Download Gregor Mendel

Punnett Squares
Monohybrid and dihybrid crosses
Quick Reminder...
 Genotype: the unobservable
genetic make-up of an organism (i.e.
The code that expresses the observable
traits)
 Phenotype: the observable traits
of an organism (i.e what you can see)
 Eg: genotype is Bb; phenotype is brown
eyes!
Gregor Mendel (1822- 1884)
 Born Johann Mendel, Austria (1822)
 Admitted to Augustinian Monastery of St. Thomas in Brno (1843)
 Studied physics and botany at the University of Vienna (1851-1853)
 Taught physics and natural science for 16 years, Brno (1854)
 Research phase 1856-1868, when he was elected Abbot of the
monastery
Gregor Mendel
 Died of a kidney disorder
(1884)
 “His death deprives the poor of
a benefactor, and mankind at
large of a man of the noblest
character, one who was a warm
friend, a promoter of the
natural sciences, and an
exemplary priest.”
“Father of Genetics” (since 1866)
 Opened our minds to inheritance
 Performed hybridization experiments with pure breeding garden
pea plants
 Why did he choose the pea plant?
 Mendel followed 7 visible features with contrasting traits
Reginald Punnett
 Reginald Punnett studied zoology
in England.
 A friend, Robert Bateson,
translated Mendel’s work into
English
 Two of them brought the new field
of genetics to Cambridge
University
 His book ‘Mendelism’ (1905) is
the first genetics ‘textbook’
Punnett Squares
 His most famous
work is the ‘Punnett
Square’
 A simple tool to
predict the
probability of
possible genotypes of
offspring
How to do Punnett Squares
 Put parent genotypes at the top and left side
 Copy the alleles from each parent to both boxes in the
row or column
 If a heterozygous condition exists, write the capital letter
first
Let’s try one!
 The presence of horns
(H) is dominant to not
having horns (h).
 Use a Punnett Square
to determine the
possible outcomes of
the offspring.
 First write the
genotype of each
parent on top and side
Let’s try one!
Prediction of offspring:
 As a phenotypic ratio:
 4:0 horned
 As a genotypic ratio:
 4:0 Hh
 As a phenotypic
percent: 100% horned
Dihybrid Crosses
• Same as monohybrid,
we are now just
looking at 2 traits at
the same time.
• Assumes they are on
different genes and
can occur at the same
time
• Eg. Red flowers (R)
or white flowers (r)
and Tall plants (Y) and
short plants (y)
Selecting alleles for dihybrids
• Parent 1
genotype is:
AaBb
•
Parent 2
genotype is
AaBb
Steps in completing a dihybrid
1.
 Find the genotype of both parents.
 Eg. AaBb x AaBb
2.
3.
4.
 Determine which alleles could be
passed on to the offspring (criss
cross)
 Complete the Punnett Square
 State the phenotypic or genotypic
ratios
A Dihybrid example
If a straight thumb (T) is dominant over a hitch-hikers thumb
(t) and long eyelashes (L) are dominant to short (l):
 Step 1: Assume parent genotypes are TTLl and TtLl
 Step 2: First parent can only give TL or Tl; Second parent
could give TL, Tl, tL or tl
 Step 3: Build a Punnett Square using these allele
combinations…
A Dihybrid example (cont)
TL
Tl
tL
tl
TL TTLL
TTLl
TtLL
TtLl
Tl
TTll
TtLl
Ttll
TTLl
Format notes:
•
group the pairs of letters in the same order as Step 2
•
if a heterozygous condition exists, write the capital letter first
A Dihybrid example (cont)
TL
Tl
tL
tl
TL TTLL TTLl TtLL TtLl
Tl TTLl TTll TtLl
Ttll
Step 4: Phenotypic ratio is 6:2:0:0
- 6 have both dominant traits (Straight thumb and long
eyelashes
- 2 have the straight thumbs but short eyelashes
- Genotype doesn’t matter here (TT vs Tt)
A unique case:
Mendel’s F2 gen.
 For yellow (Y) and green
(y) seeds; round (R) and
wrinkled (r)
 When 2 heterozygous
parents are crossed, then…
 Phenotypic Ratio is always
9:3:3:1