Dihybrid Crosses and Independent Assortment
Experiment 3
• Mendel wanted to know if inheriting
one gene would affect the inheritance of
other genes.
– Ex: Does the seed shape gene
influence the seed color gene?
– Mendel needed to follow 2 diff.
alleles from one generation to the
next.
– Dihybrid Cross: Genetic crosses
involving two genes
(single gene crosses are known as
monohybrid crosses)
Dihybrid Crosses and Independent Assortment
P generation: Crossed a homozygous RRYY (round yellow
peas) with a homozygous rryy (wrinkled, green peas)
What was the genotype
and phenotype of the
F1 offspring?
Dihybrid Crosses and Independent Assortment
• Mendel then crossed two of the F1 plants which
were heterozygous for both traits.
• If the two genes were not linked to each other and
were inherited independently of each other, then
you would see every possible combination of alleles.
• Setup a dihybrid Punnett square to show the
predicted phenotype ratio if the genes for seed
shape and seed color do not influence each other
(assort independently)
Dihybrid Crosses and Independent Assortment
R r Yy X R r Yy
http://www.youtube.com/watch?v=Y1PCwxUDTl8
Dihybrid Crosses and Independent Assortment
• When Mendel bred the two F1 plants his results were very
close to the 9:3:3:1 ratio he predicted with the dihybrid
cross.
• Principle of Independent Assortment:
– Individual genes are passed
onto gametes independently
of each other.
– The principle of independent
assortment is only true if the
two genes are found on
different chromosomes.
– Genes found on the same
chromosome are often
inherited together
Summary of Mendel’s Principles
1. Law of Dominance: Some alleles are dominant and
cover up the recessive alleles.
2. Law of Segregation: An organism has two alleles
for each gene but they can only pass on one.
3. Law of Independent Assortment: Genes found on
separate chromosomes are inherited independently
of each other.