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Inheritance of Two Genes: Dihybrid Cross

A dihybrid cross follows the inheritance of two gene pairs at the same time and tests whether their alleles assort independently or show linkage.

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Student-friendly explanation

In Mendel's dihybrid cross, two characters such as seed shape and seed colour are followed together. If the genes assort independently, the F2 phenotypic ratio is 9:3:3:1. This occurs when allele pairs separate independently during meiosis and recombine at fertilisation. Sutton and Boveri connected Mendel's factors with chromosomes, forming the chromosomal theory of inheritance. Morgan's work on Drosophila showed that genes on the same chromosome may be linked and inherited together more often than expected. Recombination can break linkage through crossing over. Board questions often test the 9:3:3:1 ratio, linkage, and recombination frequency logic.

How to write this in exams

  1. 1

    Start with the exact idea

    A dihybrid cross follows the inheritance of two gene pairs at the same time and tests whether their alleles assort independently or show linkage.

  2. 2

    Then show how to use it

    Identify the two gene pairs. Write parental genotypes. List all gamete types using segregation and assortment. Construct or mentally combine a 4 by 4 Punnett square. Count phenotypes. If linkage is mentioned, compare parental and recombinant types instead of assuming independent assortment.

  3. 3

    Add one concrete example

    A cross between round yellow and wrinkled green pea plants can produce F2 offspring in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green when the genes assort independently.

  4. 4

    Avoid this incomplete answer

    Writing four gametes for linked genes without considering parental combinations and recombinants can give a false Mendelian ratio.

Definition

A dihybrid cross follows the inheritance of two gene pairs at the same time and tests whether their alleles assort independently or show linkage.

Example

A cross between round yellow and wrinkled green pea plants can produce F2 offspring in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green when the genes assort independently.

Rule to remember

Independent dihybrid ratio: 9:3:3:1. Linked genes deviate from this ratio because parental combinations appear more frequently than recombinant combinations.

Memory hook

Independent assortment mixes two gene pairs freely; linkage keeps nearby genes travelling together more often.

Examples and method

Worked example

For RrYy x RrYy, each parent can form RY, Ry, rY, and ry gametes. Combining these gives 16 boxes and the phenotype ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

Method to apply

Identify the two gene pairs. Write parental genotypes. List all gamete types using segregation and assortment. Construct or mentally combine a 4 by 4 Punnett square. Count phenotypes. If linkage is mentioned, compare parental and recombinant types instead of assuming independent assortment.

Diagram support

A dihybrid Punnett square should label four gamete types from each parent. A linkage diagram should show genes on the same chromosome and crossing over between homologous chromosomes.

How CBSE asks it

The concept is asked through dihybrid ratio derivation, comparison of linkage and independent assortment, Morgan's Drosophila observations, and chromosomal theory statements.

Avoid common mistakes

Common confusion

Students often apply 9:3:3:1 to all two-gene crosses. Linked genes may not show this ratio because they do not assort independently.

Common wrong answer

Writing four gametes for linked genes without considering parental combinations and recombinants can give a false Mendelian ratio.

Exam tip

Before writing the ratio, check whether the question suggests independent assortment or linkage. Morgan-based questions usually expect linkage and recombination reasoning.

Quick check

Why does a standard Mendelian dihybrid cross give a 9:3:3:1 ratio?

It gives a 9:3:3:1 ratio because two gene pairs segregate and assort independently during gamete formation. Different gamete combinations then reunite at fertilisation to produce four phenotype classes.

Answer writing and exam use

1-mark answer

State the two gene pairs, show gametes, present the expected ratio, then explain whether independent assortment or linkage accounts for the observed result.

2-mark answer

State the two gene pairs, show gametes, present the expected ratio, then explain whether independent assortment or linkage accounts for the observed result.

3-mark answer

State the two gene pairs, show gametes, present the expected ratio, then explain whether independent assortment or linkage accounts for the observed result.
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