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CraftExam
medium difficulty7 min

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.

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

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1For RrYy x RrYy, each parent can form RY, Ry, rY, and ry gametes.
  2. 2Combining these gives 16 boxes and the phenotype ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

Method you can reuse

  1. 1Identify the two gene pairs.
  2. 2Write parental genotypes.
  3. 3List all gamete types using segregation and assortment.
  4. 4Construct or mentally combine a 4 by 4 Punnett square.
  5. 5Count phenotypes.
  6. 6If linkage is mentioned, compare parental and recombinant types instead of assuming independent assortment.

Visual tip

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

Common wrong answer

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

Don't mix this up

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.

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

State the two gene pairs, show gametes, present the expected ratio, then explain whether independent assortment or linkage accounts for the observed result.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    In Mendel's dihybrid cross, two characters such as seed shape and seed colour are followed together.

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