Four genes (A, B, C, D) are known to be located on the same chromosome. A series of crosses was completed to determine the recombination frequencies: A-B 50% A-C 18% A-D 31% B-C 39% B-D 50% C-D 49% Based on these numbers, what is the map distance between A and B

Explanation:

1. The largest possible map distance is 50, it represents that  two genes are found at opposite ends of a chromosome or are 50 cM apart from each other or they are found on different chromosomes .

2. Genes found at opposite ends of a chromosome behave as if they are on different chromosomes.

3. As given A-B 50%, means  that recombination frequency is 50%.

The recombination frequency(RF), 50% means that either two genes are located on different chromosomes inheriting independently or  when they are widely separated on the same chromosome with far distance.  

4. As  given here, four genes named A, B, C,  and D are known to be located on the same chromosome. so A-B are 50 cM or more  far  apart from each other.

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