24

What is less likely to be affected by any deleterious mutations on these X chromosomes?

Answer 24

Carriers are less likely to be affected by any deleterious mutations on these X chromosomes.

Question 25

How do half of the cells in any tissue have working copies of the gene in question?

Answer 25

As the X chromosome inactivated in each cell is random, half of the cells in ant tissue have a working copy of the gene in question.

Question 26

What is hermaphrodites?

Answer 26

Hermaphrodites are species that have functioning male and female reproductive organs in each individual.

Question 27

What do hermaphrodites produce?

Answer 27

They produce both male and female gametes and usually have a partner with which to exchange gametes.

Question 28

What is the benefit of being a hermaphrodite?

Answer 28

The benefit to the individual organism is that if the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex.

Question 29

What determines the sex and sex ratio?

Answer 29

Environmental rather than genetic factors determine sex and sex ratio.

Question 30

How can sex change?

Answer 30

Sex can change within individuals of some species as a result of size, competition or parasitic infection.

Question 31

How else can the sex ratio of offspring be adjusted?

Answer 31

In some species the sex ration of offspring can be adjusted in response to resource availability.