Genetic Linkage & Recombination Biology • PCM Notes
Enter progeny counts from a test cross. Parental types are the original combinations; recombinants are new combinations. Recombination frequency = map distance in centiMorgans (cM).

Progeny Phenotype Counts (Test Cross Data)

Parental Type 1: Parental
Parental Type 2: Parental
Recombinant Type 1: Recombinant
Recombinant Type 2: Recombinant

Understanding Genetic Linkage

Quick Answer: Genetic linkage occurs when genes are located close together on the same chromosome and tend to be inherited together. Recombination frequency during crossing over is used to map gene positions in centiMorgans (cM).

Key Concepts

Recombination Frequency (%) = (Number of recombinant progeny / Total progeny) x 100. 1% recombination = 1 cM. Genes are linked if recombination frequency is significantly less than 50%. At 50%, genes assort independently.

Solved Examples

Tight Linkage
Parental: 480, 470
Recombinant: 25, 25
RF = 50/1000 = 5%
Genes are 5 cM apart
Moderate Linkage
Parental: 420, 380
Recombinant: 100, 100
RF = 200/1000 = 20%
Genes are 20 cM apart
Independent Assortment
Parental: 250, 250
Recombinant: 250, 250
RF = 500/1000 = 50%
Not linked

FAQs

Why does recombination frequency never exceed 50%?

During meiosis, crossing over occurs between two chromatids out of four. The maximum recombination between two genes is 50%, which is indistinguishable from independent assortment.

What is the difference between linkage and crossing over?

Linkage is the tendency of genes to be inherited together. Crossing over is the physical exchange of chromosomal segments that breaks linkage and creates new combinations.

How is a genetic map constructed?

By calculating recombination frequencies between multiple gene pairs, geneticists determine relative distances and gene order. 1% recombination frequency equals 1 map unit or 1 centiMorgan.