Inheritance of Traits Sexual reproduction In organisms that reproduce sexually, traits are inherited through gametes. Gametes (sperm and eggs) are produced by meiosis. Asexual reproduction Genetically identical clones of the parent. In some exceptions, genetic material can be exchanged between clones. Example: antibiotic resistance in bacteria can be transferred via plasmid DNA. In sexual reproduction, haploid sperm and egg unite to form a diploid zygote. Bacteria reproduce asexually by binary fission.

Binary Fission and Budding Asexual reproduction: binary fission. A single parent cell dividing into two daughter cells. Asexual reproduction: budding. Part of the parent body is ‘pinched off’ to produce a small individual exactly like the parent. Binary fission in the bacterium Staphylococcus Budding in Hydra New individual

Sexual ReproductionAsexual Reproduction Produces genetic variation. Usually takes longer than asexual reproduction. Quick. No variation, so are more vulnerable when environment changes. Some organisms, e.g. aphids and water fleas (Daphnia), reproduce asexually most of the time, but also go through sexual reproduction as well. Sexual vs Asexual Reproduction Daphnia with eggs. These diploid eggs are produced mitotically and hatch into clones of the adult.

Introduction to Meiosis The purpose of meiosis is to produce haploid sex cells (gametes). They have one copy of each chromosome. In humans the haploid number is 23. Meiosis only occurs only in the ovaries and testes. Sperm surround an egg prior to fertilization

Meiosis I One chromosome from each homologous pair is donated to each intermediate cell. In prophase 1, homologues pair up to form tetrads in a process called synapsis. The sister chromatids can exchange genetic information during crossing over. Anaphase 1 separates homologous chromosomes. Interphase DNA replication 2N Synapsis and crossing over 2N Prophase 1 Tetrads line up on the equator of the cell. 2N Metaphase 1 Intermediate cell 1N Telophase 1 Intermediate cell Anaphase 1 Homologues separate

Meiosis II The second division of meiosis is similar to mitosis. There is no chromosome duplication in meiosis II. Sister chomatids (now separate chromosomes) are pulled apart and are donated to each gamete cell. The gametes are haploid (1N). 1N Intermediate cell Individual chromosomes separate Anaphase 2 Prophase 2 1N Telophase 2 Gamete (egg or sperm) 1N Metaphase 2 1N

Cell Division: An Overview Female embryo 2N Male embryo 2N Mitosis Somatic cell production Embryo 2N Egg 1N Sperm 1N Fertilization Zygote 2N Mitosis Somatic cell production Adult 2N Mitosis Meiosis Male adult 2N Female adult 2N Spermatogenesis Oogenesis

Aneuploidy Nondisjunction during meiosis can lead to an abnormal chromosome number. These chromosomal defects are known as aneuploidy. XY XX XXXX XX XO XXY

Aneuploidy in Sex Chromosomes Aneuploidy can result in an abnormal number of sex chromosomes. Turner syndrome affects females. X Klinefelter syndrome affects males. XXY Turner Syndrome karyotype

Aneuploidy in Autosomes Aneuploidy can also affect autosomes (non-sex chromosomes). In trisomy, the nucleus of the cells have one extra chromosome (2N+1). Three forms of trisomy survive to birth: Down syndrome (trisosmy 21) Edward syndrome (trisomy 18) Patau syndrome (trisomy 13) Down Syndrome karyotype