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Mendel and Meiosis Genetics Study of heredity Gregor Mendel Monk Studied pea plants- easy to grow, observable traits, easy to manipulate Plants also reproduce sexually-produce gametes(sex cells) Selected to breed tall plants(Parents-P) with short plants. Made hybrids Hybrid- offspring of parents with different traits(F1) Next he bred the offspring with each other to get 2nd generation(F2) P-parents F1-you F2-your kids What did Mendel figure out? Mendel’s Findings Each organism has 2 factors that control each trait. (GENES) 1 from mom,1 from dad Genes exist in different forms called alleles-sort of like choices. Ex. Eye color- 2 choices or alleles BLUE or BROWN Rule of Dominance One allele is dominant over the recessive allele. Ex. Plant height- Tall allele(T) is dominant over short allele(t) T-Dominant(Upper case) t-recessive (lower case) Law of Segregation Every individual has 2 alleles for each trait(1from mom,1 from dad) When that individual produces gametes(sex cells) each sex cell will only have 1 of the alleles for each trait Ex. Plant could be TT or Tt or tt for its allelic combination but it would only donate one to a gamete. TT- T or T Tt- T or t tt- t or t Law of Independent Assortment Each allele is inherited independently of other alleles Ex. Pea seed color and shape are inherited independantly of each other so you can have: Smooth green peas , Wrinkled green peas Smooth yellow peas ,Wrinkled yellow peas The more alleles for a trait leads to more possible combinations. Genetics vocabulary Phenotype- how the trait looks(observe) Ex- tall, short,blue eyes, brown eyes Genotype- allelic combination Ex. TT,Tt,tt BB,Bb,bb Homozygous- both alleles are the same Ex. TT,tt BB,bb Heterozygous- allele combination is different Ex. Tt Bb Punnett Squares We can use a punnett square to predict the results of breeding between 2 parents. Sometimes our predicted results do not match the actual results. T t TT Tt T t Heterozygous Cross Phenotype ratio-3:1 Tt tt Predicted results: Genotype 25% TT 50% Tt 25% tt Genotype ratio – 1:2:1 Phenotype 75% Tall 25% short More Complex Alleles Incomplete Dominance- neither allele is dominant. Red flowers X White flowers =Pink Flowers Co dominance- both alleles expressed ex Black chicken X White chicken= Checkered chicken Multiple alleles more than 2 possible alleles but there can only be 2 in each individual. Ex.Pigeon color- grey ,black, white, brown Polygenic inheritance many genes determine phenotype. Ex. Skin color and height We know that mitosis produces identical cells. In humans each body cell has 46 chromosomes(23 pairs) If we joined 2 cells each having 46 chromosomes what do we get? A cell with 92 chromosomes-TOO MANY!!! What to Do? Somehow we need to have a cell with ½ the number of chromosomes(1/2 x 46=23) so that if 2 cells joined together each having 23 chromosomes we end up with the magic number of 46! (2 x 23) All organisms have a set number of chromosomes. They get one of each from their parents. Humans have 46 or 23 pairs. We number the chromosomes 1 to 23. We have a # 1 from mom, a # 1 from dad, #2 from mom,#2 from dad, etc.The chromosome of each pair are called HOMOLOGOUS CHROMOSOMES Cells that have the pairs of their chromosomes like body cells are called DIPLOID Diploid cells are said to be 2N where N= 23 in humans. Cells that have only 1 of each chromosome like gametes are called HAPLOID Haploid cells are 1N So in humans our N =23, peas (7), Fruit fly(4) So how do we get a cell that has the correct number of chromosomes to be the gamete or sex cell ( haploid cell)? MEIOSIS- produces gametes that are 1N 23 + 23 = 46 sperm + egg = zygote (offspring) Meiosis Process of making gametes, sex cells 0r egg/sperm Humans have 46 chromosomes(23 pairs) that we get from our parents. Each pair is called a homologous pair. Remember Diploid cells contain 2 of each chromosome 2N- body cells Haploid cells contain 1 of each chromosome 1N- sex cell N = # of pairs of chromosomes 2 Stages of Meiosis Meiosis I Diploid cell 2N Meiosis II 4 Haploid cells 4 1N copy 46 split 92 46 46 23 23 23 23 Interphase Chromosomes replicate 46 92 copy Prophase I Dna coils Spindle fibers form Homologous chromosomes pair up with their sister chromatid All 4 together called TETRAD TETRAD Ends of chromosomes are sticky This is where some of the genetic material exchanges places on its homologue. This is how we get genetic variation. CROSSING OVER Metaphase I Centromere attaches to the spindle fibers Tetrads line up in the middle Anaphase I Homologous pairs split Telophase I Spindle breaks up Pairs at opposite ends Cell splits End up with cell with 46 chromosomes Not done yet- split again to get to magic #23! Meiosis II-2nd division Prophase II- spindle reforms Metaphse II- line up in middle Anaphase II- move apart Telophase II – 2 nuclei at opposite ends 4 Haploid cells at end of Meiosis IIeach has 1 copy of each chromosome. How do we all look so different? Crossing over 2 23= 70 trillion different combinations Nondisjunction- homologous pairs fail to separate during prophase I. One cell will have an extra chromosome. Ex. Down’s Syndrome has an extra #21 chromosome. Gamete can have 1 less- Ex. Turner’s Syndrome has only 1 X chromosome instead of 2