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Mendel and the Gene Idea Gregor Mendel – Austrian monk. Brought experimental and quantitative approach to genetics. Bred pea plants to study inheritance.-peas because: could control mating (self- and cross- pollination), many varieties available, short generation time hypothetical experiment:P: purple flowers X white flowers àF1: all plants purple à self or cross pollination àF2: 700 purple plants, 225 white plants P (parental) generation = true breeding plantsF1 (first filial) generation = offspringF2 (second filial) generation = F1 offspring Alleles = alternative versions of a gene Ex: on a pair of homologous chromosomes at the locus for flower-color gene, one chromosome carries the white color allele while the other chromosome carries the allele for purple 7 characters in pea plants were studieddominant (expressed) vs. recessive (hidden)flower color, flower position, seed color, seed shape, pod shape, pod color, stem length Mendel’s Principles1. Alternative version of genes (alleles) cause variations in inherited characteristics among offspring.2. For each character, every organism inherits one allele from each parent.3. If 2 alleles are different, the dominant allele will be fully expressed; the recessive allele will have no noticeable effect on the offspring’s appearance.4. Law of Segregation: the 2 alleles for each character separate during gamete formation – each resulting gamete cell gets one of the alleles. Dominant (P), recessive (p)Homozygous = 2 same alleles (PP or pp)Heterozygous = 2 different alleles (Pp) Phenotype – expressed physical traitsGenotype – genetic make-up Punnett Square – device for predicting offspring from a cross. Ex: Pp X Pp (P=purple, p=white) ***DO PRACTICE PUNNETT SQUARE PROBLEMS*** testcross: used to determine if dominant trait is homozygous (PP) or heterozygous (Pp) (since they have the same phen) by crossing with recessive (pp). Resulting offspring (all dom phen, or presence of both phens) will tell. Law of Independent Assortment:-Each pair of alleles segregates (separates) independently during gamete formation eg. color is separate from shape Monohybrid cross – study 1 character (ex: flower color only)Dihybrid cross – study 2 characters (ex: flower color and seed shape) ***DO MONO AND DIHYBRID CROSS PRACTICE PROBLEMS*** The laws of probability govern Mendelian Inheritance-Rule of Multiplication: probability that 2+ independent events will occur together in a specific combination à multiply probabilities of each event ex: probability of throwing 2 sixes: 1/6 x 1/6 = 1/36 ex: probability of having 5 boys in a row: ½ x ½ x ½ x ½ x ½ = 1/32 ex: cross AABbCc x AaBbCc, prob of offspring w AaBbcc: ½ x ½ x ¼ = 1/16-Rule of Addition: probability of 2 mutually exclusive events will occur à add together individual probabilities ex: chances of throwing a die that will land on 4 or 5: 1/6 + 1/6 = 1/3 Segregation of alleles and fertilization as chance events Extending Mendelian GeneticsThe relationship between genotype and phenotype is rarely simple-Complete Dominance – heterozygote and homozygote for dominant allele are indistinguishable (ex: YY or Yy = yellow seed color)-Incomplete Dominance – F1 hybrids have appearance that is between that of two parents (ex: red x white = pink flowers)-Codominance – phenotype of both alleles is expressed in the heterozygote (ex: red hair x white hair = roan horses)-Multiple alleles – gene has 2+alleles (ex: human ABO blood groups)blood transfusions must match blood type mixing of foreign blood à clumping à death Rh factor: protein found on RBCs (Rh+ = has protein, Rh- = no protein) *practice problems-Polygenic Inheritance – the effect of 2 or more genes acting upon a sim=ngle phenotypic character (ex: skin color, height)-Nature and Nurture: both genetic and environmental factore influence phenotype (ex: hydrangea flowers vary in shade and intensity depending on acidity/Al in soil)-Pleiotropy – one gene is able to affect multiple phenotypic characters (ex: Sickle-cell disease)-Epistasis – the phenotypic expression of one gene affects that of anotherMendelian Inheritance in HumansPedigree: diagram that shows the relationship between parents/offspring across 2+ generations***pedigree practice problems*** Genetic DisordersAutosomal recessive: Cystic fibrosis, Tay-Sachs, Sickle-cell, PhenylketonuriaAutosomal dominant: Huntington’s disease (lethal dominant allele)
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