Variation and sexual reproduction

• cost of sexual reproduction

  males unable to produce offspring; only half of each parents genome passed onto offspring, disrupting successful parental genomes
• benefit of sexual reproduction

  outweighs the cost due to an increase in genetic variation in the population
• genetic variation


  provides the raw material required for adaptation, giving sexually reproducing organisms better chance of survival under changing selection pressures
• co-evolutionary interactions between parasites and host

  may select for sexually reproducing hosts
• the red queen hypothesis can explain the persistence of sexual reproduction
• 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 from parent to offspring.
• Maintaining the genome of the parent is an advantage, particularly in a very narrow, stable niche 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
• 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.
• meiosis


  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
• Meiosis 1 (part 1)

  The chromosomes which have replicated before meiosis 1, each consist of two genetically identical chromatids attached at the centromere, these 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
• meiosis 1 (part 2)

  spindle fibres attach to the homologous chromosomes and line them up at the equator of the spindle. The orientation of the pairs of the 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.
• meiosis II


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


  when lining up at the equator, homologous pairs line up randomly relative to another pair. leads to different genetic combinations in gametes
• crossing over


  when chromosomes pair, portions of the chromatids are exchanged. the position where this is called chiasma (chiasmata plural). The area on chromosomes after crossing over is known as recombinant chromatids.
• crossing over and independent assortment increase genetic variation
• linkage


  when a cross involves 2 alleles of 2 different genes on the same chromosome the 2 genes are transmitted together are said to be linked
• crossover value


  $cov=$$(number of F2 recombinants/total number of F2 offspring) *$$100$
• 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 chromosomes present in each cell is randomly inactivated at an early stage of development. The inactivation prevents 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


  specie 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 non-hermaphrodites 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