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Download Intro to Mendelian Genetics
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Intro to Mendelian Genetics Gregor Mendel • Austrian monk and gardener • Significant work done in 1850’s • Father of genetics (study of heredity) • Heredity = passing on of traits from parent to offspring • Traits = characteristics that are inherited (ex. eye color) Mendel’s First Experiment P (Parent) Generation X Purebred tall (≥ 6 ft.) Purebred short (≤ 2 ft.) ↓ Cross - pollinate Mendel’s First Experiment F1 (filial) Generation all tall ↓ Self - pollinate Mendel’s First Experiment F2 Generation ¾ tall ¼ short Mendel’s life… • Mendel tried out different pea plant traits… Mendel’s life… • Thousands of times… Mendel’s life… • And got the same results! – In F1 generation, one trait remained and the other disappeared. – In F2 generation, the ratio of plants was ¾ to ¼ What Mendel knew… (1800s) • Each parent contributes to the traits of a plant/child. • Something is passed on from parent to child to contribute to the traits. • Some traits were disappearing, then returning in future generations. What Mendel didn’t know… • Chromosomes were being inherited from parents. (1930s) • DNA specifically is the molecule being that gives us our traits. (1920s – 1950s) • What DNA is made of. (1953) Mendel’s Rules and Laws After 1000s of experiments, Mendel came up with some “rules of heredity”: 1.Rule of Unit Factors • EACH ORGANISM HAS 2 COPIES OF A GENE THAT CONTROLS EACH TRAIT; ONE COPY CAME FROM THE MALE PARENT AND ONE COPY CAME FROM THE FEMALE PARENT • each chromosome has genes (a gene is a segment of DNA that controls a trait – ex. height) • there can be different forms (versions) of the same gene; these different forms are called alleles – ex. for the height gene, the alleles are tall and short • the offspring get 2 copies of the gene and can inherit any combination of two alleles from the parents: Allele from your mom… Allele from your dad… tall tall short short short tall short tall Mendel’s Rules and Laws 2.Rule of Dominance • ALLELES CAN BE DOMINANT OR RECESSIVE; THE DOMINANT ALLELE, IF PRESENT, COMPLETELY MASKS THE RECESSIVE ALLELE • Dominant allele - remained in the F1 generation (ex. tall) • Recessive allele - disappeared in F1 (ex. short) • So we say that tall is dominant to short • When writing alleles, follow these rules: – Use the same letter for different forms of the same gene. – Use uppercase letter for dominant allele. – Use lowercase letter for recessive allele. – Always write dominant allele first. • Example: height T _______ = tall _______ = short t A plant can be: tall TT __________ tall Tt __________ short tt __________ Important Vocab PHENOTYPE = written description of appearance or behavior Examples: tall and short BUT, two organisms can look the same but have different gene (or allele) combinations: GENOTYPE = 2 – letter gene combination Examples: TT, Tt, tt There are 3 different types of genotypes: Homozygous dominant ____________________________________ Homozygous recessive ____________________________________ Heterozygous ____________________________________ same Homo- means _______________________ different Hetero- means _______________________ FYI: homozygous is the same as PUREBRED heterozygous is the same as HYBRID (TT) (tt) (Tt) JUST A LITTLE PRACTICE Description Red hair GG mm White eyes Short toe Dd Genotype or Phenotype? phenotype genotype genotype phenotype phenotype genotype Homozygous or Heterozygous? N/A Homozygous (dominant) Homozygous (recessive) N/A N/A Heterozygous Mendel’s Rules and Laws 3.Law of Segregation THE TWO ALLELES THAT CONTROL EACH TRAIL WILL SEPARATE DURING GAMETE FORMATION Explains how two tall plants can give offspring that are ¾ tall and ¼ short! PLANT 1 Tt Plant 1’s gametes: T t X PLANT 2 Tt Plant 2’s gametes: T t All possible gamete combinations result in the following TT, Tt, and Tt (3/4 tall) tt (1/4 short) offspring:____________________ and ____________ An easier way to determine offspring… • Reginald Punnett (early 1900s) • Discovered short-hand way to solve genetics problems • Works based on Mendel’s law of segregation AND NOW WE… PRACTICE, PRACTICE, PRACTICE!