(a) Costs and benefits of sexual and asexual reproduction

• Costs of sexual reproduction: males unable to produce offspring; only half of each parent’s genome passed onto offspring, disrupting successful parental genomes.

• 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

• The Red Queen hypothesis to explain the persistence of sexual reproduction

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

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

  • Parasites better able to feed, reproduce and find new hosts 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

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

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

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

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

  • Parthenogenesis is reproduction from a female gamete without fertilisation.

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

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

• 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.

  • Mechanisms of horizontal gene transfer are not required.

(b) Meiosis

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

• 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.

Meiosis I

• 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

  • Linked genes are those on the same chromosome. 

  • Crossing over can result in new combinations of the alleles of these genes.

• 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

  • 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.

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

• Cytokinesis occurs and two daughter cells form

Meiosis II

• 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.

(c) Sex determination

• 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

• 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)

• 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

• 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

• 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

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

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

• 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