1. Using
Phenotype
Genotype
to determine

2. Which characteristic in the diagram below is dominant?
How do you know?

3. Get a copy of the Using Phenotype to Determine Phenotype handout, read the directions and complete the first 2 pages to see if you can determine your genotype based on your phenotype

4. How can I determine whether this purple flowers is . . .
homozygous (PP) or
heterozygous (Pp)

5. Animal breeders, horticulturists, and others involved in breeding organisms often need to know whether an organism with a dominant phenotype is heterozygous or homozygous for a trait. How do they determine this? For example, how might a horticulturist determine whether a pea plant with a dominant phenotype, such as purple flowers, is homozygous (PP) or heterozygous (Pp)? The horticulturist could perform a test cross.

6. In a Test Cross, an individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual.

7. Thus, the genotype of the “unknown” plant must be YY.
A plant with yellow seeds but of unknown genotype (Y?) is test-crossed with a plant with green seeds (yy).
If all of the offspring produce yellow seeds, the offspring must be Yy.
Thus, the genotype of the “unknown” plant must be YY.

8. If half of the offspring produce yellow seeds
A plant with yellow seeds but of unknown genotype (Y?) is test-crossed with a plant with green seeds (yy).
If half of the offspring produce yellow seeds
and half produce green seeds,
the genotype of the unknown plant must be Yy.

9. You can use a test cross to determine whether a plant with purple flowers is heterozygous (Pp) or homozygous dominant (PP). On a separate sheet of paper, copy the two Punnett squares shown below, and fill in the boxes in each square.

10. Pp pp Pp

11. Mendel’s Law of
Independent Assortment
& Dihybrid Crosses

12. Suppose a horticulturist has two traits that she wants to consider when crossing two plants. A cross that involves two pairs of contrasting traits is called a dihybrid cross.
For example, she may want to predict the results of a cross between two pea plants that are heterozygous for
seed shape (R = round, r = wrinkled) and seed color (Y = yellow, y = green).

13. Look at and investigate
“Crosses That Involve Two Traits” on page 171 of your Holt textbook

14. Mendel’s Law of Independent Assortment states that the alleles of different genes separate independently of one another during gamete formation.

15. Remember from last class period?

16. Due to Mendel’s Law of Independent Assortment, what allele combinations would you expect to get from an organism such as the one on the left R r Y y
Ry ,
ry ,
RY,
rY ,

17. Complete the Punnett square
Now lets write the gametes on the top and left sides of a Punnett square
R Y R Y r Y r Y
R Y
r Y
Complete the Punnett square

18. What is the phenotypic ratio of a dihybrid cross where the parents are heterozygous for both traits?
9:3:3:1

19. In the monster community, horns are controlled by an autosomal gene that can occur in the dominant form, (H) which have two horns, or the recessive form, ( h ) which has only one horn.
The length of the length of the monsters horns is controlled by another autosomal gene that occurs in the dominant form, (S) short horns, or the recessive form, (s) long horns.
Male Monster
Female Monster
HhSs
hhSS

20. Male Monster
Female Monster
HhSs
hhSS

21. Now lets see if we can complete the 3rd page of your “Using Phenotype to Determine Genotype” handout