Download Mendel`s Work

Genetics
&
Heredity
Gregor Mendel
(1822-1884)
“Father of Genetics”
Austrian monk
Studied the
inheritance of traits in
pea plants
Developed the laws of
inheritance
Mendel's work was not
recognized until the
turn of the 20th
century
What Did Mendel Do During His Spare Time?
Became curious about why
some of the pea plants in
his garden had different
physical traits.
Between 1856 and 1863,
Mendel cultivated and
tested some 28,000 pea
plants
He found that the plants'
offspring kept the traits
of the parents
Mendel’s Pea Plant
Experiments
Garden Peas
• Why study peas?
– Peas are really easy to study—not like humans.
• Peas usually self pollinate but Mendel
cross-pollinated (from one plant to
another). Took pollen from the flower of
one plant and brushed it onto a flower on a
second plant.
Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
Mendel’s Experimental Methods
Mendel hand-pollinated
flowers using a
paintbrush
He could snip the
stamens to prevent
self-pollination
He traced traits
through the several
generations
Steps of Mendel’s Experiments
1. Produced pure strains by allowing the
plants to self-pollinate for several
generations
P (Parental)Generation
–
Purebred
•
•
When they self-pollinate they always produce
offspring with the same form of a trait as the
parent.
Ex. Short parent=all short offspring, tall
parent=all tall offspring
Steps of Mendel’s Experiments
2. Crossed plants with opposite forms of a trait
• Ex. Tall plants cross-pollinated with short plants
F1 Generation (1st Filial Generation)
•
•
Offspring produced by crossing two purebred individuals.
Ex. Tall Purebred X Short Purebred = F1 Generation
3. Crossed two plants from the F1 Generation
F2 Generation (2nd Filial Generation)
• Offspring produced by crossing two F1 individuals.
Mendel’s Results
• P Generation
– 1 Tall Parent, 1 Short Parent
– What would you expect the offspring to look like?
• F1 Offspring
– All were tall.
– What would you expect if you crossed two F1
individuals?
• F2 Offspring
– Some were tall and some were short.
– Why???
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Mendel’s Thoughts on Pea Plants
•Observed that offspring sometimes
had the same traits (physical
characteristics) as their parents but
not always
•Mendel stated that physical traits
are inherited as “particles”
•Mendel did not know that the
“particles” were actually
Chromosomes & DNA
Genetic Terminology
Trait
Any characteristic that can be passed
from parent to offspring through
genes
Heredity
Passing of traits from parent to
offspring
Genetics
Study of heredity
Genes
• Genes
– Segments of DNA found on chromosomes
that contain the information for a particular
physical trait
– Genes come in pairs (1 copy from mom, 1
copy from dad)
Alleles
•Alleles
–Different forms of a gene.
–Ex. Short allele vs. tall allele
–We get one allele from father and
one from mother.
–Mendel found that one allele could
mask another.
•Ex. F1 Individuals: Received tall allele
from one parent and short allele from
another parent. But, all F1 individuals
were tall. Why?
•Some alleles are dominant and others
are recessive.
Dominant & Recessive Alleles
• Dominant Allele
– The trait that shows up in the organism
whenever the form of the gene (allele)
is present.
– Represented by a capital letter (T)
– Ex. Tall stems--T
• Recessive Allele
– The allele that is masked, or covered
up, whenever the dominant allele is
present.
– Recessive trait will show up only when
the organism does not have the
dominant allele.
– Represented by a lowercase letter (t)
– Ex. Short stems=tt
Understanding Mendel’s Crosses
• Purebreds (P Generation)
– Either TT or tt
• Hybrids (seen in F1)
– Have both a dominant and recessive allele.
– Tt
• One allele from tall parent, and one allele from
short parent.
Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
So…
• If a pea plant has a tall stem, what
possible combinations of alleles could it
have?