Variation and Sexual Reproduction 2.3

Question 1

what are the costs of sexual reproduction?

Answer 1

males unable to produce offspring
only half of each parent’s genome passed onto offspring
disrupting successful parental genomes

Question 2

why do the benefits of sexual reproduction outweigh the costs?

Answer 2

due to an increase in genetic variation in the population

Question 3

what does genetic variation provide?

Answer 3

the raw material required for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures

Question 4

what is used to explain the persistence of sexual reproduction?

Answer 4

The Red Queen hypothesis

Question 5

what may be selected for sexually reproducing hosts?

Answer 5

Co-evolutionary interactions between parasites and hosts

Question 6

what happens if a host reproduces sexually?

Answer 6

the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites

Question 7

how can asexual reproduction be a successful reproductive strategy?

Answer 7

as whole genomes are passed on from parent to offspring

Question 8

what is maintaining the genome of the parent an advantage particularly in?

Answer 8

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
• parthenogenesis in lower plants
• animals that lack fertilisation

Question 10

how can offspring be reproduced in asexual reproduction?

Answer 10

more often and in larger numbers

Question 11

why is parthenogenesis more common in cooler climates?

Answer 11

it is disadvantageous to parasites, or regions of low parasite density or diversity

Question 12

Asexually reproducing populations are not able to adapt easily to changes in their environment, but what can occur that provides some degree of variation and enables some natural selection and evolution to occur?

Answer 12

mutations

Question 13

what do organisms that reproduce principally by asexual reproduction also often have mechanisms for and what is an example of this?

Answer 13

for horizontal gene transfer between individuals to increase variation

for example the plasmids of bacteria and yeasts

Question 14

what is meiosis?

Answer 14

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

Question 15

what do chromosomes typically appear as in diploid cells?

Answer 15

homologous pairs

Question 16

what happens in meiosis I ?

Answer 16

• The chromosomes, which have replicated prior to meiosis I, each consist of two genetically identical chromatids attached at 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
• This 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 II?

Answer 17

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

Question 18

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

Answer 18

presence of sex chromosomes

Question 19

In most mammals the SRY gene on the Y chromosome determines the development of what?

Answer 19

male characteristics

Question 20

what do heterogametic (XY) males lack most of the corresponding homologous alleles on and what can this result in?

Answer 20

the shorter (Y) chromosome

results in sex-linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)

Question 21

in homogametic females (XX) what is randomly inactivated at an early stage of development?

Answer 21

one of the two X chromosomes present in each cell

Question 22

what does the X chromosome inactivation prevent and how does this affect the carriers?

Answer 22

a double dose of gene products, which could be harmful to cells

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

Question 23

as the X chromosome inactivated in each cell is random, half of the cells in any tissue will what?

Answer 23

have a working copy of the gene in question

Question 24

what are hermaphrodites ?

Answer 24

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

Question 25

what do hermaphrodites produce?

Answer 25

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

Question 26

what is a benefit of hermaphrodites?

Answer 26

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 27

For other species, environmental rather than what determine sex and sex ratio

Answer 27

genetic factors

Question 28

what can sex change within individuals of some species be a result of?

Answer 28

• size
• competition
• parasitic infection

Question 29

what can the sex ratio of offspring in some species be adjusted to in response of?

Answer 29

resource availability

Question 30

how do prokaryotes exchange genetic material and what is the result of this?

Answer 30

exchange genetic material horizontally

results in faster evolutionary change then in gametocyte

Question 31

what are homologous chromosomes?

Answer 31

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

Question 32

what are linked genes?

Answer 32

those on the same chromosome and crossing over can result in new combinations of the alleles of these genes

Question 33

what happens through recombination?

Answer 33

crossing over of DNA is random and produces genetically different chromosomes

Question 34

what is independent assortment?

Answer 34

where each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin.