The document outlines the principles of Mendelian genetics through a dihybrid cross involving pea plants with two contrasting traits: seed shape and seed color. It provides details on genotypes, gamete production, and results, culminating in a phenotypic ratio of 9:3:3:1 in the F2 generation. Moreover, it includes practice problems to reinforce understanding of genetic crosses and inheritance patterns.

1. B.Sc. III: Semester-VI
paper-XIX: Genetics and Biotechnology
Mendelian Dihybrid Cross
Dr. Anjali Naik
Head, Dept. of Botany
SBES College of Science.

2. Dihybrid cross
Plants crossed which differ in 2 pairs of contrasting characters. :
• Seed shape: Round (R)>Wrinkled (r) and
• Seed colour: Yellow (Y) > Green (y)
• Genotypes of parents: YYRR X, yyrr
• Phenotypes: Yellow, round X Green, wrinkled

4. dihybrid cross
• Parents- Genotypes= YYRR and yyrr (homozygous)
• Parents- Phenotypes: Yellow, Round X green, Wrinkled.
• Gametes produced by parents: YR X yr
• F1: All YyRr (Inbred) YyRr x YyRr
• Gametes of F1 hybrids: 4 types- YR, Yr, yR, Yr.
• F2: 4 gametes of Female parent X 4 gametes of male parent
• Total 16 combinations of genotypes and four phenotypes: Yellow, Round; Yellow,
green; Wrinkled, round and wrinkled, green.

5. Mendel’s dihybrid cross

6. Steps to follow for solving problems based on dihybrid
ratio

7. Plotting Punnett square
Male parent
Female parent
YR Yr yR yr
YR
Yr
yR
yr

8. Filling out F2 genotypes and phenotypes in punette
square
Male parent
Female parent
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr
yR YyRR
yr YyRr

9. Final Filled Punett Square
Male parent
Female parent
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr
yR YyRR YyRr
yr YyRr Yyrr

10. 9 Genotypic classes and their proportions in F2
generation of classic Dihybrid classes.

11. Things to remember in Dihybrid inheritance

12. Law of Independent Assortment.
• Definition: It states that, during formation of gametes in meiosis, the
pairs of alleles controlling 2 different characters assort independent
of each other:
• For example: Y and y, Yy pair controls the seed colour and
R and r, Rr pair controls seed shape
When RrYy is crossed with RrYy from F1 both Rr and Yy pairs be
assorted to the gametes, independent to each other.

13. Assignment -Practice problems
1. A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical
shape characteristic; s is the allele for the recessive, dented shape characteristic. Y is the allele for the dominant,
yellow color characteristic; y is the allele for the recessive, green color characteristic. Which of the following
genetic crosses would be predicted to give a phenotypic ratio of 9:3:3:1?
A. SSYY x ssyy
B. SsYY x SSYy
C. SsYy x SsYy
D. SSyy x ssYY
E. ssYY x ssyy
2. In rabbits, gray hair is dominant to white hair. Also in rabbits, black eyes are dominant to red eyes. These
letters represent the genotypes of the rabbits: 1. What are the phenotypes (descriptions) of rabbits that have the
following genotypes?
Ggbb ____________________
ggBB ________________________
ggbb ____________________
GgBb _________________________

14. Assignment -Practice problems
3. In summer squash, white fruit color (W) is dominant over yellow fruit
color (w) and diskshaped 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.
(a) What will be the appearance of F1?
(b) What will be the genotypic and phenotypic proportions in F2 if WwDD is
crossed with wwdd?