Transcript Mendelian Genetics

Mendelian Genetics
Pea Plants and Punnett
Squares
Chapter 6, sections 3-5
Gregor Mendel –
“Father” of Genetics
• Austrian monk in mid-1800s
• Interested in nature
• Studied inheritance of traits in pea
plants
• Conducted experiments and
recorded data
• Developed some basic laws of
inheritance
Pea Plants because…
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fast-growing
mature quickly
easy to care for
have many easy-to-distinguish traits /
characteristics; examples: flower color
– either purple or white, plant height –
either tall or short, seed color – either
green or yellow
Mendel’s Experiments
Video clip – 4:30
Step One: Mendel allowed plants
to self-pollinate for several
generations to make sure that they
were “pure”, or true-breeding, for
flower color.
Today, we would describe each of
these “pure” plants as
homozygous.
Self-Pollination
Experiments, cont’d
Step Two: Mendel took two pure
flowers – one purple and one white
– and cross-pollinated them. He
called these two flowers the P
generation (P for parental).
Results: all offspring were purple;
he called these offspring the F1
generation (F1 for first filial)
Cross-Pollination
Experiments, cont’d
Step Three: Mendel allowed the F1
flowers – which were all purple - to
self-pollinate. The new offspring
were called the F2 generation
(second filial).
Results: Roughly 3 out of every 4
flowers was purple and 1 out of
every 4 was white.
Interpreting the Results
• Today, we know that…
–purple flower color is dominant
to white (in pea plants)
–the white flower color did not
completely disappear; it was
simply hidden (masked) in the
F1 generation
Results, cont’d
• Remember:
–Dominant = the allele that
always shows up in the
phenotype as long as at least
one copy is present
–Recessive = the allele that will
only show up in the phenotype if
there are no dominant alleles
Results, cont’d
In the F1 generation, all of the
plants were heterozygous
(genotype) for flower color – they
each had one purple allele and one
white allele, but they all had a
purple phenotype (appearance)
Results, cont’d
• In the F2 generation:
–one of the purple flowers was
homozygous dominant
–the other two purple flowers
were heterozygous
–the white flower was
homozygous recessive
Punnett Squares
• Now, we can use Punnett squares
to show what happened in
Mendel’s initial experiments:
The Punnett Square at left
shows the cross between
two pure plants; one
parent is homozygous
dominant and the other
parent is homozygous
recessive. This is like
Mendel’s P generation.
Setting up Punnett Squares
Punnett Square – F1 cross
• For Mendel’s F1
cross, he allowed
plants to selfpollinate.
• The results are
shown at right 
• 75% (3/4) were
tall and 25%
(1/4) were short.
Mendel’s Conclusions
1. Inheritance is based on genes, which
are passed on from parents to
offspring.
2. Some versions of a gene may be
dominant while others may be
recessive.
3. In most sexually reproducing
organisms, each individual has two
alleles for each gene – one from each
parent.
Conclusions, cont’d
4. When gametes (eggs and sperm)
form, the two alleles for each
gene separate (segregate) from
each other (Law of
Segregation).
5. When gametes form, different
genes separate (segregate)
independently of each other
(Law of Independent
Assortment).
Law of Segregation
Law of Independent Assortment
Factors in Mendel’s Success
1. Mendel concentrated on one trait at a
time.
2. Only after he established the
behavior of one trait (monohybrid
cross) did he consider two traits
together (dihybrid cross).
3. He conducted a large number of
crosses (2000-3000) to eliminate
chance and to obtain a valid and
accurate explanation
Success, cont’d
4. He actually counted the number of offspring
of each category and maintained accurate
records for each generation in each
experiment.
5. Luck -- though Mendel did not know this -- in
the selection of pea plants as well as in the
selection of the particular traits he studied.
• In each pair, one form of the trait
is completely dominant over the other.
• It also helped that the genes for the traits
Mendel studied are present on seven different
chromosomes in the pea plant.