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Chapter 11 Intro. to Genetics Chap. 11-4 Meiosis • Mitosis – division of body cells (somatic cells) • End result - 2 cells identical to starting cell w/same number of chromosomes • Meiosis – production of sex cells (sperm & egg) (gametes) • End result – 4 cells with ½ the chromosome number as starting cell Karyotype • A picture of one’s chromosomes arranged in pairs • Each pair are alike in that they carry genes for the same traits • Are called homologous chromosomes Homologous Pair Chromosome Number • Gametes – called haploid b/c they contain only half the number of chromosomes • Called the n number (n refers to number of chromosomes in gametes) • Gametes have only one member of each homologous pair Chromosome Number • Somatic cells – called diploid b/c they contain two of each chromosome or 23 pairs (in humans) • Each pair – called a homologous pair • Because each pair forms a 4-stranded group, they are also called a tetrad • Come together in Prophase I in a process called synapsis Divisions of Meiosis • Meiosis occurs in 2 major divisions: • 1st division – Meiosis I - separates homologous pairs of chromosomes (reduction phase) • 2nd division – Meiosis II - separates chromatids of each chromosome (division phase) & is similar to mitosis Meiosis Meiosis • Important events to remember: • Synapsis & crossing over – Prophase I • Separation of homologous chromosomes – Anaphase I • Division of chromatids – Anaphase II Importance of Meiosis • Causes variation in chromosomes by: • 1. Crossing over - when homologous chromosomes exchange genes Importance of Meiosis • 2. Usually results in random segregation (separation) of chromosomes Both result in new genetic combinations in the offspring Mitosis vs. Meiosis Mitosis Meiosis 1 division 2 divisions Produces 2 genetically identical cells Cells are diploid Produces 4 genetically different cells Cells are haploid Produces body (somatic) cells Produces sex cells (gametes) Meiosis in Males Meiosis in Females CHAP. 11-1 & 11-2 Mendel’s Laws of Heredity • Genetics – study of heredity • Heredity – passing of traits from parent to offspring • Traits transmitted by genes located on chromosomes • Most traits determined by a pair of genes located on paired chromosomes (homologous) Mendel’s Laws of Heredity • Gregor Mendel – “Father of Genetics” • Studied garden pea plants; 2 reasons: 1.Have 7 contrasting traits • ex. – tall vs. short, green pod vs. yellow, etc. Mendel • 2. Easy to cross-pollinate (the transfer of male pollen grains from one plant to the female organ of another plant) (usually pea plants selfpollinate Genetics Vocab. • Genotype – genetic makeup of an organism (Tt) • Phenotype – physical makeup of an organism (tall) • Homozygous (pure) – when 2 genes for a trait are the same (TT or tt) • Heterozygous (hybrid) – when 2 genes for a trait are different (Tt) • Monohybrid cross – one with only 1 trait crossed • Alleles – different forms of a gene (T or t) Mendel’s Experiments • Mendel first crossed pure tall plant with pure short plant • To work a genetics problem: • Step 1- Pick symbols: • T - tall, t - short • Step 2 - Diagram the cross: • TT x tt (Parents or P generation) Mendel’s Experiments • Step 3 – Do the Punnett square: • TT x tt • offspring are called 1st filial or F1 generation) • one trait shows up (tallness) & one disappears (shortness) Mendel’s Experiments • Step 4 – List ratios: • Genotypic ratio – • Phenotypic ratio - Mendel’s Experiments • Next Mendel did a second generation cross (cross 2 plants from the 1st generation) • F2 cross = Tt x Tt • Now the tall trait appears • Offspring are called F2 generation Monohybrid Crosses Mendel’s Laws • Law of Dominance – in an organism w/contrasting alleles (different forms of a gene), one gene shows up & the other disappears • Gene that shows up – dominant • Gene that disappears – recessive • Law of Segregation – pairs of genes separate when gametes are formed in meiosis (shown in the Punnett square Ch. 11-3 Mendel’s Laws • Law of Independent Assortment – pairs of genes separate independently of each other • Dihybrid cross – one with 2 different traits Dihybrid Cross – used to show Mendel’s Law of Independent Assortment In guinea pigs, black fur is dominant over white. Rough coat is also dominant over smooth. Cross a homozygous black, rough guinea pig with a heterozygous black, rough guinea pig. •F-o-i-l (first – outside – inside - last) Incomplete Dominance • Incomplete dominance – when 1 gene is not completely dominant over the other; a blending of traits occurs • Problem - In some flowers, red is incompletely dominant over white. The heterozygous condition produces pink flowers. • Pick symbols: R – red, R′ - white • Possible genotypes: • RR- red • R′R′ - white • RR′ - pink Codominance • Codominance – when neither gene is dominant; both genes in a pair are expressed • In cattle, red hair is codominant with white. The heterozygous condition produces a roan (mix of red & white) offspring. • Pick symbols – R-red; W-white R • Possible genotypes: • RR – red • WW – white • RW - roan Bio. Bingo • • • • • • • • • • • • • • Mendel Independent assortment Heterozygous Genotype Phenotype Genetics Heredity Recessive 1st filial Homozygous Peas Traits Genes Punnett square alleles segregation F2 self-pollination dominant monohybrid incomplete dominance codominance 1:2:1 cross pollination dominance dihybrid dominant 3:1 gametes hyrbid fertilization probabilityb