WHY MEIOSIS? = WHY SEX? Youtube.

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Presentation transcript:

WHY MEIOSIS? = WHY SEX? Youtube

WHY SEX? Purifying Selection Hypothesis – Changing Environment Hypothesis -

Why Meiosis? Sea Urchin Sperm and Egg Ascaris lumbricoides Meiosis = “lessening act” Ascaris lumbricoides Body Cells - 4 chromosomes (2n) Sperm and Egg – 2 chromosomes (n)

Chromosomes Come In Distinct Types! Human Karyotype Autosomes = Sex chromosomes = Homologous chromosomes / homologs = Allele = Autosomes ; sex chromosomes; two chromosomes of each type = homologous chromosomes (homologs) = similar in size and shape and carry the same genes; each homologous chromosomes has a version of a gene called an allel

Polyploidy

Haploid number (n)? Ploidy? Total number of chromosomes present?

Meiosis is a Reduction Division! Synapsis; Tetrad; Sister Chromatids; Non-sister chromatids; chiasma(ta)

How does sexual reproduction produce offspring that are genetically unique? 1. CROSSING OVER Synapsis; Tetrad; Sister Chromatids; Non-sister chromatids; chiasma(ta)

2. INDEPENDENT ASSORTMENT OF CHROMOSOMES DURING METAPHASE I 2^n different combinations possible (where n = haploid # of chromosomes) In humans = 2^23= 8.4 million gametes How would genetic variation be affected if maternal chromosomes always had to line up together and paternal chromosomes always had to line up together?

How does sexual reproduction produce offspring that are genetically unique? 3. RANDOM FERTILIZATION You are 1 in… 70,600,000,000,000!!! Number of different possibilities = product of number of different gametes produced by each parent; In humans = 8.4 million x 8.4 million = 70.6 X10^12 = greater than the total number of people who have ever lived! (does not take into account genetic variation by crossing over)

Mistakes in Meiosis 1in 666 infants (0.15 %)

Nondisjunction  Aneuploid Cells Occur frequently; researchers estimate that 25% of all conceptions produce a fertilized egg that is aneuploid  effects are almost severe  lead to spontaneous abortion / miscarriage

Nondisjunction Patterns

Klinefelter Syndrome 1 in 1000 live male births

Triple X 1 in 1000 live births  impaired mental function ; sterility

Turner Syndrome 1 in 5000 live births

Why no YO syndrome? Only 86 genes for only a handful of proteins

Nondisjunction Patterns Only 86 genes for only a handful of proteins