Meiosis Is the special form of cell division used to produce gametes It has two functions: –To form haploid cells with half the chromosome number –To re-arrange the chromosomes with a novel combination of genes

Meiosis Meiosis comprises of two successive divisions without DNA replication in between The stages of these divisions are on your worksheet… –Either draw or cut and stick the corresponding picture in the boxes

Meiosis Rearrange the following steps of meiosis –Separation of chromatids –Formation of bivalents –Pairing by homologous chromosomes –Production of haploid cells –Chiasma formation and exchange between chromatids

Genetic Variation Meiosis and sexual reproduction allows for genetic variation This allows species to adapt to their environments through evolution There are three sources of genetic variation in sexual reproduction Can you name them? –Independent Assortment –Crossing Over –Random Fertilisation

Independent Assortment Happens in metaphase I, when the bivalent line up on the equator Each bivalent is made up of two chromosomes The chromosomes can line up in any order on the equator resulting in different possible combinations

Independent Assortment In the sample below, n=2 and there are 4 possible gametes In humans n=23 so there are over 8 million possible different gametes!

Crossing Over This occurs in prophase I While the two homologous chromosomes are joined in a bivalent, bits of one chromosome are swapped (crossed over) with the corresponding bits of the other chromosome

Crossing Over There are always equal amounts crossed over so chromosomes stay the same length Crossing over means that the maternal and paternal alleles can be mixed

Random Fertilisation This takes place when two gametes fuse to form a zygote Each gamete has a unique combination of genes and any of the numerous male gametes can fertilise the egg

Gene Mutation DNA is very stable but bases can change during DNA replication Most mutation have no phenotypic effect, these are called silent mutations

Variation Below is an outline of causes of variation As well as genetic causes, individuals are influenced by the environment Genotype + environment = phenotype

Discontinuous Variation Are distinct, separate peaks/bars Are distinct categories to place individuals Tends to have no overlap between categories Are controlled by a small number of genes Are largely unaffected by the environment

Continuous Variation Is a continuous range of values so get a smooth curve Are no distinct categories to place individuals Tends to have overlap between categories Are controlled by a large number of genes Are significantly affected by the environment

Sex Determination Females produce eggs which carry the X chromosome. Sperm carry either X or Y Males therefore determine the sex of the child Females = XX Males = XY

Sex Determination It is though that Y chromosomes dislike acid conditions within the female reproductive systems Y chromosomes are smaller and will therefore float in a sample of semen if centrifuged Clinics can use this information to select sex