Fundamentals of Genetics
Gregor Mendel (1822-1884)
• Monk from Austria
• He bred pea plants in monastery
gardens and used math to interpret
his findings
• He is the “Father of Genetics”
–Even though he was never
recognized in his lifetime!
Mendel’s Garden Peas
• Mendel observed seven characteristics
when using purebred pea plants. Each
characteristic trait occurred in two
contrasting alleles.
• Traits – a distinguishing characteristic
that is inherited such as eye color, leaf
shape and tail length.
• Allele – different forms of a trait
Mendel’s Observations
• Mendel observed 7 characteristics of pea
plants. ex: - seed color (green or yellow)
- seed texture (smooth or wrinkled)
- flower color (purple or white)
He used his knowledge of statistics to
analyze his observations.
• Crossed plants (mated), collected seeds,
recorded observations, then planted seeds,
then recorded new plant characteristics.
Mendel’s Methods
• Pollination – occurs when pollen grains
produced in the male reproductive parts
of a flower (called anthers) are transferred
to the female reproductive part of a
flower (called stigma).
• Self-pollination – occurs when pollen is
transferred from the anthers of a flower to
the stigma of either the same flower or a
flower on the same plant.
• Cross-pollination – involves pollen
transfer with flowers of two separate
plants.
• First Mendel snipped off the anthers
• Then he dusted the stigma with pollen
taken from the desired father.
• Finally he tied bags over the flowers
to keep out any stray pollen.
This gave him control over the
pollination of his pea plants (no
wind or insects could interfere!)
• He observed that purple-flowering
plants grew from the seeds of purple
plants, but… he noticed that some
white-flowering plants also grew from
the seeds of purple-flowering plants.
• Mendel wanted to find an explanation
for such variations.
Mendel’s Laws
• AKA – Patterns of Inheritance
Mendel Discovered Dominance
• 1st Cross= pure Tall x pure Short
Tall X Short

All Tall
Tallness = Dominant
Short = Recessive
• Dominance- allele
expressed when
present in genotype
• Recessive- allele
expressed ONLY
when two copies are
present
Now Breed the Hybrid
• 2nd Cross= hybrid Tall x hybrid Tall. Plants self
pollinate (self-fertilize)
Hybrid- containing contrasting alleles
Tall x Tall  ?
Results in math:
3/4 tall : 1/4 short plant
Recessive trait
reappears!!!
1. Law of Segregation
• Organisms inherit two copies of
each gene/trait = alleles.
• Alleles segregate (separate)
during the formation of sperm
and eggs.
• Organism donates ONE copy of
each gene in fertilization.
Alleles
Def: Allele is an alternative form of a gene
(ex: tall and short for PLANT HEIGHT)
• One allele T is for tallness
• One allele t is for shortness
• T allele is dominant over t allele.
Tt = Tall
TT = Tall
Certain traits do not
tt = short
blend!! (no medium
height)
Same Phenotype
Different Genotype
TT
Tt
Homozygous – when both alleles of a
pair are alike. (Ex: TT or tt)
(can be homozygous dominant or
homozygous recessive)
Heterozygous – when the two alleles in
the pair are different. (ex: Tt)
Punnet Square
• A grid system to predicting all possible
genotypes and phenotypes that result from a
cross.
• Is a shorthand way of showing how the
gametes behave in a cross.
Monohybrid Cross
– a cross
between one
trait (ex:
plant height,
OR hair
texture)
t
T
Tt
tt
Tt
tt
t
t
Testcross- cross between an organism
with homozygous recessive genotype
and a second organism with an
unknown genotype.
• The offspring
results will show
whether the
organism with
unknown genotype
is heterozygous or
homozygous
dominant.
Dihybrid Cross
a cross between two traits
(ex: plant height and seed
color)
Mendel’s question… are traits
inherited together?
Ex: Do people with Blond hair always have blue eyes?
Do purple flowers always make smooth seeds?
Do seeds that are round have to be yellow???
2. Law of Independent
Assortment
- Traits segregate (separate)
independently during the
formation of sperm and eggs.
- They then come together
again in the new offspring
(kid)