Transcript Genetics: The Work of Gregor Mendel

Mendelian Genetics
Genetics and heredity
For a long time, general ideas of inheritance
were known
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What was really lacking was a quantitative
understanding of how particular traits were
passed down to their offspring.
Gregor Mendel (1822-1884)
• German monk born in
1822
• Lived in what is now the
Czech Republic
• Tended the garden at
his monastery,
conducted experiments
with pea plants
• Studied peas for 7 years,
published results 1866;
ignored until 1900
• Now considered the
“Father of Genetics”
What is a trait?
• A trait is a characteristic that can vary from one
individual to the next (e.g., eye color)
• Mendel chose 7 pea plant traits to study:
– seed shape, seed color, flower color, pod shape, pod color,
flower position, plant height
• He was lucky that each trait happened to be located
on different chromosomes (people didn’t know about
chromosomes back then)
Mendel's experiments
• First, he created
true (pure) breeding
pea plants
• Then he
experimented
with different
traits (one at a
time)
– He cross-pollinated
plants with
different traits,
and observed the
offspring
(Normally done by bees…
Mendel used scissors and a
paint brush)
Important vocabulary
• True (pure) breeding = offspring always have
same trait as parent (every time, no matter
what!)
• Self-pollination = plant fertilizes itself
• Cross-pollination = one plant fertilizes another
• P generation = Parent generation
• F1 generation = 1st generation offspring (“filius”
is Latin for “son”; offspring of P generation)
• F2 generation = 2nd generation offspring
(offspring of F1 generation)
Mendel’s crosses
• When Mendel crossed true-breeding tall plants
with true-breeding dwarf plants (P generation)…
…what size were the
offspring (F1 generation)?
• The F1 generation were all
tall
Mendel was surprised!
• Mendel had expected the traits to blend:
tall + dwarf = medium
• Instead, the tallness of parent had masked the
shortness of the other parent
tall + short = tall????
• Mendel then let the F1 generation self-pollinate
to create the F2 generation …what size were the
offspring (F2 generation)?
• Some of the F2 offspring were tall, and some
were dwarf
• The ratio was always 3 tall to 1 dwarf (3:1)
Mendel's findings
• When Mendel crossed true-breeding plants
with different traits, all of the F1 plants
showed only one trait (e.g., all tall); the F2
plants showed a 3:1 ratio
• He did not get “medium” plants!
• He called the traits dominant (showed in F1
generation) or recessive (didn’t show up in the
F1 generation)
Mendel’s conclusions
Mendel realized that:
1. Inheritance of traits is controlled by genes
contributed by each parent (e.g., flower color)
Mendel’s conclusions
2. These genes come in different forms, called
alleles (e.g., purple or white flowers)
Mendel’s conclusions
3. Some alleles are
dominant while
others are recessive
– When both types of
alleles are present,
dominant alleles mask
(cover up) recessive
alleles
Mendel’s
Discoveries
& Results
Mendel’s Law of Dominance:
• Generally, one allele is dominant to another
• Dominant allele always expressed, even if
combined with recessive allele
– written as uppercase letter of the trait
– e.g., tall = dominant: TT, tT, or Tt
• Recessive allele expressed only if dominant
allele is not present.
– written as a lowercase letter of the dominant trait
– e.g., dwarf = recessive: tt
More vocabulary…
• Genotype = genetic combination of alleles
(e.g., TT, Tt, tt)
• Phenotype = physical characteristic or trait
(e.g., tall, short)
• Homozygous = both alleles are the same
(e.g., TT or tt) → “true breeding”
• Heterozygous = alleles are different (e.g., Tt)
• Hybrid = offspring of crosses of parents with
different traits (e.g., offspring of TT and tt)
Mendel’s Law of Segregation
• When gametes form, the alleles from each
parent are segregated (separated) from each
other
• Each gamete carries a single allele for each
gene
Parents (P)
1
2
TT
tt
3
gametes
F1
4
T
T
Tt
Tt
T
t
t
Tt
t
T
10
TT
Tt
Tt
tt
11
12
13
14
Circle #
Genotype
Hetero/Homozygous
1
TT
homozygous
3
T
7
Tt
tt
14
t
8
9
F2
6
Tt
7
gametes
5
Phenotype
tall
Generation
Probability
of F2
generation
being TT?
Gamete?
(y / n)
no
n/a
P
n/a
yes
heterozygous
tall
F1
no
homozygous
short
F2
no
n/a
Punnett Squares
• Instead of doing all of that hard work for each
of the possible outcomes under the Law of
Segregation, we can do a Punnett Square
(geneticist Reginald Punnett created it)
Father's Gametes
T
T
t
Mother’s Gametes
TT
Tt
t
Tt
Tt
There is a 1 in 4 chance
of the offspring being TT
There is a 2 in 4 chance
of the offspring being Tt
There is a 1 in 4 chance
of the offspring being tt

3 out of 4 plants = tall
1 out of 4 plants = dwarf

Tall to dwarf = 3:1

Mendel’s Law of Independent Assortment
• Genes for two different traits are inherited
independently
• There is no connection between them (e.g.,
plant height and seed color)
Animation