Variation and Sexual Reproduction

Question 1

What are costs of sexual reproduction ?

Answer 1

Costs of sexual reproduction:
-Males unable to produce offspring, slower population increase (The Paradox of the existence of males)
-Only half of each parent’s genome passed onto offspring, disrupting successful parental genomes

Question 2

What are the benefits of sexual reproduction ?

Answer 2

Benefits outweigh costs due to an increase in genetic variation in the population

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

Question 3

Use The Red Queen hypothesis to explain the persistence of sexual reproduction (between parasites and hosts)

Answer 3

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

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

Question 4

Describe how fitness is measured in a host and a parasite ?

Answer 4

Hosts better able to resist and tolerate parasitism have greater fitness.

Parasites better able to feed, reproduce and find new hosts have greater fitness.

Question 5

What is asexual reproduction and why can it be successful ?

Answer 5

In asexual reproduction, just one parent can produce daughter cells and establish a colony of virtually unlimited size over time.

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

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

Question 6

When can asexual reproduction be most successful ?

Answer 6

Maintaining the genome of the parent (in asexual reproduction) is an advantage particularly in very narrow, stable niches or when re-colonising disturbed habitat

Question 7

What are examples of asexual reproduction in eukaryotes ?

Answer 7

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

Question 8

What is parthenogenesis ?

Answer 8

Parthenogenesis is reproduction from a female gamete without fertilisation

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

Question 9

How do asexually reproducing populations adapt ?

Answer 9

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.

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

Prokaryotes can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer.

Question 10

What is meiosis ?

Answer 10

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

Question 11

What are homologous chromosomes ?

Answer 11

In diploid cells, chromosomes typically appear as homologous pairs

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

Question 12

Describe the stages of meiosis I

Answer 12

1) The chromosomes, which have replicated prior to meiosis I, each consist of two genetically identical chromatids attached at the centromere

2) The chromosomes condense and the homologous chromosomes pair up

3) Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged (homologous pair = easy touching each other for crossing over)

4) Linked genes are those on the same chromosome. Crossing over can result in new combinations of the alleles of these genes.

5) This crossing over of DNA is random and produces genetically different recombinant chromosomes

6) Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle

7) The orientation of the pairs of homologous chromosomes at the equator is random. Each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin. This is known as independent assortment.

8) The chromosomes of each homologous pair are separated and move towards opposite poles

9) Cytokinesis occurs and two daughter cells form

Crossing over doesn’t always occur, but can happen

Question 13

How can the number of different combinations and different potential gametes produced by independent assortment be calculated ?

Answer 13

Using the formula 2^n, where n is the haploid number

If has 23 pairs of chromosomes, 2^23

Assume no crossing over

Independent assortment results in new combinations of allelles and increased variation

Question 14

Describe meiosis II

Answer 14

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

A total of four haploid cells are produced

Question 15

What can determine sex ?

Answer 15

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

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

Question 16

What are the implications of males being heterogametic ?

Answer 16

-Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome

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

Question 17

What is X chromosome inactivation and why does it occur ?

Answer 17

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

-X chromosome inactivation is a process by which most of one X chromosome is inactivated.

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

Question 18

Explain the impacts of X chromosome inactivation on mutations

Answer 18

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

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

Question 19

What is a hermaphrodite ?

Answer 19

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

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

-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 20

What can determine sex in other species ?

Answer 20

-For other species, environmental rather than genetic factors determine sex and sex ratio

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

-In some species the sex ratio of offspring can be adjusted in response to resource availability

-Environmental sex determination in reptiles is controlled by environmental temperature of egg incubation.