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Chapter 4-1
Mendel’s Work
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Some Important Terms
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Heredity – the passing of physical
characteristics from parents to offspring
Trait – a characteristic that an organism
can pass on to its offspring through its
genes
Genetics – the scientific study of
heredity
Gregor Mendel
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Priest and gardener
at a monastery in
Europe
Mid-1800s
Worked with pea
plants
Mendel’s
Experiments
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Crossing Pea Plants
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Began by crossing
plants with contrasting
traits
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Example: tall and short
plants
Used purebred plants
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F1 Offspring
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F1 = first filial (son/daughter) generation
The offspring of a cross of purebred tall
and purebred short plants
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Parent plants are called P generation
All offspring in F1 were tall, despite 1
parent being short
F2 Offspring
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F2 = second filial
Plants from F1 were bred
Offspring were a mixture of tall and short
plants
 ¾ were tall;
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¼ were short
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Experiments
with other traits
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Mendel also did
experiments with
other contrasting traits
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In all crosses, only 1
form of the trait
appeared in the F1
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However, in F2, the
“lost” trait always
reappeared in ¼ of
the plants
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Dominant and Recessive
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Mendel reasoned that individual factors, or
sets of genetic “information” must control
the inheritance of traits in the peas
These factors exist in pairs
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Female gives 1 part of pair
Male gives 1 part of pair
A factor can “mask” or “cover” another factor
Genes and Alleles
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Gene – factors that
control a trait
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Alleles – different
forms of the gene
• Example: in the gene that controls height,
there is an allele for tall and short
• Each plant inherits 2 alleles; one from the
mother and one from the father
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An organism’s traits are controlled by the
alleles it inherits
Some are dominant; others recessive
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Dominant – trait always shows up
Recessive – hidden whenever dominant is
present
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This trait will only show up if the organism doesn’t
have the dominant
Alleles in Mendel’s work
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P generation – tall had 2 dominant, short
had 2 recessive
F1 generation – all plants had 1 of each
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F1 all hybrids
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Hybrid – organism that has 2 different alleles for a
trait
F2 generation – ¾ dominant and ¼
recessive
Symbols for Alleles
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Geneticists use letters to represent alleles
Dominant alleles are capital letters (ex: T)
Recessive alleles are lowercase (ex: t)