SCIENCE MENDELIAN GENETICS

1. Mendelian Genetics:
Dihybrid cross
Explore the foundational principles of
genetics. Discover Gregor Mendel's
groundbreaking experiments.
Understand the key terms of heredity,
including genes and alleles. Also review
dominant and recessive traits.
Examine Mendel's Laws, including
Segregation and Independent Assortment.

2. Dihybrid Crosses
Dihybrid crosses involve two different genes or traits.
Independent Assortment: Alleles assort independently during gamete formation.
The phenotypic ratio in the F2 generation is typically 9:3:3:1.
Dihybrid Cross
• Two Traits
• Independent Assortment
• 9:3:3:1 Ratio
Mendel's Work
• Pea Plants
• Seed Color
• Seed Shape

3. What is a dihybrid cross?
 a breeding experiment involving two organisms that
are identical hybrids for two traits or characters. A
hybrid organism is heterozygous, which means it
carries two alleles of a particular gene. An allele is an
alternative gene form inherited from each parent
during sexual reproduction. A dihybrid cross
determines the genotypic and phenotypic combination
of offspring for two particular genes that are unlinked.

4. How to Set Up Dihybrid Cross
1. Signify each allele using characters. Capital letters
are used for the dominant allele, lower case letters for
the recessive allele.
2. Write the genotype and phenotype of the parents
(P generation). Always pair alleles for the same gene
and write capital letters first and then small letters
(like AaBb).

5. How to Set Up Dihybrid Cross
4. Use a Punnett square to work out potential
genotypes of offspring. Include the different
gamete combinations for each parent (like
AaBB has two combinations, AB and aB).
5. Write the phenotype ratios of potential
offspring.

6. Setting Up a Dihybrid Cross
Start with the parental (P) generation.
Cross two true-breeding parents that differ in two traits.
Example: YYRR (yellow, round) x yyrr (green, wrinkled).
P Generation
YYRR x yyrr
F1 Generation
YyRr
F2 Generation
YyRr x YyRr

7. Punnett Square for
Dihybrid Cross
F1 individuals (YyRr) form four possible gametes: YR, Yr, yR, yr.
Construct a 4x4 Punnett square to show the 16 possible F2 genotypes.
Determine genotypes and phenotypes of offspring.
Gametes
YR, Yr, yR, yr
Punnett Square
4x4 grid
F2 Genotypes
16 possibilities

8. Seed Color & Shape Example
Consider the cross: YyRr x YyRr (F1 generation).
Four possible phenotypes in F2.
9/16 Yellow, Round; 3/16 Yellow, Wrinkled; 3/16 Green, Round; 1/16 Green, Wrinkled
9/16
Yellow, Round
1 3/16
Yellow, Wrinkled
2
3/16
Green, Round
3
1/16
Green, Wrinkled 4

9. Practice Problem: Guinea
Pigs
Black fur (B) is dominant to brown fur (b).
Rough coat (R) is dominant to smooth coat (r).
Cross a heterozygous black,
heterozygous rough guinea pig
(BbRr) with a brown, smooth
guinea pig (bbrr).
Set Up Square
Determine Gametes
Find Genotypes

10. Tomato & Dog Crosses
Problem 2: Red fruit (R) is dominant to yellow (r); tall plants (T) are dominant to dwar
Problem 3: Black nose (B) is dominant over pink (b); short hair (S) is dominant over long ha
Tomatoes
RrTt x RrTt
Dogs
BbSs x bbss

11. Group Activity: Solve Dihybrid Crosses
Divide into groups to solve five dihybrid cross problems.
Show all work, including Punnett squares and ratios.
Present your group's solutions to the class.
Corn
Roses
Chickens