BIO340 Activity #4: 3–Point Cross, Gene Order and Mapping
I. Determining single–crossovers (SCOs), double–crossovers (DCOs) and gene order.
(Initially, you do not know what the order of the genes is on the chromosome. Your DCOs will
be compared to your parental types to determine which gene is in the middle.)
We want to map the distance between genes for:
leaf color (A – green, dominant; a – yellow, recessive),
leaf texture (B – rough, dominant; b – smooth, recessive), and
plant fertility (F – normal, dominant; f – variable, recessive).
Your mapping cross is like a test cross. One parent (mapping individual) is heterozygous for all
genes (so crossover events can be assessed) while the other parent (“cryptic” or “tester”) is
homozygous recessive for all genes (so crossover events will be cryptic, i.e. undetectable).
Gene order is not yet determined.
Below, (in the order in which the genes are presented above) write the genotypes of the
mapping cross individual (i.e. the heterozygote) and the cryptic individual:
___________________ X _____________________
(mapping) (cryptic)
Table 1: Progeny from Mapping Cross
Phenotype Frequency Phenotype Frequency
1. Green, rough, normal 95 5. Green, smooth, normal 900
2. Yellow, rough, normal 50 6. Yellow, smooth, normal 5
3. Green, rough, variable 5 7. Green, smooth, variable 50
4. Yellow, rough, variable 900 8. Yellow, smooth, variable 95
Based on these results, are the genes linked or not? ___________
How would the phenotypic frequencies look if the genes were not linked? ________________
_____________________________________________________________________________
Which two numbered phenotypes in Table 1, represent offspring of the parental types? _____
and _____
Using slash notation, the parental type offspring have genotypes: _______________ and
_______________.
The actual chromosomal configuration of the mapping individual in the cross must have been
________ /________.

Which two numbered phenotypes in Table 1, represent offspring from DCOs? _____ and _____
Using slash notation, the DCO offspring have genotypes: _______________ and
_______________.
Comparing the mapping individual’s chromosome configuration with the chromosome
configuration of the DCOs, you can tell which gene is in the center of the other two based upon
which gene “switched sides” from parental type chromosomes to DCO type chromosomes?
Which gene is in the middle? __________ The gene order then is: _____________
II. Map Distance
To figure out the distances, we must identify SCOs between the gene in the middle and each of
the other two genes.
Looking at the chromosomes inherited by each progeny type above in Table 1, identify the
numbered offspring types that result from a single–crossover between the gene on the far left
(as you drew them above) and the middle gene: ___________ and ___________.
The map distance between these genes = ([(the number of SCOs between these genes) + (the
number of DCOs)] / (Total number of offspring) X 100.
Calculate the map distance between the gene on the far left and the middle gene:
Complete the figure illustrating the products of a double–crossover (in meiosis)
Mapping Individual Double–Crossover DCO Products

Looking at the chromosomes inherited by each progeny type above in Table 1, identify the
numbered offspring types that result from a single–crossover between the middle gene and the
gene on the far right (as you drew them above): ___________ and ___________.
Again, the map distance between these genes = ([(the number of SCOs between these genes) +
(the number of DCOs)] / (Total number of offspring) X 100.
Calculate the map distance between the middle gene and the gene on the far right:
III. Draw the final map (include all distances):