Inheritance and Sexual Life Cycles Chapter 13 p. 238-247

Intro to Genetics Genetics: the study of heredity Heredity: passage of traits from parents to offspring (“inheritance”) Leads to variation of species

Offspring acquire genes from parents Gene: hereditary unit in form of DNA Code for specific traits by controlling cell protein production Composed of specific sequence nucleotides Each chromosome contains 100’s to 1,000’s genes Locus: a gene’s location on a chromos. Human Genome Project

Asexual vs. Sexual Reproduction Asexual: 1 parent cell Copy entire genome → offspring Produces clone (genetically identical being) Unicell = Mitosis or Binary Fission; multicell = “budding” Sexual: 2 parent cells ½ genome from each parent → offspring Produces genetically different offspring ↑ genetic variation of species

The Human Life Cycle “Life Cycle” – conception → produce offspring 1) Somatic Cells: all cells except reproductive 44 autosomes in pairs & 2 sex chromosomes in a pair Determine sex of offspring Male: XY; Female: XX Homologous Chromosomes: pair of chromos. carrying genes for same trait Diploid: cell w/ 2 sets chromos. (2n=46) Karyotype: chart of autosomes arranged in homologous pairs

The Human Life Cycle 2) Gametes: “sex cells” sperm & ovum 23 unpaired chromosomes Haploid: 1 set chromos. (n=23) Created through meiosis (this cuts chromos. in ½) Fertilization: joining of sperm & ova → zygote Results in diploid cell (2n=46)

Variety of Sexual Life Cycles Differ in timing of meiosis & fertilization 1) Animals: meiosis produces gametes; diploid zygote divides by mitosis 2) Fungi & Protists: Gametes fuse → diploid zygote Meiosis produces haploid cells Haploid cells divide by mitosis → adult orgnsm.

Variety of Sexual Life Cycles 3) Plants & some algae: “Alternation of Generations” Alternate btwn diploid/haploid stages 1) Sporophyte: diploid stage Meiosis occurs, making haploid spores Spores develop into adult orgnsm. (gametophyte) 2) Gametophyte: haploid stage Undergoes mitosis to make gametes 2 gametes fuse → diploid zygote (spororphyte)

Meiosis & Genetic Variation Ch 13 p. 243-249

Meiosis Reduces Chromos. # from Diploid → Haploid Meiosis occurs in 2 stages, resulting in 4 genetically different daughter cells 1) Inherit 1 set chromos. from each parent (pair up to form homologous chromos.) 2) during Interphase, copy each chromos. → sister chromatids of each 3) Meiosis I: homol. chromos. separate → diploid cell 4) Meiosis II: sister chromatids separate → haploid cell w/ unreplic. chromos.

Meiosis Overview Refer to p. 244-245 for the stages of Meiosis

Unique Meiosis Events 1) Synapsis: pairing of duplicated chromos. w/ homologue Held completely together by synaptonemal complex At end of Prophase I, 4 individual chromatids appear as tetrad Sometimes non-sister chromatids overlap, creating X-shaped region called chiasmata

Unique Meiosis Events 2) Metaphase I: homologous chromos. line up (not sister chromatids) 3) Anaphase I: homo. chromos. separate (not sisters) 4) Meiosis II: separates sister chromatids → 4 genetically different daughter cells Almost identical to mitosis

Genetic Variation Meiosis results in genetic variation of species populations by: 1) Independent Assortment 2) Crossing Over 3) Random Fertilization

Independent Assortment When chromos. line up along metaphase plate, they do so independently of one another i.e.: D M OR D M D M M D D M D M D M M D # possible combos = 2n (humans = 223 = >8 mill)

Crossing Over Produces Recombinant Chromosomes: contain DNA from BOTH parents Begins in Prophase I, before synaptonemal complex forms btwn homologous chromos. Non-sister chromatids exchange portions of DNA No longer identical sister chromatids assort independently

Random Fertilization ANY ovum may be fertilized by ANY sperm # possible ovum = ~8 mill # possible sperm = ~8 mill # possible zygotes = 8 mill x 8 mill ~ 64 trillion

Genetic Variation Determines Evolutionary Adaptation Populations of species evolve through the reproductive success of individuals Organisms best adapted to environment have best success Genes good for that environment get passed Makes evolution possible