Mendelian Genetics
Father of Modern Genetics
• Austrian monk, high
school teacher, and
part-time garden
keeper
• First to propose
biological inheritance of
traits
• Work not recognized
until after his death
Gregor Mendel
Why pea plants?
• It’s what he had
• They grow fast
• Pea plants are true
breeding
• Gregor noticed that one
stock of seed would
produce only tall plants
and another only short
plants
Traits…
• Specific characteristics that vary from one
individual to the next
– Hair color
– Eye color
– Seed color
– Flower color
Mendel’s Experiment
Cross-pollination
P generation
Next, Mendel wanted to find out if the white
alleles had disappeared, or were they still present
in the F1 generation.
Vocabulary…
• P generation: parental
• F1: “first son” – the offspring of the P
generation
• F2: “second son” – the offspring of the F1
cross
• Phenotype: the physical characteristics, what
the trait “looks” like
• Hybrid: cross between parents with different
versions of the trait.
• Mendel crossed the
F1 generation with
itself
• He found that the
white flower trait
reappeared in some
of the F2
generation
Perplexed?
• Conclusion One: inheritance is due to factors
that are passed from one generation to the
next
– Today, we call those factors GENES
– Allele: different forms of a gene (like pink vs white
for flower color)
• Conclusion Two: some alleles are dominant
and others are recessive
How did the white trait
“reappear”?
• He proposed this was
due to segregation of
alleles during formation
of gametes.
What are these
called? What
process made
them?
What are these called?!
• Homozygous: two copies of the same allele
– Dominant: AA
– Recessive: aa
– TRUEBREEDING
• Heterozygous: different alleles (Aa)
Practice: Cats
1.
2.
3.
4.
What trait is represented by “T” and “t”?
“NN” = phenotype or genotype?
Long hair = phenotype or genotype?
A homozygous tabby cat would have what
genotype?
5. A cat that is “nn” would have what
phenotype?
Applying and practicing your vocab
• Crazy Traits
• Exit Slip
• Vocab quiz on Wednesday
Probability
• Mendel categorized and
counted the many
offspring of each of his
experiments.
• He noticed that each
time he repeated a
particular cross he
obtained similar results.
For example:
Every time he crossed two
plants that were
heterozygous for stem
height (Tt), about ¾ of
the offspring were tall
and ¼ were short.
Probability and Punnett Squares
• Punnett squares are
used as a tool to predict
this probability
• Monohybrid crosses can
be performed to
determine probability
of phenotype for one
particular trait
Genotypes
AA
Aa
aa
Homozygous Dominant
Heterozygous
Homozygous Recessive