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Chapter 4- Genes and Heredity
dihybrid cross- a cross
involving more than one
trait
Nelson Biology 11
Section 4.6 Dihybrid Crosses
Mendel’s Second Experiment: A Dihybrid Cross
 Crossed two pea plants that differed in two traits
round, yellow x wrinked, green
 F1 all round, yellow
 F1 must have been heterozygous for both traits (RrYy)
 F2 results showed a ratio of 9:3:3:1 which would be the same ratio if each
trait was examined individually (3:1 and 3:1)
Conclusions:
Law of Independent Assortment
The inheritance of alleles for one trait does not affect the inheritance of alleles
for another trait.
Solving Dihybrid Crosses
Skill: Determining Gametes of Parents
Remember that a gamete contains one copy of each gene. Therefore it will
contain one copy of each letter.
AaBb
possible gametes are A and B, A and b, a and B and a and b
Simply written: AB, Ab, aB, ab
How to find the gametes: draw a rainbow line connecting the first allele with
each partner, repeat for second.
Practice Problem
Chapter 4- Genes and Heredity Nelson Biology 11
What are the results of a cross between a heterozygous round yellow pea plant
and a wrinked green pea plant?
Step 1: Define alleles.
Round- dominant- R
Wrinkled- recessive- r
Yellow- dominant- Y
Green- recessive- y
Step 2: Write genotypes and phenotypes of parents.
Round yellow RrYy
Wrinkled green rryy
Step 3: Determine gametes of parents.
RrYy- RY Ry rY ry
rryyry
Round Yellow RrYy
Step 4: Complete the punnet square.
Wrinked green rryy
F1 ry
ry
ry
RY
RrYy
RrYy
RrYy
ry
RrYy
Ry
Rryy
Rryy
Rryy
Rryy
rrYy
rrYy
rrYy
rrYy
rryy
rryy
rryy
rryy
rY
ry
Step 5: Summarize results. As you count a genotype, cross it out.
Genotype
RrYy
Rryy
rrYy
rryy
4/16
4/16
4/16
4/16
genotypic ratio: RyYr:Rryy:rrYy:rryy=1:1:1:1
Phenotype Round Yellow 4/16
Round green 4/16
Wrinked yellow 4/16
Wrinkled green 4/16
Phenotypic ratio
Practice p. 154#1-5
1:1:1:1
Chapter 4- Genes and Heredity
Nelson Biology 11