Page 2 - LN

 On crossing the same for an F2 generation, we would observe four combinations
of characters in the ratio of 9:3:3:1.
 Thus, 9:3:3:1 is the dihybrid ratio.

Laws of Mendel associated with Dihybrid cross
Law of Independent Assortment says that the traits can segregate independently of
different characters during gamete formation.
Dihybrid Cross Examples
 Mendel took a pair of contradicting traits together for crossing, for example colour
and the shape of seeds at a time.

 He picked the wrinkled-green seed and round-yellow seed and crossed them. He
obtained only round-yellow seeds in the F1 generation.

 This indicated that round shape and yellow colour of seeds are dominant in
nature.

 Meanwhile, the wrinkled shape and green colour of seeds are recessive traits.
Then, F1 progeny was self-pollinated.

 This resulted in four different combinations of seeds in the F2 generation. They
were wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the
phenotypic ratio of 9:3:3:1.
 During monohybrid cross of these traits, he observed the same pattern of
dominance and inheritance. The phenotypic ratio 3:1 of yellow and green colour
and of round and wrinkled seed shape during monohybrid cross was retained in
dihybrid cross as well.

 Consider “Y” for yellow seed colour and “y” for green seed colour, “R” for round
shaped seeds and “r” for wrinkled seed shape. Thus, the parental genotype will
be “YYRR” (yellow-round seeds) and “yyrr” (green-wrinkled seeds).
DIFFERENCE BETWEEN MONOHYBRID AND DIHYBRID CROSS

Properties Monohybrid Dihybrid

Means Mono refers to single and hybrid Di refers to two or double and
means mixed breed hybrid means breed

Cross Monohybrid cross is used to study Dihybrid cross is used to study the
the inheritance of a single pair of inheritance of 2 different alleles
alleles

Used to the dominance of genes Offspring assortment
study

2