Mendel studied inherited traits in pea plants through controlled breeding experiments in the mid-1800s. He discovered that traits are passed from parents to offspring through discrete factors that he called genes. His experiments demonstrated that genes come in different versions called alleles that are separated during gamete formation and transmitted independently of one another based on simple mathematical probabilities seen in his Punnett square diagrams. Mendel was thus able to deduce the basic principles of heredity and inheritance that became the foundation of modern genetics.

1. • What is the difference between
acquired and inherited traits?

2. • Austrian monk considered
the “Father of Genetics”
• Did his experiments in 1866
• He did plant experiments
using the garden pea plant
• He discovered the pattern in
which organisms inherit
traits

3. • Mendel chose to use
the garden pea in his
experiments for
several reasons.
1. Availability
2. Easy to manipulate for breeding
3. Distinct traits
Mendel chose his subject carefully

4. What traits did Mendel study?

5. •Two different forms of the same gene =
•The factors that carry all traits =

6. • When he wanted
to breed, or cross,
one plant with
another, Mendel
opened the petals
of a flower and
removed the male
organs.
Remove
male parts
Mendel chose his subject carefully

7. • He then dusted the female organ with pollen
from the plant he wished to cross it with.
Female
part
Transfer pollen
Pollen
grains
Male
parts
Cross-pollination
Mendel chose his subject carefully

8. PP pp
Pp Pp Pp Pp
Hybrids
Purebred = Homozygous
Hybrid = Heterozygous

9. Pp Pp
PP Pp Pp pp

10. • Mendel called the observed trait dominant
(represented by a CAPITAL letter) and the
trait that disappeared recessive (represented
by a lowercase letter).
The rule of dominance
P = purple, p = white

11. • Principle of Dominance: when
there is a dominant allele and a
recessive allele inherited together
for a particular gene, the dominant
allele will be expressed.
Mendel’s 1st
Law of Inheritance
• Dominant
•Round
• Recessive
•wrinkled
• Express the dominant allele for
Round

12. • PHENOTYPE –
physical
characteristics that
you SEE
• Ex. Round,
wrinkled, green,
yellow, purple
flower, white
flower
♦GENOTYPE – genetic
makeup represented
by letters
♦Ex. RR, rr, Rr, Yy,
PP, Ppyy

13. •Paternal •Maternal
wrinkledROUND

14. You inherit two alleles for each gene (trait).
1 from MOM! 1 from DAD!

15. • The law of segregation states that every
individual has two alleles of each gene and
when gametes are produced, each gamete
receives one of these alleles.
The law of segregation
• During fertilization, these gametes randomly
pair to produce four combinations of alleles.
RR rr
Rr Rr Rr Rr

19. Mendel’s 3Mendel’s 3rdrd
Law of InheritanceLaw of Inheritance
Principle of Independent Assortment: the
alleles for different genes usually separate and
inherited independently of one another. So, in
dihybrid crosses you will see more combinations
of the two genes.
•BBbbTTtt
•diploid (2n)
•sperm
•haploid (n)
•meiosis II
•BBTT
•BBtt
•bbTT
•bbtt

20. The first generation
Dihybrid Cross round yellow x wrinkled green
Round yellow Wrinkled green
All round
yellow
Round yellow Round green Wrinkled yellow Wrinkled green
9 3 3 1
P1
F1
F2

22. Reginald Punnett
1.(1875-1967)
2.In 1902, created the
Punnett Square - a chart
which helped to determine
the probable results of a
genetic cross
•http://vector.cshl.org/dnaftb/concept_5/con5bio.html

23. Punnett Square
3.each parent can only
contribute one allele per
gene
4.these genes are found on
the chromosomes carried
in the sex cells.
5.offspring will inherit 2
alleles to express that
gene
•Male gametes
•Female gametes

24. Punnett SquaresPunnett Squares
•Steps to Solve Punnett SquaresSteps to Solve Punnett Squares
•Step 1Step 1: Assign letters for alleles.
•Step 2Step 2: Write down the cross you will make.
What are the genotypes of the parents?
•Step 3Step 3: Set up and solve the Punnett square.
MONOHYBRID CROSSESMONOHYBRID CROSSES - 1 trait crossed

