3 map units if length of chromosome is 10 map units. as frequency of linkage is inversely proportional to distance between the genes.

Introduction:
In this cross, we are studying the recombination frequency between two genes, a and b. We have homozygous individuals for genes a and b, and we are crossing them with individuals that have the wild type allele for both genes. The objective is to determine the distance between genes a and b based on the percentage of parental-type offspring.

Parental Types:
Out of 1000 individuals in the cross, 700 individuals were of parental type. Parental type refers to individuals that possess the same combination of alleles as one of the parents. In this case, the parental types are individuals homozygous for genes a and b (aaBB or AAbb) and individuals with the wild type allele for both genes (AABB or aaBB).

Recombinant Types:
The remaining 300 individuals are considered recombinant types. Recombinant types have a different combination of alleles compared to both parents. In this case, the recombinant types would be individuals that have one gene from each parent (AaBB or aaBb).

Calculating the Recombination Frequency:
To determine the distance between genes a and b, we need to calculate the recombination frequency. The recombination frequency is the percentage of recombinant types among the total offspring.

Recombination frequency = (Number of recombinant types / Total number of offspring) * 100

In this case, the recombination frequency is:
(300 / 1000) * 100 = 30%

Determining Genetic Distance:
The recombination frequency is directly proportional to the distance between genes on a chromosome. As the distance between genes increases, the likelihood of recombination events occurring between them also increases.

The formula to calculate the genetic distance (in map units) between two genes is:
Genetic distance = Recombination frequency * 100

In this case, the genetic distance between genes a and b is:
30% * 100 = 30 map units.

Conclusion:
Based on the given cross, we determined that the distance between genes a and b is 30 map units. This means that there is a 30% chance of recombination occurring between these two genes. The higher the recombination frequency, the further apart the genes are on the chromosome. This information is valuable in understanding the genetic linkage and distance between genes, which can provide insights into inheritance patterns and genetic mapping.