Mendelian
Genetics
1
Hypothetically: if you crossed a
tall plant with a short plant what
would the offspring look like?
A. All tall
B. All short
C. All would be medium sized
D. Half tall and half short
When Mendel crossed pure-breeding tall
plants with pure breeding short plants
what traits were observed in in their
offspring?
A. All offspring were short
B. All offspring were tall
C. Half of the offspring were tall and
the other half was short.
D. All offspring were medium height.
When Mendel crossed this first
generation of plants with itself what
traits were observed in in their
offspring?
A. All offspring were short
B. All offspring were tall
C. Half of the offspring were tall and
the other half was short.
D. ¾ of the offspring were tall and ¼
were short
Mendel concluded that some
“factor” was responsible for
the traits that were observed in
his pea plants. Today we know
that these “factors” he referred
to are called genes.
Gene
A discrete unit of hereditary
information consisting of a specific
nucleotide sequence in DNA (or RNA
in some viruses).
7
Alleles
• Alleles are alternate forms of a gene.
• Examples: tall and short for plant height or
purple or white for flower color.
two
• Every trait has at least
allelesone from each parent.
• The location of an allele on a chromosome
is known as its locus (loci = plural form).
9
Genotype
• The letters that represent (symbolize)
the trait being investigated. The
genetic make-up of an organism.
• Examples: Bb, BB, bb
11
Phenotype
The actual representation of the
genes. The Physical appearance or
traits in an organism resulting from
its genetic makeup (what you see).
Examples: tall, purple flower or whit
flower, blond hair, freckles, etc.
12
Dominant
The allele that is fully expressed in
an organism (observed).
• Represented by capital letters.
• Tall = T
13
Recessive
• The allele that is masked by the
dominant allele.
• Represented by lower case
letters.
• Short = t
14
Homozygous
• When both alleles (letters) are
the same.
• BB = Homozygous Dominant
• bb = Homozygous recessive
15
Heterozygous
• When the alleles (letters) are
different.
• One upper case letter and one that
is lower case.
• Bb = Heterozygous
16
If a plant is homozygous dominant
for tallness what is its genotype?
Use the symbol T for tallness.
(Tallness is dominant)
A.
B.
C.
D.
TT
tt
Tt
I don’t know
If a plant is heterozygous for tallness
what is its genotype?
Use the symbol T for tallness.
(Tallness is dominant)
A.
B.
C.
D.
TT
tt
Tt
I don’t know
Example Problem
• Round = R
• wrinkled = r
• If a plant has round seeds, do we
know what its genotype is?
19
• It could be RR or Rr
• If a plant has wrinkled seeds, do
we know what its genotype is?
• Yes!!!: it is r r
Punnett Square
• A method for finding predicted
outcomes and probabilities for
offspring from any cross.
• A chart for predicting the traits
of offspring.
22
Some more terms:
P-generation is the parental generation.
The p-generation produce the F1
generation.
The F1 generation crossed with itself
produces the F2 generation.
Example Problem
24
In foxes, red coat color is determined
by the dominant gene R; silver-black
coat is determined by the recessive
gene r. A homozygous (pure) red
male is crossed with a silver-black
female. (The P generation).
1.What is the genotype of the female?
What are the genotype percentages
of their offspring?
• First…make a Punnett square for
showing your work
R
R
r
Rr
Rr
r
Rr
Rr
Punnett Squares
• Why are punnett squares useful?
• We can use a punnett square to predict the
probable genotypes and phenotypes for
offspring from a genetic cross.
• Genotype = What is inside the genes, the
make-up.
• Phenotype = The outward expression of
the genes.
28
Assignment
• “Using Punnett Squares to
Predict the Outcomes of
Crosses” worksheet and
“Monohybrid Crosses”
worksheet – Due Wed 2/11 and
Friday 2/13 respectively
29