Problems of Quantitative Genetics

Question: An individual heterozygous for three genes XxYyZz test crossed to xxyyzz and 1000 progenies are classified by the genetic contribution of the heterozygous parent as follows

XYZ	390
xyz	374
XyZ	27
xYz	30
XYz	5
xyZ	8
Xyz	81
xYZ	85
Total	1000

Draw a linkage map of the linked genes, showing the order and distance in centimorgan (cM).

Solution: 

XYZ	390	Parental type
xyz	374
XyZ	27	Single crossover between Z and Y
xYz	30
XYz	5	Double crossover
xyZ	8
Xyz	81	Single crossover between X and Z
xYZ	85
Total	1000

1.             1. XYZ and xyz are parental type

2.         2. Order of the genes

·       Double crossover are always in the lowest frequency in this case  XYz and xyZ are in the lowest frequency

·       A DCO event moves the middle allele from one sister chromatid to the other.

·       This event places the non-parental allele of the middle gene onto a chromosome with the parental alleles of the two flanking genes.

·       Z gene must be in the middle because the recessive z allele is now on the same chromosome as the X and Y allele and the dominant Z allele on the same chromosome as the recessive x and y alleles.

Therefore, gene order is XZY.