Genetic Linkage

Analyzing Three-Point Test Cross Data

Genotype	Observed	Type of Gamete
ABC	390	Parental
abc	374	Parental
AbC	27	Single-crossover between genes C and B
aBC	30	Single-crossover between genes C and B
ABc	5	Double-crossover
abC	8	Double-crossover
Abc	81	Single-crossover between genes A and C
aBC	85	Single-crossover between genes A and C
Total	1000

What are the parental genotypes?

• The genotypes most frequently found are the parental genotypes
• ABC and abc are the parental genotypes

What is the gene order?

• The double crossover moves a non-parental allele of the central gene between two parental alleles
• Gene C is between genes A and B (gene order = A C B)

What are the linkage distances?

• Linkage distance equals the sum of the appropriate single cross plus all double crosses divided by total number of gametes
• A - C = (100* (81+85+5+8))/1000 = 17.9 cM
• C - B = (100* (27+30+5+8)/1000) = 7.0 cM