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

1

4

5

3

2

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