Mendel I NotesMendel I Notes

CP BiologyCP Biology

Ms. MorrisonMs. Morrison

Genetics: Genetics: scientific study scientific study

of heredityof heredity

Gregor MendelGregor Mendel

Austrian monk – in mid 1800s taught high Austrian monk – in mid 1800s taught high school and took care of the monastery school and took care of the monastery gardensgardens

Garden stocked with true breeding pea Garden stocked with true breeding pea plantsplants True-breeding = always have identical offspringTrue-breeding = always have identical offspring Ex. Tall plants always produce more tall plantsEx. Tall plants always produce more tall plants

Controlled how plants pollinatedControlled how plants pollinated Did not allow self-pollinationDid not allow self-pollination Cross pollinated between different pea plantsCross pollinated between different pea plants

Mendel’s Pea Plant Mendel’s Pea Plant ExperimentsExperiments

Studied 7 pea plant traits Studied 7 pea plant traits Trait = specific characteristic, ex. Plant Trait = specific characteristic, ex. Plant

heightheight Crossed two true-breeding pea plantsCrossed two true-breeding pea plants

Starting plants = P (parental) generation, Starting plants = P (parental) generation, one tall and one shortone tall and one short

All offspring = hybrids (offspring of parents All offspring = hybrids (offspring of parents with different traits)with different traits)

Offspring generation = FOffspring generation = F11 (first filial) (first filial) Had expected blend of parents’ traits – Had expected blend of parents’ traits –

thought plants would have medium heightthought plants would have medium height Actual results – all plants tall, short trait Actual results – all plants tall, short trait

disappeareddisappeared

Mendel’s ConclusionsMendel’s Conclusions

First – biological inheritance is First – biological inheritance is determined by factors passed determined by factors passed from one generation to the nextfrom one generation to the next Genes = chemical factors that Genes = chemical factors that

determine traitsdetermine traits Genes have two contrasting forms Genes have two contrasting forms

(tall, short)(tall, short) Alleles = different forms of a geneAlleles = different forms of a gene

Mendel’s ConclusionsMendel’s Conclusions

Second – principle of Second – principle of dominance: some alleles are dominance: some alleles are dominant and some are dominant and some are recessiverecessive Dominant allele always showsDominant allele always shows Recessive allele only shows when Recessive allele only shows when

not dominant allele presentnot dominant allele present Tall = dominant, short = recessiveTall = dominant, short = recessive

Mendel’s Further Mendel’s Further ExperimentsExperiments

Crossed FCrossed F11 hybrid offspring to hybrid offspring to determine if recessive allele still determine if recessive allele still presentpresent Offspring = FOffspring = F22 generation generation 75% tall75% tall 25% short – recessive allele 25% short – recessive allele

reappearedreappeared

Mendel’s Final Mendel’s Final ConclusionsConclusions

At some point recessive allele was At some point recessive allele was separated from the dominant allele in Fseparated from the dominant allele in F11 = segregation of alleles= segregation of alleles

Suggested that segregation of alleles Suggested that segregation of alleles occurred during formation of gametes – occurred during formation of gametes – meiosismeiosis

Gametes only carry single copy of each Gametes only carry single copy of each genegene

Offspring inherit one allele from each Offspring inherit one allele from each parent (so they have two alleles total)parent (so they have two alleles total)

ProbabilityProbability

Is likelihood that an event will occurIs likelihood that an event will occur Mendel realized outcomes of genetic Mendel realized outcomes of genetic

crosses could be predicted using crosses could be predicted using probabilityprobability

Punnett square – diagram that shows Punnett square – diagram that shows the genetic combinations that might the genetic combinations that might result from a genetic crossresult from a genetic cross Dominant alleles are capitals, ex. Tall, TDominant alleles are capitals, ex. Tall, T Recessive alleles are lowercase, ex. Short, tRecessive alleles are lowercase, ex. Short, t

Genetics TermsGenetics Terms Homozygous (true-breeding): have Homozygous (true-breeding): have

two identical alleles, ex. TT or tttwo identical alleles, ex. TT or tt Heterozygous (hybrid): have 2 Heterozygous (hybrid): have 2

different alleles, ex. Ttdifferent alleles, ex. Tt Phenotype = physical appearance Phenotype = physical appearance

(what organism looks like)(what organism looks like) Genotype = genetic makeup Genotype = genetic makeup

(organism’s actual alleles)(organism’s actual alleles) NOTE – can have same phenotype but NOTE – can have same phenotype but

different gentotype, ex. TT and Tt different gentotype, ex. TT and Tt both look tallboth look tall

Punnett Square Example Punnett Square Example 11

FF11 results: results: Phenotype: 100% Phenotype: 100%

talltall Genotype: 100% TtGenotype: 100% Tt

TT TT

tt TtTt TtTt

tt TtTt TtTt

Punnett Square Example Punnett Square Example 22

FF22 results: results: Phenotype: (3:1)Phenotype: (3:1)

75% tall75% tall 25% short25% short

Genotype: (1:2:1)Genotype: (1:2:1) 25% TT25% TT 50% Tt50% Tt 25% tt25% tt

TT tt

TT TTTT TtTt

tt TtTt tttt