1. Gregor Mendel
 When two heterozygous
plants are crossed
the expected and
observed phenotypic
ratio will always be
3:1
 This was the same
for all 7 of his tests

2. Gregor Mendel
 Mendel concluded that the two alleles for each
characteristic separate during gamete production.
 This segregation of alleles corresponds to the
distribution of homologous chromosomes to different
gametes in meiosis
 Known as the law of segregation

3. Gregor Mendel
Genotype of alleles:
R = red flower
r = orange flower
All genes occur in pairs, so 2 alleles affect a
characteristic
Possible combinations are:
Genotypes RR Rr rr
Phenotypes RED RED ORANGE

4. Practice
For each genotype: heterozygous (He) or homozygous (Ho)
Ho
TT_____ He
Bb______ Ho
DD______ He
Ff ______ Ho
tt _____
He
Dd_____ Ho
ff ______ He
Tt ______ Ho
BB______ Ho
dd_____
Which of the genotypes would be purebred?
TT, ff, DD, BB, tt, dd
__________________________________________________
Which of the genotypes would be hybrid?
Dd, Bb, Tt, Ff
__________________________________________________

5. Practice
Determine the phenotype for each genotype
 Yellow body color is dominant to blue
Yellow Yellow Blue
YY_______________ Yy _______________yy _______________
 Square shape is dominant to round
Square Square Round
SS________________Ss_______________ ss________________

6. Practice
For each phenotype, give the possible genotypes
 A tall head (T) is dominant to short (t)
TT, Tt
Tall_______________ tt
Short_______________
 Pink body color (P) is dominant to yellow (p)
PP, Pp
Pink_______________ pp
Yellow______________

7. Genetic Crosses
 Monohybrid cross: cross involving a single trait
 Flower color, plant height
 Dihybrid cross: cross involving two traits
 Flower color & plant height
 Punnett squares help determine the possible
combinations of genotypes that can occur in the
offspring.
 It also shows the probability of each genotype occurring

8. Genetic Crosses

9. Genetic Crosses
Solving Punnett squares only takes a few steps:
1. Determine the genotypes of the parent organisms
2. Write down your ‘cross’
3. Draw your Punnett square
4. ‘Split’ the letters of the genotype for each parent &
put them ‘outside’ the Punnett square
5. Determine the possible genotypes of the offspring by
filling in the Punnett square
6. Summarize the results (both genotype and
phenotype of the offspring)

10. Genetic Crosses
1. Sometimes the question already tells you, other
times you need to understand the vocabulary. Read
carefully!
2. Write down the genotypes of the parents
Ex: Tt x tt
3. Draw the Punnett square

11. Genetic Crosses
4. Split the letters of our cross Tt x tt
This ‘split’ represents the process of meiosis
The vertical represents the male gamete
The horizontal represents the female gamete

12. Genetic Crosses
5. Each box represents a possible genotype of the
offspring

13. Genetic Crosses
6. Summarize the results of the offspring; both
genotype and phenotype
Genotype:
50% TT 50% tt
Phenotype:
50% tall 50% short

14. Practice
SpongeBob is heterozygous for his square shape, but
SpongeSusie is round. Create a Punnett square to show the
possibilities if they had children. (look at old questions!)
Ss ss
Cross: ___________ x ___________
Punnett Square ____
s ____
s
S
____ Ss Ss
____
s Ss ss
Genotypes: Ss, ss Genotypic ratio: 3:1
Phenotypes: Square, Round Phenotypic ratio: 3:1

15. Practice
Patrick met Patti at a dance. They are both heterozygous for
their pink body color, create a Punnett square to show the
possibilities if they had children. (look at old questions!)
Pp Pp
Cross: ___________ x ___________
Punnett Square ____
P ____
p
P
____ PP Pp
____
p Pp pp
Genotypes: PP, Pp, pp Genotypic ratio: 1:2:1
Phenotypes: Pink, yellow Phenotypic ratio: 3:1

16. Gregor Mendel
 In one dihybrid cross, Mendel studied the inheritance
of seed color and seed shape
 The allele for yellow seeds (Y) is dominant to the
allele for green seeds (y).
 The allele for round seeds (R) is dominant to the
allele for wrinkled seeds (r).

17. Gregor Mendel
 He crossed true-breeding plants that has yellow, round
seeds (YYRR) with true-breeding plant that had green,
wrinkled seeds (yyrr)
 One possibility is that the two characteristics are
transmitted from parents to offspring as a package
 The Y and R alleles and the y and r alleles stay
together

18. Gregor Mendel
 The Y and R alleles and
the y and r alleles stay
together
 This was not consistent
with Mendel’s results

19. Gregor Mendel
 An alternative hypothesis is that the two alleles
segregate independently of each other
 Known as the law of independent assortment
 The presence of one specific allele for one trait has no
impact in the presence of a specific allele for the second
trait.

20. Gregor Mendel
 In our example, the F1 offspring would still produce
yellow, round seeds. (YyRr)
 However, when the F1’s produced gametes, genes would
be packaged into gametes with all possible allelic
combinations.
 Four classes of gametes (YR, Yr, yR, and yr) would be
produced in equal amounts.

21. Gregor Mendel