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This overview discusses the pioneering work of Gregor Mendel, the Austrian monk whose experiments with pea plants laid the groundwork for the study of genetics. In the mid-1800s, Mendel discovered key principles of heredity through controlled pollination of true-breeding plants. His findings revealed the concepts of dominant and recessive alleles, segregation of alleles during gamete formation, and the use of probability models, such as Punnett squares, to predict genetic outcomes. Mendel's work established the foundation for modern genetics and our understanding of inheritance.
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Mendel I Notes CP Biology Ms. Morrison
Gregor Mendel • Austrian monk – in mid 1800s taught high school and took care of the monastery gardens • Garden stocked with true breeding pea plants • True-breeding = always have identical offspring • Ex. Tall plants always produce more tall plants • Controlled how plants pollinated • Did not allow self-pollination • Cross pollinated between different pea plants
Mendel’s Pea Plant Experiments • Studied 7 pea plant traits • Trait = specific characteristic, ex. Plant height • Crossed two true-breeding pea plants • Starting plants = P (parental) generation, one tall and one short • All offspring = hybrids (offspring of parents with different traits) • Offspring generation = F1 (first filial) • Had expected blend of parents’ traits – thought plants would have medium height • Actual results – all plants tall, short trait disappeared
Mendel’s Conclusions • First – biological inheritance is determined by factors passed from one generation to the next • Genes = chemical factors that determine traits • Genes have two contrasting forms (tall, short) • Alleles = different forms of a gene
Mendel’s Conclusions • Second – principle of dominance: some alleles are dominant and some are recessive • Dominant allele always shows • Recessive allele only shows when not dominant allele present • Tall = dominant, short = recessive
Mendel’s Further Experiments • Crossed F1 hybrid offspring to determine if recessive allele still present • Offspring = F2 generation • 75% tall • 25% short – recessive allele reappeared
Mendel’s Final Conclusions • At some point recessive allele was separated from the dominant allele in F1 = segregation of alleles • Suggested that segregation of alleles occurred during formation of gametes – meiosis • Gametes only carry single copy of each gene • Offspring inherit one allele from each parent (so they have two alleles total)
Probability • Is likelihood that an event will occur • Mendel realized outcomes of genetic crosses could be predicted using probability • Punnett square – diagram that shows the genetic combinations that might result from a genetic cross • Dominant alleles are capitals, ex. Tall, T • Recessive alleles are lowercase, ex. Short, t
Genetics Terms • Homozygous (true-breeding): have two identical alleles, ex. TT or tt • Heterozygous (hybrid): have 2 different alleles, ex. Tt • Phenotype = physical appearance (what organism looks like) • Genotype = genetic makeup (organism’s actual alleles) • NOTE – can have same phenotype but different gentotype, ex. TT and Tt both look tall
Punnett Square Example 1 • F1 results: • Phenotype: 100% tall • Genotype: 100% Tt
Punnett Square Example 2 • F2 results: • Phenotype: (3:1) • 75% tall • 25% short • Genotype: (1:2:1) • 25% TT • 50% Tt • 25% tt
