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Genetics Unit 1
I. Background Stuff
• Meiosis and fertilization pass the genetic
code from parents to offspring
• The genetic code is found in genes of
chromosomes (DNA)
• Heredity and the environment are the 2
factors that create an individual. You can
control your environment but not your
heredity
II. Gregor Mendel
• What is Genetics?
- The study of heredity
• What is heredity?
- Passing of traits from parents to
offspring.
• Father of Modern Genetics
• Mendel was a monk
• Studied pea plants
A. Peas
• Why peas?
- Cheap, easy to grow, could self-pollinate,
didn’t take much room, you can eat them, 7
black and white contrasting traits
• Look at the 7 contrasting traits on page 215
• Allele – contrasting forms of a trait
• Phenotype – physical traits / green peas, purple
flowers, round seeds are all phenotypes
III. Mendel’s Experiment
We will use a tall and a short pea
plant for our example
A. Step 1
• He “created” a pure-bred tall and a purebred short pea plant.
• He did this by taking a tall plant and
breeding it to itself and seeing if it had any
short offspring. If it did he knew it was not
pure-bred.
B. Step 2
• He crossed a pure tall Pea plant with a
pure short pea plant.
• This is called the P generation / P stands
for parental not Pea
• What did he get?
• He did not get medium because they do
not exist. He did not get some tall and
some short.
• He got all tall / The offspring of the P
generation is called the F1 generation.
C. Step 3
• Mendel wondered what happened to the
short allele.
• He crossed two F1 generation plants.
• What did he get?
• He got 3 tall plants to every short plant or
a 3tall:1short ratio
• This is known as the F2 generation
IV. Mendel’s 4 Hypotheses
Mendel developed 4 hypotheses from
his research which have been
universally proven so they are really
laws.
A. Genes occur in pairs
• Mendel stated that you get one gene from
mom and one from dad
• So each phenotype is controlled by two
genes, one from each parent
• Genotype – the genetic makeup of an
organism / the two genes you receive from
your parents
B. Principle of Dominance and
Recessive
• One gene may mask the effects of the other.
• The one that does the masking is the dominant
trait / capital letter
• The masked trait is recessive / lowercase letter
• TT – Tall Pea Plant / Homozygous Dominant
• tt – Short Pea Plant / Homozygous Recessive
• Tt – Tall Pea Plant / Heterozyous
C. Principle of Segregation
• Genes separate individually in meiosis
• If one parent is Tt then half of the gametes
will be T and the other half will be t
D. Principle of Independent
Assortment
• Traits are not linked or attached
• Just because you are a tall pea plant does
not mean you will have purple flowers.
• Just because you have blond hair does
not mean you will have blue eyes.
V. Predicting the Results of
Genetic Crosses (Mating)
A. Basic Info
• When Mendel crossed his F2 generation and
saw a 3:1 ratio, the only pea’s genotypes he
knew were the short ones (tt)
• The tall ones could have been TT or Tt
• R.C. Punnett created a system to determine the
probable outcome of the cross.
• Probability - # of one outcome / # possible
outcomes
• Everyone flip a penny 50 times and get the class
average of heads vs. tails
A. Basic Info Continued
• What is the predicted ratio of heads to
tails?
• What did the class get? Is that wrong?
B. Monohybrid Cross
• A cross between two individuals looking at
1 pair of triats (tall vs short)
• Use a Punnett Square to show TT x tt and
give the genotypic and phenotypic
percents and ratios
• Each box represents a possible offspring
• Do the same with your F1, Tt x Tt