Mendel I Notes
CP Biology
Ms. Morrison
Genetics: scientific
study of heredity
Gregor Mendel
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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
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Controlled how plants pollinated
Did not allow self-pollination
 Cross pollinated between different pea plants
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Mendel’s Pea Plant Experiments
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Studied 7 pea plant traits
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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
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Mendel’s Conclusions
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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
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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
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Crossed F1 hybrid offspring to determine
if recessive allele still present
 Offspring
 75%
= F2 generation
tall
 25% short – recessive allele reappeared
Mendel’s Final Conclusions
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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
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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
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Genetics Terms
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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
T
T
t
Tt
Tt
t
Tt
Tt
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F1 results:
 Phenotype: 100% tall
 Genotype: 100% Tt
Punnett Square Example 2
T
T
t
TT
Tt
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F2 results:
 Phenotype: (3:1)
75% tall
 25% short
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t
Tt
tt
Genotype: (1:2:1)
25% TT
 50% Tt
 25% tt
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