Gene Linkage

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Linked genes are found on the same chromosome, and are therefore usually inherited together. Linked genes may be separated from one another if crossing over occurs between homologous chromosomes. The closer
together two genes are on a chromosome, the less frequently crossing over will occur between them. In other words, determining the frequency of cross-over (%CO) gives us information about the relative distance between two genes.

The basic question is - are the genes - A or a linked to the genes B or b? A typical linkage question will ask (1) whether genes are linked, (2) which genes are linked, and (3) what is the % cross-over.

A Punnett square gives us the expected ratio. If you wish to determine if genes are linked, you must first assume they are not and determine if four genotypes are produced in a 25%, 25%, 25%, 25% ratio in the hybrid backcross. Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. 

• If the ratio is 1:1:1:1 (all 25% genotypes), the genes are NOT linked and the cross over % = 0. 
• If the ratio is NOT 1:1:1:1 (usually significantly more of two major products, two equal and much larger numbers, and significantly less of two minor/mutant products), the genes must be linked. Calculate % cross-over by dividing the sum of the minor mutant products by the total number of offspring, then converting to a percentage by multiplying by 100:

Ex: Parent 1 AAbb x Parent 2 aaBB -> all hybrid offspring AaBb in the F1 generation. 

The F1 or P2 individuals are backcrossed with a pure recessive to bring out any recessive genes (individuals). 

Cross AaBb x aabb and check the results: 

Aabb and aaBb show up more frequently than expected in the example data above. We can assume these are our major products. AaBb and aabb are our minor/mutated products resulting from unlikely crossing over. 

Practice

Take the practice quiz to try some linkage problems.