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Aristotle thought that genes came
from throughout the body to make up
pangenes, called pangenesis
Spermists believed the entire human
was contained in the humunculus and
that the uterus was used to incubate
Ovists believed the entire human was
contained in the egg and the sperm
was used for its nutrients
Homunculus
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Grew up in Austria in a poor
farming family in the Czech
Republic
He wanted a better life for
himself and thought the only
way he could do so was by
becoming a monk
He left home in 1843, changing
his name from Johann to
Gregor
The monastery decided to help him get an
education by sending him to become a teacher
…but he failed
 So later he was sent to a university in 1851 he
studied under Christian Doppler, who taught him
to learn science through experimentation
 He also had good math and plant science
professors
 He decided to start studying genetics through
peas at the monastery
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1.
2.
3.
Peas come in many different
varieties- colours, shapes,
sizes, etc.
They have a short life span
They are easy to grow
Between 1856 and 1863, Mendel grew
some 29,000 pea plants.
http://tinyurl.com/MendelsPeas
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Mendel “cross-pollinated” the peas (artificial
pollination) by using pollen from one plant to
fertilize a different plant.
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Used TRUE-BREEDING PLANTS: all new plants have
same traits as parent plants. (“breed true” =
homozygous)
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Mendel used a few traits, and watched to see
what happened when he crossed “parent plants”
with opposite traits. (eg. Round x wrinkled pea)
1st generation: “Parents” (P1)
2nd generation: “First filial
generation” (F1)
3rd generation: “Second filial
generation” (F2)
4th generation: “Third filial
generation” (F3)
Filial means child or offspring in latin.
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For his experiments, Mendel crossed two
true-breeding plants that had different traits
for the same characteristic.
Eg. He would cross a plant that was true-breeding
for green pods with a plant that was true-breeding
for yellow pods. When two true-breeding plants
are crossed, this is called hybridization.
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The offspring from this cross are
heterozygous, meaning they contain the two
different genes for one trait.
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When he crossed wrinkled with round, the F1
were all round.
He crossed the F1 plants, and some of the F2
were wrinkled, and some were round.
He made hundreds more similar crosses,
recorded the results, analyzed them
mathematically, and came up with 4
principles of heredity.
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Mendel presented his work in a
paper in 1865, “Experiments in
Plant Hybridization”
However, it was not well received
and given little attention
His work was significant not just
because of his discoveries in
genetics, but also because he
used mathematical and statistical
analysis to analyze his results.
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Soon after this he gave up science when he
became the abbot of the monastery.
He died in 1884.
In 1900 three new scientists, Hugo DeVries,
Erich Von Tschermak, and Carl Correns, who
worked on genetics independent of one
another finally gave Mendel the credit he
deserved.
1. Genes In Pairs: Genetic characters are
controlled by unit factors (genes) that exist
in pairs in individual organisms and are
passed from parents to their offspring.
When two organisms produce offspring,
each parent gives the offspring one of the
factors from each pair.
Mother contributes
one allele and father
contributes the other
allele for a trait.
An allele is the biological element located on a
chromosome that carries the expression of a trait. We
represent them with letters.
One allele
for hair
colour from
mom
…and one
from dad.
2. Dominance and Recessiveness: When two
unlike factors responsible for a single character
are present in a single individual, one factor can
mask the expression of another factor. That is,
one factor is dominant to the other, which is
said to be recessive.
Dominant alleles are represented
with capital letters
Dominant genes
Recessive alleles are
represented with small
letters
The baby is heterozygous
for eye colour. She carries
the dominant colour
brown and recessive
colour blue. Hetero means
different.
Bb
bb
BB
The parents are both
homozygous for eye colour.
Homozygous means both alleles
for a characteristic are the same.
Homo means the same.
Recessive genes
bb
Bb
bb
On this slide who is a
heterozygote and who is a
homozygote?
3. The Principle of Segregation: During
the formation of gametes, the paired
factors separate (segregate) randomly so
that each gamete receives one factor or
the other.
Gamete: A cell containing half of the genes for their
offspring. Eggs and sperm are gametes in humans.
4. Independent Assortment: During
gamete formation, segregating pairs
of different factors assort
independently of each other.
Blonde hair &
blue eyes
Brown hair and
brown eyes
Blonde hair from mother
and brown eyes from
father. Each characteristic
is separate from another.
Modern Recessive traits:
•Many modern traits and disease
are recessive
•Cystic fibrosis, sickle cell anemia,
albinism, Tay Sachs are all recessive
autosomal diseases (genes are
found on chromosomes other than
the 23rd)
Modern Mendelian Genetics: crossbreeding Belgian Blue Cattle