BIOLOGY 12 Meiosis: Continuity and Variation

Recap How meiosis works

meiosis ensures genetic continuity between generations and promotes variation how? humans have traits of humans not all humans are exactly alike

Genetic Variation four methods independent assortment crossing over or genetic recombination random fertilization non-disjunction why is variation important? so we are not all the same

Independent Assortment orientation of the homologous chromosomes at the equator is random two possibilities for each pair: fifty-fifty chance of receiving maternal or paternal

Independent Assortment 2 n = number of possible combinations n is number of haploid chromosomes humans: 2 23 = 8 million possible combinations!

Crossing Over the exchange of genetic material between two homologous chromosomes produces individual chromosomes that combine genes inherited from both parents

Crossing Over in humans, an average of two or three crossing over events occur per chromosome pair Crossing Over / Genetic Recombination

Random Fertilization two parents: one egg  1 of 8 million combinations one sperm  1 of 8 million combinations 8 million x 8 million = 64 trillion diploid combinations!

Non-Disjunction the failure of chromosome pairs to separate properly (either chromosome pairs or sister chromatids) results in cells with an imbalance of chromosomes

Non-Disjunction loss of a single chromosome (2n – 1) daughter cell will have one chromosome missing from one of its pairs Monosomy gaining a single chromosome (2n + 1) daughter cell will have an extra chromosome in a pair Trisomy

Non-Disjunction examples: Down syndrome (Trisomy-21) Patau syndrome (Trisomy-13) Patau Edward syndrome (Trisomy-18) Edward Turner syndrome (Monosomy X chromosome) Turner

Questions 1. Why does meiosis occur? 2. Explain how Meiosis increases genetic variation among a species. 3. Sketch and label Meiosis for a cell with 2n = Explain the difference between a tetrad, chromatid, and chromosome. 5. When do the following events occur? 1. Homologous chromosomes pair up and form tetrads. 2. Spindle fibres move homologous chromosomes to opposite poles. 3. Sister chromatids line up at the equator. 4. Crossing over occurs. 5. Chromatids separate. 6. Homologous pairs line up at the equator. 7. Cytoplasm divides, 2 daughter cells are formed.