3. Monohybrid Cross
a method of determining the inheritance pattern
of a trait between two single organisms.
a cross between parents who are true-breeding
for a trait; i.e., both are homozygous for one allele
of the
gene, for example AA x aa, in
which A is the dominant allele
for a trait and a is the recessive
allele for that same trait.
4. Sample Problem
In pea plants, spherical seeds (S) are
dominant to dented seeds (s). In a
genetic cross of two plants that are
heterozygous for the seed shape trait,
what fraction of the offspring should
have spherical seeds?
5. Analysis
The figure above represents a monohybrid
cross of F1-hybrid plants.
Both parent plants are heterozygous (Ss) for
an allele that determines seed shape.
Presence of the dominant allele (S) in
homozygous (SS) or heterozygous (Ss) plants
results in spherical seeds.
Homozygous recessive (ss) plants have
dented seeds.
6. Analysis
To solve the sample problem, you'll
need to set up a Punnett square.
Punnett square - a diagram that is used
to predict an outcome of a particular
cross or breeding experiment
7. Steps to Solve the Sample
Problem
Set up a 2 by 2
Punnett square.
8. Steps to Solve the Sample
Problem
Write the alleles for
parent 1 on the left
side of the Punnett
square.
Each gamete will have
one of the two alleles of
the parent. In this
particular cross, half of
the gametes will have
the dominant (S) allele,
and half will have the
recessive (s) allele. We
will use blue and brown
to keep track of the
alleles of each parent.
S
s
9. Steps to Solve the Sample
Problem
Write the alleles
from parent 2 above
the Punnett square.
For this heterozygous
parent (Ss), half of the
gametes will have the
dominant (S) allele, and
half will have the
recessive (s) allele.
S
S
s
s
10. Steps to Solve the Sample
Problem
Fill the squares for
parent 1.
Fill each square with
the allele from Parent
1 that lines up with
the row.
Fill the squares for
parent 2.
Fill each square with
the allele from Parent
2 that lines up with
the column.
S
S
s
s
S S
s sS
S
s
s
11. Steps to Solve the Sample
Problem
Interpreting the
results of a Punnett
square
Genotypes that
resulted from this
monohybrid cross
(Ss x Ss)
25% homozygous
dominant
50% heterozygous
25%
homozygous
recessive
S
S
s
s
S S
s sS
S
s
s
12. Steps to Solve the Sample
Problem
Interpreting the
results of a Punnett
square
Phenotypes that
resulted from this
monohybrid cross
(Ss x Ss)
75% Spherical
25% Dented
S
S
s
s
S S
s sS
S
s
s
14. Dihybrid Cross
a cross between F1 offspring (first generation
offspring) of two individuals that differ in two traits
of particular interest.
used to test for dominant and recessive genes in
two separate characteristics
The rules of meiosis, as they apply to the
dihybrid, are codified in Mendel's First Law and
Mendel's Second Law, which are also called the
Law of Segregation and the Law of Independent
Assortment, respectively
15. Example Problem
In summer squash, white fruit color (W) is
dominant over yellow fruit color (w) and disk-
shaped fruit (D) is dominant over sphere-
shaped fruit (d).. If a squash plant true-
breeding for white, disk-shaped fruit is
crossed with a plant true-breeding for yellow,
sphere-shaped fruit, what will the phenotypic
and genotypic ratios be for:
a. the F1 generation?
b. the F2 generation?
16. Steps to Solve the Sample
Problem
Write down the cross in terms of the
parental (P1) genotypes and
phenotypes:
WWDD (white, disk-shaped
fruit) X wwdd (yellow, sphere-
shaped fruit)
17. Steps to Solve the Sample
Problem
Determine the P1 gametes, place
them in a Punnett Square and fill in
the resulting genotypes:
18. Steps to Solve the Sample
Problem
Determine the genotypic and
phenotypic ratios for the F1
generation:
All F1 progeny will be
heterozygous for both characters
(WwDd) and will have white, disk-
shaped fruit .
19. Steps to Solve the Sample
Problem
Write down the cross between
F1 progeny:
WwDd (white, disk-shaped
fruit) X WwDd (white,
disk-shaped fruit)
20. Steps to Solve the Sample
Problem
Determine the
F1 gametes,
place them in a
Punnett Square
and fill in the
resulting
genotypes:
21. Steps to Solve the Sample
Problem
Determine the genotypic and phenotypic ratios
for the F2 generation:
Genotypic ratios:
1/16 will be homozygous dominant for both traits
(WWDD)
2/16 will be homozygous dominant for color and
heterozygous for shape (WWDd)
2/16 will be heterozygous for color and homozygous
dominant for shape (WwDD)
1/16 will be homozygous dominant for color and
homozygous recessive for shape (WWdd)
4/16 will be heterozygous for both traits (WwDd)
22. Steps to Solve the Sample
Problem
2/16 will be heteozygous for color and
homozygous recessive for shape (Wwdd)
1/16 will be homozygous recessive for color
and homozygous dominant for shape
(wwDD)
2/16 will be homozygous recessive for color
and heterozygous for shape (wwDd)
1/16 will be homozygous recessive for both
traits (wwdd)
This is a 1:2:2:1:4:2:1:2:1 genotypic ratio
23. Steps to Solve the Sample
Problem
Determine the genotypic and phenotypic
ratios for the F2 generation:
Phenotypic ratios:
9/16 will have white, disk-shaped fruit
3/16 will have white, sphere-shaped
fruit
3/16 will have yellow, disk-shaped fruit
1/16 will have yellow, sphere-shaped
fruit
This is a 9:3:3:1 phenotypic ratio.