25. Monohybrid CrossesMonohybrid Crosses
•CrossCross
•Bb x BbBb x Bb
•B •b
•female gametes
•B
•b
•male gametes
•BB
•Bb
•Bb
•bb •Phenotypes
•%
•75% Brown eyed, 25% Blue eyed
•Ratio
•3 Brown eyed: 1 Blue eyed
•Genotypes
•%
•25% BB, 50% Bb, 25% bb
•Ratios
•1BB:2Bb:1bb
•STEP 
•STEP 
•STEP 
•STEP 
BB = brown eyes Bb = brown eyes bb = blue eyes
Cross two Alaskan huskies that are heterozygous for
brown eyes (Bb).

26. Your Turn
YY x Yy (Y-yellow, y-green)

27. Steps to Solve DIHYBRID Punnett
Square word problem.
DIHYBRID CROSSES –
2 traits crossed
1. Assign letters to the different alleles for the trait.
2. Write the cross that you will be making.
3. Use the FOIL method to determine the combination of
alleles for each gamete.
4. Set up the Punnett Square by separating your female and
male gametes. Solve.

28. DIHYBRID CROSS
•Problem: In pea plants round seeds are dominant to wrinkled
seeds and green pods are dominant to yellow. If 2 plants that
were heterozygous for both traits were crossed, what would be
the phenotypic and genotypic ratios?
•Cross: RrGg X RrGg
Gene Trait Appearance
R Round
r Wrinkled
G Green
g Yellow
•STEP 
•STEP 

29. DIHYBRID CROSSES
•
•Cross: RrGg X RrGg
•EXTRA STEP - Finding the possible gametes
•(R r) (Gg)
•F.O.I.L
First
Outer
Inner
•STEP 

30. •
•PRACTICE F.O.I.L.
•Finding the possible gametes
•(r r) (Y y)
•F.O.I.L
First
Outer
Inner
Last
•Try These:
TtBB
 DDee
 ffPp

31. •Phenotypic ratioPhenotypic ratio: 9 round, green: 3 round, yellow: 3 wrinkled, green: 1 wrinkled, yellow
 (9:3:3:1)
•
•RG •Rg •rG •rg
•RRGg •RrGG •RrGg
•RRGg •RRgg •RrGg •Rrgg
•RrGG •RrGg •rrGG •rrGg
•RG
•Rg
•rG
•rg •RrGg •Rrgg •rrGg •rrgg
•STEP 
•STEP 
•RRGG
•Genotypic ratioGenotypic ratio: 1 RRGG: 2 RRGg: 2 RrGG: 4 RrGg: 1 RRgg: 2 Rrgg: 2 rrGg: 1 rrGG: 1
rrgg

32. PROBLEM: A pea plant that is
heterozygous for round and yellow is
crossed with one that is wrinkled and
green. What are the genotypic and
phenotypic ratios for this cross?
R = round
r = wrinkled
Y = yellow
y = green
Cross: RrYy X rryy

33. •Gametes: RrYy rryy
RY ry
Ry ry
rY ry
ry ry
• •RY •Ry •rY •ry
•ry •RrYy •Rryy •rrYy •rryy
•Genotypic ratioGenotypic ratio - 1RrYy : 1 Rryy: 1 rrYy: 1 rryy  (1:1:1:1)
•Phenotypic ratio - 1 round, yellow: 1 round, green: 1 wrinkled,

34. Practice Problems
1. TTyy x TtYY (T = tall, t = short, Y = yellow, y =
green)
2. In llamas, floppy ears (F) are dominant to pointy
ears (f). Also, bowlegs are recessive (l). A
bowlegged llama heterozygous for floppy ears is
crossed with a llama that is heterzygous for both
traits. Show the genotypes of these two parents.
3. In starfish, pink body color (P) is dominant to
orange (p), and thick eyebrows (T) are dominant
over thin (t). Patrick, who is heterozygous for body
color but purebred for thick eyebrows, has met
Patti, who is recessive for both traits. Is it possible
for the new couple to have an offsprings that
resembles their mother?

35. Punnett Square
Click image to view movie.