1. Monohybrid and dihybrid crosses
Punnett Squares
Monohybrid and dihybrid crosses

2. 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!

3. 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 ( )
Taught physics and natural science for 16 years, Brno (1854)
Research phase , when he was elected Abbot of the monastery

4. 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.”

5. “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

6. 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’

7. Punnett Squares His most famous work is the ‘Punnett Square’
A simple tool to predict the probability of possible genotypes of offspring

8. 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

9. 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

10. 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

13. 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)

14. Selecting alleles for dihybrids
Parent 1 genotype is:
AaBb
Parent 2 genotype is AaBb

15. Steps in completing a dihybrid1. 2. 3. 4.
Find the genotype of both parents.
Eg. AaBb x AaBb
Determine which alleles could be passed on to the offspring (criss cross)
Complete the Punnett Square
State the phenotypic or genotypic ratios

16. 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…

17. A Dihybrid example (cont)TL Tl tL tl
TTLL
TTLl
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

18. A Dihybrid example (cont)TL Tl tL tl
TTLL
TTLl
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)

19. 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