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अभ्यासक्रम

Types of Linkage > Incomplete Linkage

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Estimated time: 10 minutes
CISCE: Class 12

Definition: Incomplete linkage

The condition in which genes present on the same chromosome show partial separation due to crossing over during meiosis, resulting in both parental and recombinant combinations, is called 'incomplete linkage'.

CISCE: Class 12

Incomplete Linkage

Mechanism (Crossing Over): The separation of linked genes is caused by crossing over. This is the physical exchange of parts of non-sister chromatids of homologous chromosomes following synapsis during meiosis.

Deviation from Mendel: In a standard test cross for independently assorting genes, a 1:1:1:1 ratio is expected (25% frequency for each phenotype). Conversely, if genes were fully linked, a 1:1 ratio (50% for each parental type, 0% recombinants) would be expected.

The Incomplete Linkage Signature: Incomplete linkage yields a ratio that falls between these two extremes. It produces a significantly higher percentage of parental types and a lower, but non-zero, percentage of recombinant types.

CISCE: Class 12

Sweet Pea Experiment

In sweet peas, the traits for flower colour and pollen shape exhibit incomplete linkage.

  • The Cross: A sweet pea variety with blue flowers and long pollen (BBLL) is crossed with a variety having red flowers and round pollen (bbll). All F1 individuals (BbLl) produce blue flowers and long pollen.
  • The Test Cross: The F1 hybrids (BbLl) are test-crossed with the double recessive parent (bbll).
  • The Result: Instead of the expected 1:1:1:1 ratio, a 7:1:1:7 ratio is obtained. This highlights a tendency for the dominant alleles to remain together but proves they can still be separated.

Test Cross Frequency Breakdown

Phenotype Gene Combination Observation Expected (Independent Assortment) Expected (Fully Linked)
Blue-long Parental 43.7% 25% 50%
Blue-round Recombinant 6.3% 25% 0%
Red-long Recombinant 6.3% 25% 0%
Red-round Parental 43.7% 25% 50%

The total recombinant frequency (blue-round + red-long) is exactly 12.6%, proving the genes are linked but still undergo crossing over.

Incomplete linkage in sweet pea:

CISCE: Class 12

Key Points: Incomplete Linkage

  • Incomplete linkage allows linked genes to occasionally separate during meiosis.
  • The separation is directly caused by crossing over between non-sister chromatids.
  • It produces both parental and recombinant progeny (unlike complete linkage, which produces zero recombinants).
  • The test cross ratio heavily deviates from Mendel's expected 1:1:1:1, favouring a higher proportion of parental traits (e.g., a 7:1:1:7 ratio).
  • The total frequency of new combinations is greater than 0% but always less than 50%.
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