Mendel I Notes CP Biology Ms. Morrison

Genetics: scientific study of heredity

Gregor Mendel Austrian monk – in mid 1800s taught high school and took care of the monastery gardens Austrian monk – in mid 1800s taught high school and took care of the monastery gardens Garden stocked with true breeding pea plants Garden stocked with true breeding pea plants True-breeding = always have identical offspring True-breeding = always have identical offspring Ex. Tall plants always produce more tall plants Ex. Tall plants always produce more tall plants Controlled how plants pollinated Controlled how plants pollinated Did not allow self-pollination Did not allow self-pollination Cross pollinated between different pea plants Cross pollinated between different pea plants

Mendel’s Pea Plant Experiments Studied 7 pea plant traits Studied 7 pea plant traits Trait = specific characteristic, ex. Plant height Trait = specific characteristic, ex. Plant height Crossed two true-breeding pea plants Crossed two true-breeding pea plants Starting plants = P (parental) generation, one tall and one short Starting plants = P (parental) generation, one tall and one short All offspring = hybrids (offspring of parents with different traits) All offspring = hybrids (offspring of parents with different traits) Offspring generation = F 1 (first filial) Offspring generation = F 1 (first filial) Had expected blend of parents’ traits – thought plants would have medium height Had expected blend of parents’ traits – thought plants would have medium height Actual results – all plants tall, short trait disappeared 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 First – biological inheritance is determined by factors passed from one generation to the next Genes = chemical factors that determine traits Genes = chemical factors that determine traits Genes have two contrasting forms (tall, short) Genes have two contrasting forms (tall, short) Alleles = different forms of a gene Alleles = different forms of a gene

Mendel’s Conclusions Second – principle of dominance: some alleles are dominant and some are recessive Second – principle of dominance: some alleles are dominant and some are recessive Dominant allele always shows Dominant allele always shows Recessive allele only shows when not dominant allele present Recessive allele only shows when not dominant allele present Tall = dominant, short = recessive Tall = dominant, short = recessive

Mendel’s Further Experiments Crossed F 1 hybrid offspring to determine if recessive allele still present Crossed F 1 hybrid offspring to determine if recessive allele still present Offspring = F 2 generation Offspring = F 2 generation 75% tall 75% tall 25% short – recessive allele reappeared 25% short – recessive allele reappeared

Mendel’s Final Conclusions At some point recessive allele was separated from the dominant allele in F 1 = segregation of alleles At some point recessive allele was separated from the dominant allele in F 1 = segregation of alleles Suggested that segregation of alleles occurred during formation of gametes – meiosis Suggested that segregation of alleles occurred during formation of gametes – meiosis Gametes only carry single copy of each gene Gametes only carry single copy of each gene Offspring inherit one allele from each parent (so they have two alleles total) Offspring inherit one allele from each parent (so they have two alleles total)

Probability Is likelihood that an event will occur Is likelihood that an event will occur Mendel realized outcomes of genetic crosses could be predicted using probability 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 Punnett square – diagram that shows the genetic combinations that might result from a genetic cross Dominant alleles are capitals, ex. Tall, T Dominant alleles are capitals, ex. Tall, T Recessive alleles are lowercase, ex. Short, t Recessive alleles are lowercase, ex. Short, t

Genetics Terms Homozygous (true-breeding): have two identical alleles, ex. TT or tt Homozygous (true-breeding): have two identical alleles, ex. TT or tt Heterozygous (hybrid): have 2 different alleles, ex. Tt Heterozygous (hybrid): have 2 different alleles, ex. Tt Phenotype = physical appearance (what organism looks like) Phenotype = physical appearance (what organism looks like) Genotype = genetic makeup (organism’s actual alleles) Genotype = genetic makeup (organism’s actual alleles) NOTE – can have same phenotype but different gentotype, ex. TT and Tt both look tall NOTE – can have same phenotype but different gentotype, ex. TT and Tt both look tall

Punnett Square Example 1 F 1 results: Phenotype: 100% tall Genotype: 100% Tt TT tTtTt tTtTt

Punnett Square Example 2 F 2 results: Phenotype: (3:1) 75% tall 25% short Genotype: (1:2:1) 25% TT 50% Tt 25% tt Tt TTTTt tTttt