Transcript Sect. 5.1

5.1 Mendel’s Work
Ch.5 – What were the results of
Mendel’s experiments?
Heredity: passing of physical characteristics
from parents to offspring
Trait: Each different form of a characteristic,
such as stem height or seed color
Genetics: the scientific study of heredity
Mendel’s experiments
Mendel’s experiments
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Flower’s petals surround the pistil and the
stamens. The pistil produces female sex
cells (eggs). The stamens produce pollen,
which contains male sex cells (sperm).
A new organism begins to form when egg
and sperm join in the process called
fertilization.
Crossing Pea Plants
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Mendel crossed two pea plants (crosspollination): he removed pollen from a flower
on one plant, then brushed the pollen onto a
flower on a second plant.
Purebred organism is the offspring of many
generations that have the same trait – EX.
Purebred short pea plants always come from
short parent plants.
The F1 Offspring
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Parent generation : P generation
Offspring : Filial (child) generation – F1 and
F2 and so on.
In all of Mendel’s crosses, only one form of
the trait appeared in the F1 generation.
However, in the F2 generation, the “lost”
form of the trait always reappeared in about
¼ of the plants.
F1 and F2 generation
Dominant and Recessive Alleles
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Mendel concluded that individual factors, or
set of genetic “information” must control the
inheritance of traits in the peas.
The factors that control each trait exist in
pairs. The female parent contributes one
factor, while the male parent contributes the
other factor.
Genes and Alleles
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Gene: factors that control a trait.
Alleles: different forms of a gene. – EX. The
gene that controls stem height in peas has
one allele for tall stems and one allele for
short stems.
An organism’s traits are determined by the
alleles it inherits from its parents. Some
alleles are dominant, while others are
recessive.
Alleles
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Dominant allele’s trait always shows up and
recessive allele is hidden whenever
dominant allele is present.
Only pea plants that inherit two recessive
alleles for short stems will be short.
Alleles in Mendel’s Crosses
Examples of Alleles
P - TT and tt
F1 – Tt and Tt
F2 – TT, Tt, Tt, and tt
Tall allele - Capital T,
Short allele – lowercase t
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Significance of Mendel’s Contribution
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Mendel’s work changed scientist’s ideas
about heredity.
Before Mendel, people thought you can
blend genetic information to produce new
traits (ex. Red + white = Pink), but now we
know that traits are determined by individual,
separate alleles inherited from each parent.
If a trait is from a recessive allele, the trait
can seem to disappear in the offspring.