Variation and sexual reproduction

a. Costs and benefits of sexual and asexual reproduction In this section the learning will be about comparing the costs and benefits of sexual and asexual reproduction

Sexual reproduction Sex cells (gametes) produced by meiosis Two parents required to contribute genetic material 50% of genetic material comes from each parent Variation is produced in offspring Offspring often protected Can take a long time to produce offspring Occurs in higher invertebrates, all vertebrates and many plants Raw material for evolution (Red Queen)

Asexual reproduction Involves binary fission, budding, vegetative reproduction, fragmentation, spore formation and parthenogenesis One parent is required 100% of genetic material from parent No or very little variation in offspring Occurs quite rapidly Occurs mainly in lower organisms and many plants

Parthenogenesis Females produce offspring from unfertilized eggs Includes some nematodes, crustaceans, insects and desert lizards More common in cooler climates that parasites find a disadvantage More common where parasite density or diversity is low

Horizontal gene transfer Occurs in bacteria and yeast often using plasmids (and viruses)

b. Meiosis forms variable gametes In this section the learning will be looking at the process of meiosis and how it produces variation Meiosis sheet with questions

Meiosis is the process of making haploid gametes from gamete mother cells. Meiosis increases variation

Meiosis I – first meiotic division

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

Meiosis I – first meiotic division Pairing of homologous chromosomes

Meiosis I – first meiotic division Random crossing over at chiasmata resulting in exchange of DNA between homologous pairs and recombination of linked genes

Meiosis I – first meiotic division Independent assortment and separation of parental chromosomes irrespective of their maternal and paternal origin takes place

Meiosis II – second meiotic division Separation of sister chromatids/ chromosomes and gamete formation

Meiosis – the whole Haploid gametes from diploid gamete mother cells

In many organisms gametes are formed directly from the cells produced by meiosis In some groups, mitosis may occur after meiosis to form a haploid organism and gametes form later by differentiation such as ferns and lilies

c. Sex determination In this section the learning will be looking at sex determination and how it effects inheritance and variation

Many species are hermaphroditic Many invertebrates and a smaller number of vertebrates are hermaphrodites. A hermaphrodite possesses both male and female reproductive organs during their life span. Some of these animals self-fertilize, while others require a partner. Hermaphroditism is a varied mode of reproduction that manifests differently depending on the affected species. Snails, worms, echinoderms, some fish, most plants

Sex and sex ratios can be determined by environmental rather than genetic factors in some species Turtles and crocodiles sex is determined by temperature during development in the egg. Low temperatures (22oC - 27oC) often give one sex whilst higher temperatures (above 30oC) give the other It appears that the enzyme aromatase (which can convert testosterone into oestrogen) is important in temperature dependent sex determination

Sex and sex ratios can be determined by environmental rather than genetic factors in some species i.e. temperature

Sex and sex ratios can be determined by environmental rather than genetic factors in some species i.e. location (slipper snail Crepidula fornicata). Individuals pile up on top of one another to form a mound. Young individuals are always male. This phase is followed by the degeneration of the male reproductive system. The next phase can be either male or female, depending on the animal's position in the mound. If the snail is attached to a female, it will become male. If such a snail is removed from its attachment, it will become female. Similarly, the presence of large numbers of males will cause some of the males to become females

Sex can change within some individuals of a species as a result of size, competition or parasitic infection Some species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male, whilst in bluehead wrasse it is the reverse. In the marine worm (Bonellia viridis), larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin

Sex chromosomes XY determine sex in live-young bearing mammals and some insects including drosophila. Other organisms are also heterogametic in their sex chromosomes.

Male In many mammals a gene on the Y chromosome determines the development of maleness In live-young bearing mammals the heterogametic (XY) male lacks homologous alleles on the smaller (Y) chromosome Males have only 1 allele to express from the X chromosome and this can have implications for recessive detrimental trait expression in the phenotype (XbY)

Females The expression of 2 alleles from the X chromosomes means they can carry a trait but not be sufferers of it in their phenotype (XBXb) A portion of the X chromosomes that are non homologous to the Y chromosome are randomly inactivated in female cells (in only one of the X chromosomes) This prevents double gene expression of possible harmful traits in cells Carriers are less likely to be affected by any deleterious mutations on X chromosomes as inactivation is random, so half of the cells in any tissue will have a working gene