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PinkMonkey Online Study Guide-Biology

7.7 Dihibrid Ratio

Mendel established the law of segregation through monohybrid crosses involving only one pair of alleles at a time. However, in an organism, so many characters are present together and each character is represented by a pair of alleles. So, Mendel wanted to know, "Whether one pair of alleles affects or influences the inheritance pattern of other pairs of alleles in the organism or, each pair is inherited independently as if in a monohybrid cross?" To find the answers to these questions, Mendel performed dihybrid crosses.

A cross between two pure, true breeding parents in which the inheritance pattern of two allelomorphic pairs is considered (studied) simultaneously is called a dihybrid cross. The phenotypic ratio obtained in the F2 generation of a dihybrid cross is called the dihybrid ratio.

A dihybrid is an individual which is double heterozygous (i.e. heterozygous for two pairs of alleles).

Mendel’s dihybrid cross : Mendel considered two characters in the pea plants simultaneously, e.g. cotyledon color (yellow / green) and seed shape (round / wrinkled). He selected one variety of pea which was pure (true breeding) for yellow round seeds and crossed it with another variety pure for green wrinkled seeds.

All the F1 of this cross were yellow round seeds (green and wrinkled characters did not appear in F1 hybrids). Mendel anticipated this because, from the earlier monohybrid experiments he knew that yellow was dominant over green and round was dominant over wrinkled.

P     Yellow Round     X     Green Wrinkled


F1                    Yellow Round

Similarly, a cross between yellow wrinkled and green round also produced only yellow round seeds in F1

P     Yellow Wrinkled     X     Green Round


F1                        Yellow Round

Moreover, the reciprocal crosses (interchanging male and female parents) also gave the same results.

Further, when the F1 dihybrids were self-pollinated or inbred, the F2 generation was always the same, e.g.

Click here for enlarge

The analysis of F2 progeny showed four different kinds of phenotypes. These were (1) Yellow round (2) Yellow Wrinkled (3) Green round and (4) Green Wrinkled in the ratio of 9:3:3:1 respectively. It will be seen that out of these four types, two show the same combinations as the parents whereas the remaining two are new combinations (recombinants).

The phenotypic ratio of 9:3:3:1 in the F2 progeny of a dihybrid cross is called the dihybrid ratio. Same results were obtained by Mendel using other pairs of alleles in different combinations.

On the basis of these experiments and their results, Mendel formulated the law of independent assortment of characters and explained it as follows.

Table of Contents

7.0 Introduction
7.1 Gregor Mendel
7.2 Mendel's Experiment on Sweet Pea
7.3 Terminology Used
7.4 Law of Dominance
7.5 Monohybrid Ratio
7.6 Law of Segregation
7.7 Dihybrid Ratio
7.8 Law of Independent Assortment
7.9 Test Cross or Back Cross
7.10 The Concept of "Factor"

Chapter 8


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